Friday, December 25, 2009
Tuesday, October 20, 2009
Physiological defect: Presbyopia.(2) Causes and risk factors
Enlace a entrada en español.
Causes
As I explain in a previous post, presbyopia is a visual disorder that is product of the time as it goes by.
When we look something at near something in order to focus and to see clear, a phenomenon is caused in our eyes called ACCOMMODATION, and the lens has to change its shape so that the image of what we see, is focused on the retina. But over the years, on one hand, the ciliary muscle, as the rest of body muscles, gets more rigid and shorter; and on the other hand, the lens increases the thickness of its faces, thus losing its elasticity (this therory is the most approved one). So, the muscle loses the ability to contract itself to allow that the lens to be more convex (at the same time, the crystalline lens does not do it because of its stiffness), and the accommodation does not happen.
This loss of accommodation process is gradual. It begins affecting the vision in the closest distance, and the changes of focus in different near distances are slower and with worse quality (those changes of focus are what we call “accommodative flexibility”). Little by little the ability to focus on near distances and increasingly farther from oneself its more difficult; this happens when our arms begin to stretch and in a certain moment they seem short. When it happens, this indicates that our focus is failing and we need a lens that compensates the accommodative effort that we can not perform anymore.
When a child is 10 years old, has an accommodation of around 14 diopters (that means, the closest distance where she can focus something very small is at around 7 cm); this quantity decreases linearly with the age, until that, at 50 years old, the accommodation is around 2.50 diopters (the closest distance where she can focus something very small is at around 40 cm, as you see the focusing ability has decreased almost 6 times the one of the child).
This process begins between the 40’s and 50’s (depending on the refractive error that we already have at a young age; at the beginning the presbyopia starts adding a little positive lens for near distance -1.00 diopter approximately - over the graduation that we need to see far away); it usually stops around 58 years old (with a addition of around 2.25 or 2.50 diopters).
Presbyopia equally affects everyone, but hyperopic people start to suffer it at an earlier age than the emmetropic one; and these ones earlier than myopic people. Some of these myopic people (low and medium), when the presbyopia appears in their lifes, they are capable to read without using their eyeglasses, because of the compensation that is caused between both phenomenons.
From the moment presbyopia appears, this increases during a period of 10 or 12 years and then it is strabilized. Some of you, seeing how fast presbyopia increases when it begins, might think that it will never stop, but I assure you that it does.
In the case of some emmetropic or low hyperopic people, they suffer an increase of hyperopia after using reading eyeglasses of +1.00 dipoter during some time, when presbyopia is evident; so that after a couple of years, that graduation is not enough for the close up tasks, but they are only useful for distance vision. This is called HYPEROPIA OF PRESBYOPIA.
Risk Factors
According to American Optometric Association
RELATED POST
Physiological defect: Presbyopia.(1) Vision, Appearance and Symptoms
Refractive visual disorders. Some clarifications.
Some numbers...
Causes
As I explain in a previous post, presbyopia is a visual disorder that is product of the time as it goes by.
When we look something at near something in order to focus and to see clear, a phenomenon is caused in our eyes called ACCOMMODATION, and the lens has to change its shape so that the image of what we see, is focused on the retina. But over the years, on one hand, the ciliary muscle, as the rest of body muscles, gets more rigid and shorter; and on the other hand, the lens increases the thickness of its faces, thus losing its elasticity (this therory is the most approved one). So, the muscle loses the ability to contract itself to allow that the lens to be more convex (at the same time, the crystalline lens does not do it because of its stiffness), and the accommodation does not happen.
This loss of accommodation process is gradual. It begins affecting the vision in the closest distance, and the changes of focus in different near distances are slower and with worse quality (those changes of focus are what we call “accommodative flexibility”). Little by little the ability to focus on near distances and increasingly farther from oneself its more difficult; this happens when our arms begin to stretch and in a certain moment they seem short. When it happens, this indicates that our focus is failing and we need a lens that compensates the accommodative effort that we can not perform anymore.
When a child is 10 years old, has an accommodation of around 14 diopters (that means, the closest distance where she can focus something very small is at around 7 cm); this quantity decreases linearly with the age, until that, at 50 years old, the accommodation is around 2.50 diopters (the closest distance where she can focus something very small is at around 40 cm, as you see the focusing ability has decreased almost 6 times the one of the child).
This process begins between the 40’s and 50’s (depending on the refractive error that we already have at a young age; at the beginning the presbyopia starts adding a little positive lens for near distance -1.00 diopter approximately - over the graduation that we need to see far away); it usually stops around 58 years old (with a addition of around 2.25 or 2.50 diopters).Presbyopia equally affects everyone, but hyperopic people start to suffer it at an earlier age than the emmetropic one; and these ones earlier than myopic people. Some of these myopic people (low and medium), when the presbyopia appears in their lifes, they are capable to read without using their eyeglasses, because of the compensation that is caused between both phenomenons.
From the moment presbyopia appears, this increases during a period of 10 or 12 years and then it is strabilized. Some of you, seeing how fast presbyopia increases when it begins, might think that it will never stop, but I assure you that it does.
In the case of some emmetropic or low hyperopic people, they suffer an increase of hyperopia after using reading eyeglasses of +1.00 dipoter during some time, when presbyopia is evident; so that after a couple of years, that graduation is not enough for the close up tasks, but they are only useful for distance vision. This is called HYPEROPIA OF PRESBYOPIA.
Risk Factors
According to American Optometric Association
RELATED POST
Physiological defect: Presbyopia.(1) Vision, Appearance and Symptoms
Refractive visual disorders. Some clarifications.
Some numbers...
Physiological defect: Presbyopia.(1) Vision, Appearance and Symptoms
Enlace a entrada en español
Presbyopia is what everybody knows as “old sight”. It is not considered as refractive disorder or an ocular illness.
Vision of presbyopic
The typical complaint in presbyopia is “BLURRED VISION AT CLOSE DISTANCE”. A presbyopic person stretches her arms in order to see better, but in a certain moment “THEY ARE TOO SHORT”.
Distance vision may have not any changes, at least in the first stages.
Appearance of presbyopic eye
The shape of the eye is like a myopic or hyperopic or even emmetropic one at young age; but the difference is given by the crystalline lens. When a person looks at objects up close, she is not capable of focusing them on the retina, but behind (as hyperopic people), therefore seeing them blurred.
Prevalence
Everybody will have suffered presbyopia sooner or later, after 45 years old.
As presbyopia is age-related, its prevalence is higher in societies in which larger proportions of the population survive into old age.
For instance, from all Spanish population, 43% suffers “old sight”. And from this 43%, 7% of presbyopic people have never checked their vision in any previous occasion. Because of the increased life expectancy, within 10 years, more than half of the Spanish population will suffer from this condition.
It does not exist any way tested to be true in order to prevent it.
Symptoms
Incipient presbyopia (the first symptoms)
Evident presbyopia
RELATED POST
Refractive visual disorders. Some clarifications.
Some numbers...
Refractive disorders: Hyperopia , Hypermetropia or Farsightedness. (1) Vision and Accommodation
Presbyopia is what everybody knows as “old sight”. It is not considered as refractive disorder or an ocular illness.
Vision of presbyopic
The typical complaint in presbyopia is “BLURRED VISION AT CLOSE DISTANCE”. A presbyopic person stretches her arms in order to see better, but in a certain moment “THEY ARE TOO SHORT”.
Distance vision may have not any changes, at least in the first stages.
Appearance of presbyopic eye
The shape of the eye is like a myopic or hyperopic or even emmetropic one at young age; but the difference is given by the crystalline lens. When a person looks at objects up close, she is not capable of focusing them on the retina, but behind (as hyperopic people), therefore seeing them blurred.
Prevalence
Everybody will have suffered presbyopia sooner or later, after 45 years old.
As presbyopia is age-related, its prevalence is higher in societies in which larger proportions of the population survive into old age.
For instance, from all Spanish population, 43% suffers “old sight”. And from this 43%, 7% of presbyopic people have never checked their vision in any previous occasion. Because of the increased life expectancy, within 10 years, more than half of the Spanish population will suffer from this condition.
It does not exist any way tested to be true in order to prevent it.
Symptoms
Incipient presbyopia (the first symptoms)
- It is difficult to maintain the performance of near task in a continuous way.
- Headache because of the continuing effort.
- The letters seem to move on the paper.
- More light is necessary in order to see better.Near vision is worse at the end of the day.
- Sometimes it seems that we “do not control” our eyes (as the accommodation and convergence are related, the gradual loss of accommodation involves the progressive worsening of our fusion, resulting in Convergence insufficiency, and even in a double vision when we are more tired).
Evident presbyopia
- We stretch our arms in order to find the distance where we can read whatever we want, until a certain moment where we do not get to read although we stretch them completely.
- At the beginning presbyopia only affects near distance (documents, handicrafts,…), and little by little it starts affecting intermediate distance too (computer,…)
- We tend to avoid the small texts and precision tasks in near vision whenever we can (we do not sew anymore, we do not read so much, we do not perform certain leisure activities that we liked… :-( )
- We always look for natural light or a good light lamp.
- It is usually accompanied by “Dry eye”.
RELATED POST
Refractive visual disorders. Some clarifications.
Some numbers...
Refractive disorders: Hyperopia , Hypermetropia or Farsightedness. (1) Vision and Accommodation
Thursday, September 17, 2009
World Sight Day 2009 (WSD)
Información en español
World Sight Day (WSD) is an international day of awareness, held annually on the second Thursday of October to focus attention on the global issue of avoidable blindness and visual impairment. This year, it focuses on gender and eye health–equal access to care.
VISION 2020 is the global initiative for the elimination of avoidable blindness, a joint programme of the World Health Organization (WHO) and the International Agency for the Prevention of Blindness (IAPB) with an international membership of NGOs, professional associations, eye care institutions and corporations.
Over the next two decades, Vision 2020 will take steps to prevent an estimated 100 million people from becoming blind.
Vision 2020 focuses on creating adequate eyecare facilities, a foundation of well-trained eyecare
workers, implementing programmes to control the major causes of blindness, and integrating
eye-care into general health care services... (more)
More information:
VISION 2020: The Right to Sight
What is VISION 2020: The Right to Sight?
International Key Messages
8 OCTOBER 2009
World Sight Day (WSD) is an international day of awareness, held annually on the second Thursday of October to focus attention on the global issue of avoidable blindness and visual impairment. This year, it focuses on gender and eye health–equal access to care.
VISION 2020 is the global initiative for the elimination of avoidable blindness, a joint programme of the World Health Organization (WHO) and the International Agency for the Prevention of Blindness (IAPB) with an international membership of NGOs, professional associations, eye care institutions and corporations.
Over the next two decades, Vision 2020 will take steps to prevent an estimated 100 million people from becoming blind.
Vision 2020 focuses on creating adequate eyecare facilities, a foundation of well-trained eyecare
workers, implementing programmes to control the major causes of blindness, and integrating
eye-care into general health care services... (more)
More information:
VISION 2020: The Right to Sight
What is VISION 2020: The Right to Sight?
International Key Messages
Monday, August 31, 2009
Refractive disorders: Astigmatism.(5) Solutions
Enlace a entrada en español
Solutions
Just as myopia and hyperopia, astigmatism can be corrected with eyeglasses, contact lenses and/or refractive surgery.
In any treatment, the correction is more complex because of the asymmetry of this refractive error.
Refractive disorders: Astigmatism.(1) Vision
Refractive disorders: Astigmatism. (2) Appearance
Refractive disorders: Astigmatism. (3) Different features
Refractive disorders: Astigmatism. (4) Symptoms
Refractive visual disorders. Some clarifications.
Some numbers...
Solutions
Just as myopia and hyperopia, astigmatism can be corrected with eyeglasses, contact lenses and/or refractive surgery.
In any treatment, the correction is more complex because of the asymmetry of this refractive error.
- Eyeglasses, with usually the same correction to look far away and to look up close (although there are some cases in which the astigmatism may be different in one distance or another).
The lenses used for compensating astigmatism depend on the kind of astigmatism we have.
So far, the compensation of a refractive error was simple, because a spherical lens modified equally every ray that crossed it at any point of it; that is, any ray that got to the lens, got to the retina. In the astigmatism, this is MORE COMPLICATED.
Because all surface (of cornea or crystalline lens) has not got a symmetrical curvature (there will be a meridian with more curvature and other with a lesser one), the light that gets into the eye, gets to different points regarding the retina. Therefore, if we put a spherical lens in front of a astigmatic eye, it will only correct a meridian. This way, all rays, that go through the lens and later through the eye, will keep on getting to different points with respect to the retina, because some of rays will be focused on it, but others will be focused behind or in front of the retina (depending on the kid of astigmatism).
To make it simpler. Think that an astigmatic person sees the image distorted; on the other hand, it is usually accompanied by hyperopia or myopia that causes seeing blurred images. So, for you to understand it better, “this person will see blurred through one meridian, and distorted through the other”. Consequently, we need a lens with two different powers to compensate both effects: one spherical lens to compensate the blur and other cylindrical or toric lens to compensate the distortion. Each one will focus the rays that go through by each meridian, INTO the retina.
The eyeglasses with astigmatism may usually make you feel a little sick when you begin to use them, mainly with refractions over 1.00 or 1.50, that is why it is advisable to start using them gradually. In fact, in high degree, firstly diagnosed astigmatism, the graduation is usually prescribed gradually, increasing it little by little.
- In the past, astigmatism could only be compensated with eyeglasses, because the cylindrical lens demands it to be worn with specific angle degrees. The contact lenses on the eye ares in constant movement because of the blinking, and in the past, this prevented that stability from happening. When said contact lenses were made, these were made with a material that lasted for a long time, because manufacturing was expensive. Latter research lead to the current situation, where there are many systems to stabilize them, and the cylindrical power in disposable contact lenses is higher and higher as time goes by.
Therefore, nowadays, this refractive disorder may be compensated with soft contact lenses as well as with lenses; and in adults as well as in children.
In the cases of astigmatism caused in keratoconus, they are usually compensated with rigid gas permeable contact lenses, with the purpose of holding the process of growth of the cornea a little bit. But sometimes we choose the soft contact lenses in the cases where the other lenses are impossible to wear due to the blinking; on one hand, because the eyelids may expel the lenses; and on the other, because they are not stabilized in the best position possible, therefore causing blurred image. Anyway, these patients usually achieve better Visual Acuities with contact lenses than with eyeglasses.
Also, the astigmatism caused by a problem such as a deformation of the eyeball by palpebral disorders (as chalazion), treating the underlying cause will resolve the astigmatism. If the patient suffers a severe astigmatism, her best option is the semi-rigid permeable contact lens.
- In the other hand, vision therapy is useful when the visual system has been compensating a little difference of graduation from between some meridians and others, and the focusing (accommodation) and team coordination (fusion) are tired.
We use vision therapy when symptoms exist and we have to teach the visual system to use its accommodation correctly again.
Many times simply wearing some eyeglasses is just not enough because they do not resolve completely the problem.
Although initially you see well with them or maybe you have started to use them and everything seems to be going better, shortly you may be uncomfortable again and unable to perform at work or when studying.
During a time you were using your visual system incorrectly and you have to re-educate it; the eyeglasses by themselves will not do it.
- As the previous refractive disorders, the VISUAL HYGIENE AND ERGONOMIC RULES must always be taken into account.
- And finally, the refractive surgery is another option. It consist of changing the shape of the cornea, and therefore correcting the astigmatism in the cornea or in the crystalline lens.
Refractive disorders: Astigmatism.(1) Vision
Refractive disorders: Astigmatism. (2) Appearance
Refractive disorders: Astigmatism. (3) Different features
Refractive disorders: Astigmatism. (4) Symptoms
Refractive visual disorders. Some clarifications.
Some numbers...
Monday, August 10, 2009
Refractive disorders: Astigmatism. (4) Symptoms.
Enlace a entrada en español
Behavior of astigmatic people
A low degree of astigmatism USUALLY causes more symptomatology, because the visual system tries to compensate it, and does not show blurred vision symptoms; therefore, at the beginning people do not usually associate the symptoms to a vision problem.
Refractive disorders: Astigmatism.(1) Vision
Refractive disorders: Astigmatism. (2) Appearance
Refractive disorders: Astigmatism. (3) Different features
Refractive disorders: Astigmatism. (5) Solutions
Refractive visual disorders. Some clarifications.
Some numbers...
Behavior of astigmatic people
A low degree of astigmatism USUALLY causes more symptomatology, because the visual system tries to compensate it, and does not show blurred vision symptoms; therefore, at the beginning people do not usually associate the symptoms to a vision problem.- The main symptom is the distorted and blurred vision in high astigmatism; and headache, visual strain, ocular itching and red eyes in low astigmatism.
As I explained in a previous post, a young hyperopic person with the low degree is able to “disguise” her disorder if her accommodative function works correctly. But in the case of an astigmatic person, although her accommodation is in correct condition, it “gets crazy”, because it has several points or images that fall in different planes regarding the retina; accommodation is constantly trying to focus all of them, without distinction. The closest points of the retina will cause less tension, but those that are farther will be more difficult to compensate. Visual system will be exhausted and the symptoms (red eyes, itching, burning, tearing, headache, visual strain,…) will appear.
- Although a person with astigmatism is usually born with it, if this is low, she could compensate it during the infancy. But as the child grows up and the school request is higher (more number of homework or study hours, more reading, more understanding,…), and her accommodation naturally decreases, the child has got more problems to compensate it; and this moment is when the astigmatism is obvious, showing several symptoms. That is why, many people say their astigmatism appeared between 10 and 20 years old.
- So, an astigmatism will cause more symptoms if it is associated with a hyperopia than with myopia, because hyperopic people have a more exhausted accommodative system due to the excessive effort that she makes to compensate her refractive error.
- That is why astigmatism can also cause learning disabilities.
- It can develop wrong postural habits, like tilting her head when looking far away or looking up close, since when the astigmatic person tilts her head, can find the position in which she sees better, or where the image is less distorted. This can cause cervical problems in the long run.
- “Blurred” or double vision greater at near than at distance.
- While she reads, she jumps the line or the letters seem to be moving.
- Visual strain and feeling sleepy.
- Headache mainly in the fronthead and in the eyes.
- Photophobia (light sensitivity).
- Frequent conjunctivitis or blepharitis.
Refractive disorders: Astigmatism.(1) Vision
Refractive disorders: Astigmatism. (2) Appearance
Refractive disorders: Astigmatism. (3) Different features
Refractive disorders: Astigmatism. (5) Solutions
Refractive visual disorders. Some clarifications.
Some numbers...
Friday, July 24, 2009
Ocular injuries after solar exposure
Enlace a entrada en español
A digression on the “Astigmatism series”.
Summer is here and many of you had already been on holiday or you are thinking about that, preparing everything to enjoy a well-deserved time for rest :-)
Do not forget to put in the suitcase sunglasses for all your family members (adults and children). Just as you do not forget the sun protection lotions, sunglasses are something indispensable to enjoy some safe holiday.
Think about the time that you are going to be performing outdoor activities, without your eyes being protected. Furthermore, there are more harmful radiations in the beach, because we have to add the ones that are reflected in the sea; but many surfaces reflect those harmful rays (sand, buildings, sidewalks,…). Even in cloudy days, with high clouds, the solar radiation that crosses them is nearly the same one that when there are no clouds at all. Only rain, fog and low clouds reduce UV radiation in a significant way.
Any tissue of the eye, as it happens with the skin, can be burnt. Solar UVA or UVB radiations on the tissues cause a cellular death process and a transformation of cell DNA, which can create irreversible disorders. Therefore, we must take care of our eyes by wearing lenses that comply with health regulations of visual quality. In the case of European sunglasses, the CE mark identifies glasses fulfilling quality regulations.
Atmospheric ozone is a barrier against very detrimental UVC radiation and other UV types; and this moderates the quantity of UVB that arrives to the earth (more harmful than UVA).
The constant slimming of ozone layer causes an increase of the UVB radiation that we receive daily. So, while this keeps being like that (unfortunately), we will have to do everything in our power to protect our eyes.
Some injuries that are caused in the eye due to solar radiation are the following ones:
IN CONJUNCTIVA (5)
IN CORNEA (1)
Cornea and crystalline lens absorb most of UVA and UVB radiation that get into the eye, damaging these structures; therefore, if a person has got a keratoconus or she has undergone a refractive surgery, her cornea will be thinner and it will be able to absorb less radiation, so, this radiation might get into the eye, causing some injuries.
IN AQUEOUS HUMOR (4)
The aqueous humor of the anterior chamber has a lot of Vitamin C (ascorbic acid), which is responsible for filtering UV radiation, and this way, achieving that the least amount possible of radiation gets to the crystalline lens.
IN CRYSTALLINE LENS (8)
The same as solar exposure can cause that your skin aged sooner, the same happens with the lens; it might suffer from a premature aging, because of the damages in DNA.
IN RETINA (10)
Looking directly at sun (in a eclipse or not), without adequate eye protection, causes a photochemical damage of the photoreceptors, inducing a burn at macular level (area of maximum vision of the retina) and being the reason of an irreversible blindness.
A digression on the “Astigmatism series”.
Summer is here and many of you had already been on holiday or you are thinking about that, preparing everything to enjoy a well-deserved time for rest :-)Do not forget to put in the suitcase sunglasses for all your family members (adults and children). Just as you do not forget the sun protection lotions, sunglasses are something indispensable to enjoy some safe holiday.
Think about the time that you are going to be performing outdoor activities, without your eyes being protected. Furthermore, there are more harmful radiations in the beach, because we have to add the ones that are reflected in the sea; but many surfaces reflect those harmful rays (sand, buildings, sidewalks,…). Even in cloudy days, with high clouds, the solar radiation that crosses them is nearly the same one that when there are no clouds at all. Only rain, fog and low clouds reduce UV radiation in a significant way.
Any tissue of the eye, as it happens with the skin, can be burnt. Solar UVA or UVB radiations on the tissues cause a cellular death process and a transformation of cell DNA, which can create irreversible disorders. Therefore, we must take care of our eyes by wearing lenses that comply with health regulations of visual quality. In the case of European sunglasses, the CE mark identifies glasses fulfilling quality regulations.
Atmospheric ozone is a barrier against very detrimental UVC radiation and other UV types; and this moderates the quantity of UVB that arrives to the earth (more harmful than UVA).
The constant slimming of ozone layer causes an increase of the UVB radiation that we receive daily. So, while this keeps being like that (unfortunately), we will have to do everything in our power to protect our eyes.
Some injuries that are caused in the eye due to solar radiation are the following ones:
IN CONJUNCTIVA (5)
IN CORNEA (1)
Cornea and crystalline lens absorb most of UVA and UVB radiation that get into the eye, damaging these structures; therefore, if a person has got a keratoconus or she has undergone a refractive surgery, her cornea will be thinner and it will be able to absorb less radiation, so, this radiation might get into the eye, causing some injuries.
- Painful superficial keratitis
- Photokeratitis or snow blindness
- Climatic droplet keratopathy or Spheroidal Degeneration
IN AQUEOUS HUMOR (4)
The aqueous humor of the anterior chamber has a lot of Vitamin C (ascorbic acid), which is responsible for filtering UV radiation, and this way, achieving that the least amount possible of radiation gets to the crystalline lens.
- A prolonged exposure to sunlight decreases the quantity of vitamin C in the aqueous humor.
IN CRYSTALLINE LENS (8)
The same as solar exposure can cause that your skin aged sooner, the same happens with the lens; it might suffer from a premature aging, because of the damages in DNA.
IN RETINA (10)
Looking directly at sun (in a eclipse or not), without adequate eye protection, causes a photochemical damage of the photoreceptors, inducing a burn at macular level (area of maximum vision of the retina) and being the reason of an irreversible blindness.
- Early development of Macular degeneration
- Melanomas
Tuesday, July 21, 2009
Refractive disorders: Astigmatism. (3) Different features
Enlace a entrada en español
Development
Astigmatism relatively changes a little throughout all life.
Astigmatism is not very frequent during school age and it has got few changes of frequency and degree. One study made in Orinda, California, showed that the frequency of increase in an astigmatism of 1 diopter or more, at 6 years old, rises gradually from 2% by 3%, at 14 years old.
Higher levels of astigmatism are associated with moderate to high hyperopia during infancy, but both tend to decrease by the age of 5 years.
If a child is going to have a high astigmatism, it should already exist before beginning school stage.
In adult age, astigmatism does not usually change; if it do it, it usually indicates tension-related one, as myopia (in both cases vision therapy is very useful in order to structure a correct vision).
The little astigmatism that appears during infancy can be due to the strength that the upper eyelid exerts on the cornea causing that the vertical meridian to be more curve than the horizontal one.
In a more mature age, this astigmatism may change its shape, turning the vertical axis flatter, because of the laxity of palpebral muscle that rests on the eyeball. That is the reason why the axis or degrees of our astigmatism change through the years.
Prevalence
According to an American study published in Archives of Ophthalmology, nearly 30% of children between the ages of 5 and 17 have astigmatism.
On the other hand, a recent Brazilian study found that 34% of the students in one city of the country were astigmatic.
The National Autonomous University of Mexico revealed that astigmatism is the visual problem with most prevalence among people younger than 23 years old, and even 23% of population younger than 14 years old, put up with it.
Regarding the prevalence in adults, a study in Bangladesh found that nearly 32.4% of those over the age of 30 had astigmatism.
Also, several studies have found that the prevalence of astigmatism increases with age.
Factors and Causes
FACTORS:
If a child is going to have a high astigmatism or hyperopia, these will appear from birth or in early age. This means the factors are hereditary.
CAUSES:
Refractive disorders: Astigmatism.(1) Vision
Refractive disorders: Astigmatism. (2) Appearance
Refractive disorders: Astigmtaism. (4) Symptoms
Refractive disorders: Astigmatism. (5) Solutions
Refractive visual disorders. Some clarifications.
Some numbers...
Development
Astigmatism relatively changes a little throughout all life.
Astigmatism is not very frequent during school age and it has got few changes of frequency and degree. One study made in Orinda, California, showed that the frequency of increase in an astigmatism of 1 diopter or more, at 6 years old, rises gradually from 2% by 3%, at 14 years old.
Higher levels of astigmatism are associated with moderate to high hyperopia during infancy, but both tend to decrease by the age of 5 years.
If a child is going to have a high astigmatism, it should already exist before beginning school stage.
In adult age, astigmatism does not usually change; if it do it, it usually indicates tension-related one, as myopia (in both cases vision therapy is very useful in order to structure a correct vision).
The little astigmatism that appears during infancy can be due to the strength that the upper eyelid exerts on the cornea causing that the vertical meridian to be more curve than the horizontal one.In a more mature age, this astigmatism may change its shape, turning the vertical axis flatter, because of the laxity of palpebral muscle that rests on the eyeball. That is the reason why the axis or degrees of our astigmatism change through the years.
Prevalence
According to an American study published in Archives of Ophthalmology, nearly 30% of children between the ages of 5 and 17 have astigmatism.
On the other hand, a recent Brazilian study found that 34% of the students in one city of the country were astigmatic.
The National Autonomous University of Mexico revealed that astigmatism is the visual problem with most prevalence among people younger than 23 years old, and even 23% of population younger than 14 years old, put up with it.
Regarding the prevalence in adults, a study in Bangladesh found that nearly 32.4% of those over the age of 30 had astigmatism.
Also, several studies have found that the prevalence of astigmatism increases with age.
Factors and Causes
FACTORS:
If a child is going to have a high astigmatism or hyperopia, these will appear from birth or in early age. This means the factors are hereditary.
CAUSES:
- High weight of the upper eyelid.
- Slightly fallen upper eyelid (Ptosis).
- Ocular contusions.
- Corneal scars or lacerations, due to hits, injuries and infections in the eye.
- Changes in corneal shape following eye surgery (refractive one, of catarata,…)
- KERATOCONUS (this disorder will have its own post later, but let me briefly explain that the cornea acquires a conical shape as time goes by, and each time it gets thinner).
- Metabolic changes, as for instance high sugar levels in the blood that changes the shape of the lens of the eye, and this causes astigmatism. When this sugar level is normalized, the lens usually gets back to its shape and said astigmatism disappears.
Refractive disorders: Astigmatism.(1) Vision
Refractive disorders: Astigmatism. (2) Appearance
Refractive disorders: Astigmtaism. (4) Symptoms
Refractive disorders: Astigmatism. (5) Solutions
Refractive visual disorders. Some clarifications.
Some numbers...
Monday, July 06, 2009
Refractive disorders: Astigmatism.(2) Appearance
Enlace a entrada en español
Appearance of astigmatic eye
As I wrote in a previous post, one characteristic of the astigmatic eye is that the external face of its cornea -1- is not spherical (as a pure myopic or hyperopic eye), but elliptical. It is similar an American football cut in half (CORNEAL ASTIGMATISM).
But astigmatism may be more complicated than all that, since because the cornea has a thickness, the curvature of the internal face of the cornea may also cause an astigmatism that is more complicated to diagnose and treat; the same happens with the one that is caused also because of an unusual slope of the crystalline lens -8 (INTERNAL ASTIGMATISMS). Those last cases are less frequent.
But both of them (corneal and internal ones), may exist at the same time; and the addition of both, is the result of the total astigmatism that one person has got.
This typical shape of the ocular surfaces causes that their different "meridians" (horizontal and vertical ones) do not have the same power; therefore, some of them are more curve than others, and this causes the light is focused on two or more planes regarding the retina, instead of only on the retina itself. The light is clearly focused along one plane but is blurred along the other. The result is blurred vision at all distances.
More ...
RELATED POST
Refractive disorders: Astigmatism.(1) Vision
Refractive disorders: Astigmatism. (3) Different features
Refractive disorders: Astigmtaism. (4) Symptoms
Refractive disorders: Astigmatism. (5) Solutions
Refractive visual disorders. Some clarifications.
Some numbers...
Appearance of astigmatic eye
As I wrote in a previous post, one characteristic of the astigmatic eye is that the external face of its cornea -1- is not spherical (as a pure myopic or hyperopic eye), but elliptical. It is similar an American football cut in half (CORNEAL ASTIGMATISM).
But astigmatism may be more complicated than all that, since because the cornea has a thickness, the curvature of the internal face of the cornea may also cause an astigmatism that is more complicated to diagnose and treat; the same happens with the one that is caused also because of an unusual slope of the crystalline lens -8 (INTERNAL ASTIGMATISMS). Those last cases are less frequent.But both of them (corneal and internal ones), may exist at the same time; and the addition of both, is the result of the total astigmatism that one person has got.
This typical shape of the ocular surfaces causes that their different "meridians" (horizontal and vertical ones) do not have the same power; therefore, some of them are more curve than others, and this causes the light is focused on two or more planes regarding the retina, instead of only on the retina itself. The light is clearly focused along one plane but is blurred along the other. The result is blurred vision at all distances.
More ...
RELATED POST
Refractive disorders: Astigmatism.(1) Vision
Refractive disorders: Astigmatism. (3) Different features
Refractive disorders: Astigmtaism. (4) Symptoms
Refractive disorders: Astigmatism. (5) Solutions
Refractive visual disorders. Some clarifications.
Some numbers...
Tuesday, June 30, 2009
Refractive disorders: Astigmatism.(1) Vision
Enlace a entrada en español
This third refractive disorder is the most common of them all, although the most known is myopia. This can coexist with myopia as well as hyperopia (13 percent of population has only got astigmatism, and 20 percent has astigmatism associated with myopia or hyperopia).
Astigmatic person’s vision
The typical complaints in astigmatism are “`BLURRED VISION´ MAINLY WHEN TRYING TO VIEW DISTANT OBJECTS, AND FREQUENT FRONT HEADACHE”. Only when the astigmatism is high, the complaint is also, “`BLURRED VISION´ AT CLOSE TASKS”.
If you realize I write 'BLURRED VISION' in quotation marks, because this refractive error, usually congenital, causes the outlines of the letters or the objects to be seen distorted; as if there would be some shadows behind every letter or every object, or as if they were seen double or distorted. Actually it is not a BLURRED VISION.
For low and medium quantities of refraction:
I am going to show you two simple ways for checking if you or your child has got astigmatism or not:
RELATED POST
Refractive disorders: Astigmatism. (2) Appearance
Refractive disorders: Astigmatism. (3) Different features
Refractive disorders: Astigmtaism. (4) Symptoms
Refractive disorders: Astigmatism. (5) Solutions
Refractive visual disorders. Some clarifications.
Some numbers...
This third refractive disorder is the most common of them all, although the most known is myopia. This can coexist with myopia as well as hyperopia (13 percent of population has only got astigmatism, and 20 percent has astigmatism associated with myopia or hyperopia).
Astigmatic person’s vision
The typical complaints in astigmatism are “`BLURRED VISION´ MAINLY WHEN TRYING TO VIEW DISTANT OBJECTS, AND FREQUENT FRONT HEADACHE”. Only when the astigmatism is high, the complaint is also, “`BLURRED VISION´ AT CLOSE TASKS”.If you realize I write 'BLURRED VISION' in quotation marks, because this refractive error, usually congenital, causes the outlines of the letters or the objects to be seen distorted; as if there would be some shadows behind every letter or every object, or as if they were seen double or distorted. Actually it is not a BLURRED VISION.
For low and medium quantities of refraction:
- An uncorrected myopic person has low Visual Acuity at distance but it is good at near.
- An uncorrected hyperopic person can have a good Visual Acuity at any distance as long as she has enough accommodation.
- But, in spite of that a little astigmatism of about 0.50 diopter can not appreciably interfere in Visual Acuity in general, a person with uncorrected astigmatism does not have any distance where the image shaped in her retina is perfectly clear. In the case of a hyperopic astigmatism, by using help of the accommodation, it will be easier for itself to achieve better sharpness than a myopic astigmatism.
I am going to show you two simple ways for checking if you or your child has got astigmatism or not:
- Put this image in the computer screen and get yourself away about 1 meter or 1 meter and a half, and covering one eye (and without wearing your glasses, if you have it), check if you can see ALL lines with the same contrast, that is, all of them have the same degree of “blackness” or all of them are clear or all of them are blurred.
If you do not have any astigmatism, the answer will be “yes”; you see it as it is in the drawing, or maybe all drawing is blurred (if you have some myopia or hyperopia).
But if you have it, the answer will be that you see something like this:
(Unless myopia or hyperopia, when we give the value of the astigmatism in a prescription, we give the power of the lens and also the axis of said lens).
This is the most common answer. It would belong to pure astigmatism between 180º and 150º (the astigmatic axis is perpendicular to the lines that you see better). If you have some myopia or hyperopia, as well the rest of axises will be seen out of focus.
- Another clear way to check it is looking at the full moon. If you have some astigmatism, it will be impossible to perceive its outlines and it will be seen elongated and out of focus.
RELATED POST
Refractive disorders: Astigmatism. (2) Appearance
Refractive disorders: Astigmatism. (3) Different features
Refractive disorders: Astigmtaism. (4) Symptoms
Refractive disorders: Astigmatism. (5) Solutions
Refractive visual disorders. Some clarifications.
Some numbers...
Wednesday, June 17, 2009
Refractive disorders: Hyperopia. (4) Solutions
Enlace a entrada en español.
Solutions
Low or even medium hyperopias are difficult to diagnose because, as I wrote in one previous post, they “consume” the refraction. Besides, they are difficult to be detected by parents or teachers because these hyperopic people have a more or less clear vision in any distance. That is why these people need a COMPREHENSIVE EYE EXAM from early age and a chekup once a year.
In that comprehensive visual examination the refraction that the visual system of the hyperopic person is constantly compensating is checked, along with other skills of the visual system, specifically her accommodation which is the one that is working all time, and if it (accommodation) may have disturbed other visual skills.
In the case that the refraction error can be diagnosed or that the error is causing a certain symptomatology, these are the possible solutions:
RELATED POST
Refractive disorders: Hyperopia , Hypermetropia or Farsightedness. (1) Vision and Accommodation
Refractive disorders: Hyperopia (2) Different features.
Refractive disorders: Hyperopia. (3) Symptoms.
Refractive visual disorders. Some clarifications.
Some numbers...
Solutions
Low or even medium hyperopias are difficult to diagnose because, as I wrote in one previous post, they “consume” the refraction. Besides, they are difficult to be detected by parents or teachers because these hyperopic people have a more or less clear vision in any distance. That is why these people need a COMPREHENSIVE EYE EXAM from early age and a chekup once a year.
In that comprehensive visual examination the refraction that the visual system of the hyperopic person is constantly compensating is checked, along with other skills of the visual system, specifically her accommodation which is the one that is working all time, and if it (accommodation) may have disturbed other visual skills.
In the case that the refraction error can be diagnosed or that the error is causing a certain symptomatology, these are the possible solutions:
- Eyeglasses, either if she needs them to see clear because she has a high hyperopia, or if she has a low one and needs them to avoid that constant effort for close-up tasks, and this way, avoiding certain symptomatology. In the last case, eyeglasses are not used for seeing clear.
In the case of a high hyperopia, the use of eyeglasses can make sense both for short and long distance seeing, if the visual acuity for long distance is reduced.
In the case of lower hyperopia, that is causing some symptoms, the option of eyeglasses is good because she does not require wearing them during all day.
Hyperopia is corrected with plus, positive or convex spherical lenses (thicker at the middle than in the edges of the lens), that optically falls in front the image, on the retina.
- If hyperopia is higher than +1.50 diopter, another option for compensating it, is by wearing contact lenses for a long period of time. In this case, unlike myopia, each patient may use the best contact lens for her, that is, there is no suitable specific one. There is no one that stops the hyperopia, simply because as I have explained, the hyperopia does not increase.
There exists a research about Orthokeratology, carried out specifically in Australia, which tries to find a contact lens that model the cornea and reduce the hyperopia; but even, as I say, it is under research.
Hyperopic people are usually more uncomfortable wearing contact lenses than eyeglasses, because although her visual field is bigger wearing contacts, the size of images is more real; but wearing eyeglasses with plus lenses, they magnify the objects (I will explain this effect later); therefore, as they see everything bigger wearing the glasses, they are more comfortable wearing them than with contact lenses. - Vision Therapy is the best allied in this refraction disorder. A hyperopic person does not often respond to lens correction alone, but they are required to “remediate” accommodative dysfunction. With therapy we teach her to control her accommodation and her convergence and to perform her close-up tasks without effort. We will avoid that this to happen again.
Within vision therapy, besides some simple visual activities, the use of eyeglasses with a low positive refraction will be able to relieve her symptomatology in order to be able to perform the daily close-up tasks. This way, she relieves all accommodative effort that she constantly performs. - Besides, as it happens with myopic people, some simple VISUAL HYGIENE AND ERGONOMIC RULES will help prevent her reaching that visual stress. Modification of the patient's habits and visual environment is occasionally useful as an adjunct therapy.
Mainly:- Improving lighting or glare reduction
- Using better quality printed material.
- Decreasing temporal demands, with frequent rests.
- Looking far away and focusing on something, frequently, when performing close-up tasks.
- Etc.
- Improving lighting or glare reduction
- Lastly, surgery: hyperopia is a refractive defect that can be operated as well, but except for high hyperopias, it is not considered as the best option.
RELATED POST
Refractive disorders: Hyperopia , Hypermetropia or Farsightedness. (1) Vision and Accommodation
Refractive disorders: Hyperopia (2) Different features.
Refractive disorders: Hyperopia. (3) Symptoms.
Refractive visual disorders. Some clarifications.
Some numbers...
Monday, June 01, 2009
Refractive disorders: Hyperopia. (3) Symptoms
Enlace a entrada en español.
Hyperopia degrees
Usually, if hyperopia is low, hyperopic people do not have any symptom, and the time can go by until they show any. Besides, the younger the person is, the lesser the symptoms she will have, since her accommdation works perfectly and therefore, she can compensate the problem without any effort.
Either medium hyperopic, or not so young people or in certain cases will show (without wearing the refraction) the following symptoms:
RELATED POST
Refractive disorders: Hyperopia , Hypermetropia or Farsightedness. (1) Vision and Accommodation
Refractive disorders: Hyperopia (2) Different features.
Refractive disorders: Hyperopia. (4) Solutions.
Refractive visual disorders. Some clarifications.
Some numbers...
Hyperopia degrees
- Low: 0 - (+2.00) D
- Medium: (+2.25)-(+5.00) D
- High: More than +5.00 D
Usually, if hyperopia is low, hyperopic people do not have any symptom, and the time can go by until they show any. Besides, the younger the person is, the lesser the symptoms she will have, since her accommdation works perfectly and therefore, she can compensate the problem without any effort.Either medium hyperopic, or not so young people or in certain cases will show (without wearing the refraction) the following symptoms:
- Constant or intermittent blurred near vision.
- If hyperopia is medium/high, it also affects distant vision.
- Visual inconvenience when she performs close-up tasks.
- Headache, visual strain, ocular pain, burning, itching, tearing, red eyes… (due to incapacity of keeping the effort of accommodation that is demanded).
- In the case of low hyperopia that people have always been able to compensate without any problem (both as from a distance and as close up), as time goes by or in a special period with too many close-up tasks (at work or at school), they show inconvenience and discomfort and they do not know the reasons why they are caused. They have always seen well and have not had any previous visual problems; and they do not think, that this can be the cause of their problems. Before seeing blurred at near (while they can still keep the accommodation with effort), they usually show the symptoms above (headache, visual strain, itching, red eyes,...).
- In the case of children or young people, they usually have problems with the reading: line jumping, jumping letters, letters “seem to dance”, “they don’t stop!”… (presbyopic or old sight people also say these same words). These symptoms cause that they have aversion to reading.
- They as well perform facial contortions or frequent blinking while reading, that shows the effort that they are doing in their close-up tasks.
- Poor eye-hand coordination.
- In children (younger than 3 years old), if hyperopia is high, it can cause accommodative convergent strabismus (crossed-eye). This is because as they are young, they can perform much effort in order to see the image clear; but since accommodation and convergence are related (I will explain this better later), when the eyes perform too much accommodation, they also converge a lot, and one of eyes gets crossed. Consequently, this can also cause lazy eye or amblyopia on that eye.
- In the case of low hyperopic people who have always been able to compensate without any problem and they have always had a enviable sight in their youth, they inevitably undergo presbyopia. But unfortunately for the hyperopic, they suffer it before that the rest of the people, before they are 40 years old. These people will think that their arms are shorter each time, because they need put their reading text farther (but this refractive disorder deserves its own post later).
RELATED POST
Refractive disorders: Hyperopia , Hypermetropia or Farsightedness. (1) Vision and Accommodation
Refractive disorders: Hyperopia (2) Different features.
Refractive disorders: Hyperopia. (4) Solutions.
Refractive visual disorders. Some clarifications.
Some numbers...
Wednesday, May 20, 2009
Refractive disorders: Hyperopia.(2) Different features
Enlace a entrada en español
Appearance of hypermetropic eye
The farsighted eye, unlike the myopic eye, is smaller than usual, or its lenses have less power.
Consequently, the image is focused BEHIND the retina.
Development
Some authors “…concluded that the growth of the eye during infancy is extremely rapid, and its adult size is reached by the age of 3 years…” and that “…the process of emmetropization is evident during the first year of life…”.
“…by the ages of 6 to 8 years old emmetropization has taken place, the great majority of the children being in the emmetropic group, which has its peaks at 1.00 diopter of hyperopia…”
There is a high percentage of children who were born hyperopic, because our eyes, when we were born, are not completely developed, and are small. As time goes by, if this hypermetropia is low, it disappears when the eye grows, so that sometimes, it can even turn it into myopia in school years.
Hyperopia, unlike myopia, do not usually vary until getting into adult years. At that moment, the appearance of presbyopia or “old sight” causes that the hyperopia increases much more, either hyperopia can appear or the myopia can even decrease.
Prevalence
Around 10% of people in Spain suffer from hypermetropia. 22.4% of population in USA and Australia is hyperopic. Specifically, there is a prevalence of 12.8% in American children aged 5-17 years.
In school years, hyperopia is usually lesser worrying than myopia because the clinically significant values of hypermetropia (and astigmatism) are 2-4% of the children who begin the school years, and also, these values do not increase over time.
“Hypermetropia is influenced by ethnicity. Native Americans, African Americans, and Pacific Islanders are among the groups with the highest reported prevalence of hyperopia. A study of 1.880 Chinese schoolchildren in Malaysia showed that the prevalence of hyperopia greater than +1.25 diopter, was only 1.2%”. (pag 8)
Probability
Factors
If you remember, environmental factors are very important in myopia; however, although the majority of children are hypermetropic when they start the school years, the hyperopia does not usually increase, but it decreases if anything.
Therefore, its existence is usually thought to be caused by hereditary factors.
In the other hand, a visual disorder derived from hyperopia is the presbyopia or “old sight”, that is typical of old people. In this case, this disorder is product of the time as it goes by. The ciliary muscle, as the rest of body muscles, loses elasticity and strength, and as well the capacity of being contracted; this way, the lens can not be more convex, and that prevents the accommodation to happen. That is why hyperopic people begin to suffer presbyopia earlier than emmetropic people; and these ones, earlier than myopic people.
RELATED POST
Refractive disorders: Hyperopia , Hypermetropia or Farsightedness. (1) Vision and Accommodation
Refractive disorders: Hyperopia. (3) Symptoms.
Refractive disorders: Hyperopia. (4) Solutions.
Refractive visual disorders. Some clarifications.
Some numbers...
Appearance of hypermetropic eye
The farsighted eye, unlike the myopic eye, is smaller than usual, or its lenses have less power.
Consequently, the image is focused BEHIND the retina.
Development
Some authors “…concluded that the growth of the eye during infancy is extremely rapid, and its adult size is reached by the age of 3 years…” and that “…the process of emmetropization is evident during the first year of life…”.
“…by the ages of 6 to 8 years old emmetropization has taken place, the great majority of the children being in the emmetropic group, which has its peaks at 1.00 diopter of hyperopia…”
There is a high percentage of children who were born hyperopic, because our eyes, when we were born, are not completely developed, and are small. As time goes by, if this hypermetropia is low, it disappears when the eye grows, so that sometimes, it can even turn it into myopia in school years.
Hyperopia, unlike myopia, do not usually vary until getting into adult years. At that moment, the appearance of presbyopia or “old sight” causes that the hyperopia increases much more, either hyperopia can appear or the myopia can even decrease.
Prevalence
Around 10% of people in Spain suffer from hypermetropia. 22.4% of population in USA and Australia is hyperopic. Specifically, there is a prevalence of 12.8% in American children aged 5-17 years.
In school years, hyperopia is usually lesser worrying than myopia because the clinically significant values of hypermetropia (and astigmatism) are 2-4% of the children who begin the school years, and also, these values do not increase over time.
“Hypermetropia is influenced by ethnicity. Native Americans, African Americans, and Pacific Islanders are among the groups with the highest reported prevalence of hyperopia. A study of 1.880 Chinese schoolchildren in Malaysia showed that the prevalence of hyperopia greater than +1.25 diopter, was only 1.2%”. (pag 8)
Probability
Factors
If you remember, environmental factors are very important in myopia; however, although the majority of children are hypermetropic when they start the school years, the hyperopia does not usually increase, but it decreases if anything.
Therefore, its existence is usually thought to be caused by hereditary factors.
In the other hand, a visual disorder derived from hyperopia is the presbyopia or “old sight”, that is typical of old people. In this case, this disorder is product of the time as it goes by. The ciliary muscle, as the rest of body muscles, loses elasticity and strength, and as well the capacity of being contracted; this way, the lens can not be more convex, and that prevents the accommodation to happen. That is why hyperopic people begin to suffer presbyopia earlier than emmetropic people; and these ones, earlier than myopic people.
RELATED POST
Refractive disorders: Hyperopia , Hypermetropia or Farsightedness. (1) Vision and Accommodation
Refractive disorders: Hyperopia. (3) Symptoms.
Refractive disorders: Hyperopia. (4) Solutions.
Refractive visual disorders. Some clarifications.
Some numbers...
Monday, May 04, 2009
Refractive disorders: Hyperopia, Hypermetropia or Farsightedness. (1) Vision and Accommodation
Enlace a entrada en español
Carrying on with the “refractive disorders series”, and seeing that there have not been a lot of questions or comments regarding myopia, I will keep on writing about hypermetropia or the “refractive defect of children and old people”.
Vision of Hypermetropic people
The typical complaint in hypermetropia is “INSTABILITY OF VISION WITH PROBLEMS AT CLOSE DISTANCE”.
Her distance vision is usually VERY GOOD; as many people say, a hypermetropic person has “EAGLE VISION”; but unlike myopic people, her visual system is an exhausted system, due to it is constantly trying to compensate her refractive error in order not to have any problems, both as from a distance and as close up.
Therefore, as the near effort is higher, the exhaustion will appear earlier at this distance; and therefore, blurred vision will appear first at near, and then, when the visual system is already very exhausted, she will also notice that she has poor distance vision.
But, what does this mean?
In order for you to understand it better, I need to briefly explain to you the concept of “accommodation”, which I have named it many times previously.
Accommodation
In the case of a person without refraction:
In the case of a hyperopic person:
Therefore, the ocular system can perform the effort required to carry on this retarded image to the retina, through a change of the power of its ocular surfaces. This is the reason why an hypermetropic person who sees wrong at distance (because her image is focused behind the retina), if she is a young hypermetropic person and/or her hyperopia is low, she has the capacity in order to focus the image on the retina and to see it clear, through her eye accommodation; this way, her visual system is constantly under “visual stress” situation in order to keep that image clear.
If also, we add the additional effort that hypermetropic person performs when she looks at their close-up tasks (since hers lens has to be more convex than the ones of an emmetropic person), in order to achieve that the image is maintained clear on the retina, in some cases it can be a really daily effort.
All of this causes some problems:
Well, this is only the beginning…
RELATED POST
Refractive disorders: Hyperopia (2) Different features.
Refractive disorders: Hyperopia. (3) Symptoms.
Refractive disorders: Hyperopia. (4) Solutions.
Refractive visual disorders. Some clarifications.
Some numbers...
Carrying on with the “refractive disorders series”, and seeing that there have not been a lot of questions or comments regarding myopia, I will keep on writing about hypermetropia or the “refractive defect of children and old people”.
Vision of Hypermetropic people
The typical complaint in hypermetropia is “INSTABILITY OF VISION WITH PROBLEMS AT CLOSE DISTANCE”.Her distance vision is usually VERY GOOD; as many people say, a hypermetropic person has “EAGLE VISION”; but unlike myopic people, her visual system is an exhausted system, due to it is constantly trying to compensate her refractive error in order not to have any problems, both as from a distance and as close up.
Therefore, as the near effort is higher, the exhaustion will appear earlier at this distance; and therefore, blurred vision will appear first at near, and then, when the visual system is already very exhausted, she will also notice that she has poor distance vision.
But, what does this mean?
In order for you to understand it better, I need to briefly explain to you the concept of “accommodation”, which I have named it many times previously.
Accommodation
In the case of a person without refraction:
- When she looks an object far away, the visual system must be totally relaxed so the image is focused on her retina and she sees in a clear way;
- On the other hand, if she wants to see an object (a text, a watch, anything else) which is around 40 cm far from her eyes (for instance), her visual system has to perform the action of ACCOMMODATION, that is, the ciliary muscle has to be contracted and the fibers of the Zonule of Zinn have to be relaxed (to be stretched); this way, it allows that the lens -8- to be more convex, increasing its curvature, and also allowing its power to increase and letting the image of the near object that is being watched, to be focused on the retina, so it is seen in a clear way.
In the case of a hyperopic person:
Therefore, the ocular system can perform the effort required to carry on this retarded image to the retina, through a change of the power of its ocular surfaces. This is the reason why an hypermetropic person who sees wrong at distance (because her image is focused behind the retina), if she is a young hypermetropic person and/or her hyperopia is low, she has the capacity in order to focus the image on the retina and to see it clear, through her eye accommodation; this way, her visual system is constantly under “visual stress” situation in order to keep that image clear.
If also, we add the additional effort that hypermetropic person performs when she looks at their close-up tasks (since hers lens has to be more convex than the ones of an emmetropic person), in order to achieve that the image is maintained clear on the retina, in some cases it can be a really daily effort.
All of this causes some problems:- For themselves: this constant effort in order to keep the image focused on the retina, both as from a distance as for close up objects, creates stress or exhaustion in the long run, causing a certain symptomatology.
- For Optometrists: young hypermetropic people are more complicated to diagnose when the refraction is small, because they are able to “consume” it, their accommodation makes up for any lens.
Well, this is only the beginning…
RELATED POST
Refractive disorders: Hyperopia (2) Different features.
Refractive disorders: Hyperopia. (3) Symptoms.
Refractive disorders: Hyperopia. (4) Solutions.
Refractive visual disorders. Some clarifications.
Some numbers...
Sunday, April 12, 2009
Why does a myopic person’s vision improve when she squeezes her eye lids to a narrow opening?
Enlace a entrada en español
One feature of myopic people is that when they look far away and see blurred, tey unconsciously learn that their vision improve when squeezing their eyelids to a narrow opening.
The reason why this happens is because when an eye is “focused”, the image of the point that it looks to, is equivalent to a clear point; but when the eye is “out of focus”, the image of the point is equivalent to a blur circle.
In order for you to understand better, I will explain the difference between “optical image” and “retinal image”. We should begin from two premises:
The size of this blur circle depends on two factors: the distance of the optical image from the retina and the pupil size.
So, when a myopic person squeezes her eyelids, she just reduces the quantity of light rays that get into her eyes, and with this, she achieves decreasing the size of the circle on the retina, causing a more clear image.
It is proved that for the Visual Acuity to be good, the pupil size must range between 2 and 5 mm: when it is smaller than 2 mm, the diffraction effects of the light tend to reduce Visual Acuity; whereas if it is bigger than 5 mm, spherical aberration is the factor may reduce it.
Therefore, in order to know whether a child has myopia, a test can be performed: the child covers one eye whereas with the other eye, she looks some letters far away from her, through a cardboard with a small hole of around 1 mm (it is like a homemade “pinhole”). If the child has myopia she will see better when looking through the hole (it is just as if she squeeze her eye lids to a narrow opening); if the child is emmetrope (not refractive error), she will not see any difference.
RELATED POSTS
Refractive disorders: Myopia (1).Vision and Symptoms
Refractive disorders: Myopia (2) Kinds and factors
Refractive disorders: Myopia (3). Solutions
Refractive visual Disorders. Some clarifications.
Some numbers...
The reason why this happens is because when an eye is “focused”, the image of the point that it looks to, is equivalent to a clear point; but when the eye is “out of focus”, the image of the point is equivalent to a blur circle.
In order for you to understand better, I will explain the difference between “optical image” and “retinal image”. We should begin from two premises:
- Optical image is formed by the optical system of the eye. It is always clear and it may or may not coincide with the image formed on the retina.
- Retinal image is formed on the retina, so it may be blurred or clear.
If the optical image is sharply focused on the retina, both images are one and the same; but if the optical image of a point object is not focused on the retina, the retinal image will be a blur circle.
The size of this blur circle depends on two factors: the distance of the optical image from the retina and the pupil size.
- For a constant pupil size, the distance of the optical image from the retina (or the size of blur circle), is determined by the refractive error; the lesser the refractive error, the closer the image will from the retina, and therefore, the lesser the blur circle size will be.
- In the other hand, for a given position of the optical image, if the pupil size varies (varying the brightness of the environment or squeezing the eye lids), the circle size varies too, so if the pupil size is smaller, the circle will be smaller too.
So, when a myopic person squeezes her eyelids, she just reduces the quantity of light rays that get into her eyes, and with this, she achieves decreasing the size of the circle on the retina, causing a more clear image.It is proved that for the Visual Acuity to be good, the pupil size must range between 2 and 5 mm: when it is smaller than 2 mm, the diffraction effects of the light tend to reduce Visual Acuity; whereas if it is bigger than 5 mm, spherical aberration is the factor may reduce it.
Therefore, in order to know whether a child has myopia, a test can be performed: the child covers one eye whereas with the other eye, she looks some letters far away from her, through a cardboard with a small hole of around 1 mm (it is like a homemade “pinhole”). If the child has myopia she will see better when looking through the hole (it is just as if she squeeze her eye lids to a narrow opening); if the child is emmetrope (not refractive error), she will not see any difference.
RELATED POSTS
Refractive disorders: Myopia (1).Vision and Symptoms
Refractive disorders: Myopia (2) Kinds and factors
Refractive disorders: Myopia (3). Solutions
Refractive visual Disorders. Some clarifications.
Some numbers...
Thursday, March 26, 2009
Regarding a movie: “Blindness”
Enlace a entrada en español.
A digression on the “Myopia series”.
Some days ago, I went to watch a just premiered movie “Blindness”. I will leave my personal criticism out since this blog is not about movies or books. But as a plot, for those people that have not read the book of the same name by Jose Saramago (from which the movie is adapted), or have not watched the movie, I will tell you briefly that it is about how panic and paranoia invaded a whole city when, one by one, all citizens are victims of a sudden blindness called “white illness”. The human behavior and condition give an exorbitant turn.
This novel/movie shows clearly how men are used to vision and how they give it very little importance when they own it. People that are born with vision, they do not know what they own. Fortunately only 3% of the world´s blind population are children. All of our daily activities require a good vision, from the time we wake up in the morning and we look at the watch, until we go to sleep and prepare the alarm clock for the next day. “Time concept” is very important for a blind man, that without light, has to learn to distinguish in what moment of the day he is, surrounded by that constant darkness –clarity-; in the case of sudden blindness, this causes a disturbance in the human biorhythms, and he does not know when he has to eat or to sleep.
A team of neuroscientists of the Visual Neuroscience Training Program at Johns Hopkins Institute, Baltimore (USA) has discovered (December 2008) a small number of light sensors (nerve cells) in the retina of our eyes, that communicate with the brain through very tiny and slow signals; and, unlike cones and rods, these cells contain MELANOSIN, which are not used for seeing images, but to monitor light levels in order to adjust the body’s clock and control constriction of the pupils in the eye (among other functions).
Ask yourself, what activities, simple or complex, you perform throughout the day WITHOUT THE NEED TO USE VISION… NOTHING, even when we dream, we see images. Many jobs such as computer users (a very high number of people, even those that work at home, and it is increasing), surgeons, taxi drivers, bricklayers, architects, dentists, plumbers, etc., need to have vision 100% of their time, or rather, we are used to use our vision 100% of our time.
At school, the reading and writing learning process of the children requires that 100% of the information that arrives to the brain gets in through the eyes. The school learning is purely visual. Really MEN ARE MAINLY VISUAL; human evolution has created us this way, it has created us such dependence on vision that when something like this happens and the adaptation of environment does not cause the immediate development of other senses, the first reaction of our brain is to change our behavior. In the case of the novel /movie, the change is not positive, but like in war moments, catastrophes, etc., it is proved that the human condition does not always offer its best face.
There are many people that do not like the changes in their life, regardless of whether these changes are controlled as uncontrolled (book: “Who moved my cheese”); but when you lose something that the rest of your life depends on, such as vision, this involves a change in your thinking, your habits, your hobbies, your plans and definitely your whole life.
One defect of our human condition is that WE DO NOT APPRECIATE WHAT WE HAVE.
80% OF INFORMATION THAT ARRIVES TO THE BRAIN, GETS IN THROUGH THE EYES.
This post is only an approach that I wanted to show you, in order to appreciate your vision and take care of it. Asking yourself: “What if I’d done this or that…”, always happens, but it can be late!!!!
A digression on the “Myopia series”.
Some days ago, I went to watch a just premiered movie “Blindness”. I will leave my personal criticism out since this blog is not about movies or books. But as a plot, for those people that have not read the book of the same name by Jose Saramago (from which the movie is adapted), or have not watched the movie, I will tell you briefly that it is about how panic and paranoia invaded a whole city when, one by one, all citizens are victims of a sudden blindness called “white illness”. The human behavior and condition give an exorbitant turn.
This novel/movie shows clearly how men are used to vision and how they give it very little importance when they own it. People that are born with vision, they do not know what they own. Fortunately only 3% of the world´s blind population are children. All of our daily activities require a good vision, from the time we wake up in the morning and we look at the watch, until we go to sleep and prepare the alarm clock for the next day. “Time concept” is very important for a blind man, that without light, has to learn to distinguish in what moment of the day he is, surrounded by that constant darkness –clarity-; in the case of sudden blindness, this causes a disturbance in the human biorhythms, and he does not know when he has to eat or to sleep.
A team of neuroscientists of the Visual Neuroscience Training Program at Johns Hopkins Institute, Baltimore (USA) has discovered (December 2008) a small number of light sensors (nerve cells) in the retina of our eyes, that communicate with the brain through very tiny and slow signals; and, unlike cones and rods, these cells contain MELANOSIN, which are not used for seeing images, but to monitor light levels in order to adjust the body’s clock and control constriction of the pupils in the eye (among other functions).
Ask yourself, what activities, simple or complex, you perform throughout the day WITHOUT THE NEED TO USE VISION… NOTHING, even when we dream, we see images. Many jobs such as computer users (a very high number of people, even those that work at home, and it is increasing), surgeons, taxi drivers, bricklayers, architects, dentists, plumbers, etc., need to have vision 100% of their time, or rather, we are used to use our vision 100% of our time.
At school, the reading and writing learning process of the children requires that 100% of the information that arrives to the brain gets in through the eyes. The school learning is purely visual. Really MEN ARE MAINLY VISUAL; human evolution has created us this way, it has created us such dependence on vision that when something like this happens and the adaptation of environment does not cause the immediate development of other senses, the first reaction of our brain is to change our behavior. In the case of the novel /movie, the change is not positive, but like in war moments, catastrophes, etc., it is proved that the human condition does not always offer its best face.
There are many people that do not like the changes in their life, regardless of whether these changes are controlled as uncontrolled (book: “Who moved my cheese”); but when you lose something that the rest of your life depends on, such as vision, this involves a change in your thinking, your habits, your hobbies, your plans and definitely your whole life.
One defect of our human condition is that WE DO NOT APPRECIATE WHAT WE HAVE.
80% OF INFORMATION THAT ARRIVES TO THE BRAIN, GETS IN THROUGH THE EYES.
This post is only an approach that I wanted to show you, in order to appreciate your vision and take care of it. Asking yourself: “What if I’d done this or that…”, always happens, but it can be late!!!!
Tuesday, March 17, 2009
Refractive disorders: Myopia (3). Solutions
Enlace a entrada en español
Solutions
Myopia is not an illness, that’s why it can not be “cured”.
But I will explain all these solutions that we can offer the optometrists in later posts.
As myopia keeps on awakening many questions, research is continually performing going on in order to stop its progression and remove it. Nowadays there is an European project called “My Europia”, that is focused on the creation of special eyeglasses with different refraction in the middle of the lens and in the periphery, that causes that the eye to react different ways in order to prevent that progression.
Medicines, vitamins,… Is there anything that I can have in order to stop my myopia?
Until now, there is no medicine that can certainly works in order to stop myopia. The only thing that has been tried in different schools at USA is pirenzepine . But as you can read in the article, this medicine is able to cause myopia to progress slowly but it is not able to prevent that progression. Besides this result, there are still many concerns: secondary effects, what happens if we interrupt the treatment, or how much time would a patient need to be with it. Also, even though this medicine was safe, it had to be removed from some children because it caused ocular irritation. Therefore, there is still much research to do in this subject.
In the other hand, vitamin A, lutein and zeaxanthin are supplements that are said to help eye health. I guess this is like the rules of visual hygiene, if you perform them, they are not detrimental to you; they are vitamins and they are always good for our organism, but personally, I would not support a treatment exclusively on them.
RELATED POSTS
Refractive disorders: Myopia (1).Vision and Symptoms
Refractive disorders: Myopia (2) Kinds and factors
Why does a myopic person’s vision improve when she squeezes her eye lids to a narrow opening?
Refractive visual Disorders. Some clarifications.
Some numbers...
Solutions
Myopia is not an illness, that’s why it can not be “cured”.
- If you want to “compensate” a myopia or its problem of blurred distance vision, the treatments most commonly used imply wearing eyeglasses or soft contact lenses (mainly disposable ones); in order to go to the theater, to drive, to walk, to watch TV, to practice some sport,… In this case, myopia is corrected with minus or diverging spherical lenses (thicker at the edges than in the middle of the lens), that optically falls behind the image on the retina.
- But if you would like to “try to stop” that progression as much as possible, the perfect thing would be wearing contact lenses and still better, if they are semi-rigid permeable ones.
Besides this, the ideal thing would be to perform a series of simple visual exercises, which mainly teach us how we should correctly use our ability of focusing (accommodation) and how we could teach our visual system to be less “focal”, so that, although we are performing much near work with papers, computer, books, notes, homework, and for many hours, we do not help myopia to keep on increasing. This is one of the visual functions that we can work with the vision therapy.
Besides all it has been said before, if the myopic person with few diopters (less 1 diopter), removes her eyeglasses in order to perform that near work, she will help her visual system (specifically, her ciliary muscle and her crystalline lens -8-, structures that are responsible for focusing works the right way) not to be stressed. In the case of a bigger myopia, she would need another pair of eyeglasses with less degree of myopia, in order to work at near with it without performing unnecessary effort. Although it is totally proved that the use of near eyeglasses with less refraction is useful in order to reduce the myopia or to stop it, it improves the environment factors without a doubt.
- Many people choose the solution that “will put an end” to myopia, tired of using eyeglasses or contact lenses during much time, resorting to Refractive Surgery. This solution is given by ophthalmologists (not by optometrists). But I have to let you know that not all patients are suitable for this procedure. What I advice you is to go to a clinic specialized in this kind of surgery and that after a complete ocular exam, they advice you your best option.
- In the case someone wants to “reduce myopia” without getting into the operating room, there exists the option of a treatment performed with semi-rigid permeable contact lenses, called ORTHOKERATOLOGY or “corneal refractive therapy”. This is performed by optometrists. This technique consists of leveling little by little the excess of myopic corneal curvature (when the main cause of the myopia is the cornea), until the point of best vision is obtained, and respecting the health of this ocular structure. Thereby, contact lenses can be used only when we sleep (and not all nights) and then, during the day we can enjoy good vision without wearing any refractive compensation. This option is perfect for those who candidates that need to achieve a certain Visual Acuity without refraction in the medical examination, and they do not want to get into the operating room.
- Lastly, in order to “prevent” not only that myopia rises or increases, but also to avoid any symptom of visual strain, we should take into an account some RULES OF VISUAL HYGIENE, that are not detrimental to anybody, but, on the other way, can help EVERYBODY, in the carrying out of the near tasks. Again, these rules improve the environmental factors.
But I will explain all these solutions that we can offer the optometrists in later posts.
As myopia keeps on awakening many questions, research is continually performing going on in order to stop its progression and remove it. Nowadays there is an European project called “My Europia”, that is focused on the creation of special eyeglasses with different refraction in the middle of the lens and in the periphery, that causes that the eye to react different ways in order to prevent that progression.
Medicines, vitamins,… Is there anything that I can have in order to stop my myopia?
Until now, there is no medicine that can certainly works in order to stop myopia. The only thing that has been tried in different schools at USA is pirenzepine . But as you can read in the article, this medicine is able to cause myopia to progress slowly but it is not able to prevent that progression. Besides this result, there are still many concerns: secondary effects, what happens if we interrupt the treatment, or how much time would a patient need to be with it. Also, even though this medicine was safe, it had to be removed from some children because it caused ocular irritation. Therefore, there is still much research to do in this subject.
In the other hand, vitamin A, lutein and zeaxanthin are supplements that are said to help eye health. I guess this is like the rules of visual hygiene, if you perform them, they are not detrimental to you; they are vitamins and they are always good for our organism, but personally, I would not support a treatment exclusively on them.
RELATED POSTS
Refractive disorders: Myopia (1).Vision and Symptoms
Refractive disorders: Myopia (2) Kinds and factors
Why does a myopic person’s vision improve when she squeezes her eye lids to a narrow opening?
Refractive visual Disorders. Some clarifications.
Some numbers...
Subscribe to:
Posts (Atom)
Posts
