Why does a myopic person’s vision improve when she squeezes her eye lids to a narrow opening?

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Ref: BOOK: PRIMARY CARE OPTOMETRY

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:

• 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.



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Regarding a movie: “Blindness”

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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!!!!

Refractive disorders: Myopia (3). Solutions

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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.



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Refractive disorders: Myopia (2). Kinds and Factors

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Different kinds of myopia, which one is yours?


CONGENITAL MYOPIA – It is the myopia of the newborn, mainly those who are born with low weight or are premature babies (“20% of preterm infantas suffer from myopia; as the greater the degree of prematurity is, the level of myopia is higher, reaching 8 to 10 diopters, according to a study by the International Center of Optometry –IOC-”). This myopia persists in the childhood, and remains when the school stage begins (6 years) (2%). It is not usually a low myopia degree and as time goes by, it will increase.


ACQUIRED MYOPIA -

Myopia caused in the youth – This myopia appears between 6 and 12 years old. This percentage increases from 2% at 6 years old to 20% at 20 years old :-O


Usually, it is a low myopia degree (-0.50 diopters or more), mainly if it appears from the age of 12. Normally these myopia will decrease, turning into emmetropia or even hyperopia in adult age.

Myopia of young adult person – This appears between 20 and 40 years old, as well in low degree, but the prevalence increases up to 30%. As the previous one, this myopia will decrease too, turning into emmetropia or even hyperopia at the last stage of the life.

These two last myopias are low and they are usually due to environmental factors (*), as for instance, excessive near-point visual activities or bad conditions of visual hygiene at school/home/work/… These myopias may increase but they will not do it in high degree.


Myopia at the beginning of the maturity – This appears from the age of 40-50 years old and the prevalence increases gradually at the last years of the life. It is the case of those people that when reaching this age, say they do not have any problem at reading a book unlike their friends of the same age. The reason is that this myopia is usually associated with the changes of the density of the ocular structures, typical of the age. When the density changes, the “refractive index” of these structures changes too, and therefore, as well the power of the whole ocular optic system. This kind of myopia is directly associated with the development and the progression of Cataracts. At a first stage myopia may be compensated through eyeglasses, but if the cataract keeps on going the natural course, the last step is surgery.


NOCTURNAL MYOPIA or NIGHT MYOPIA – It becomes apparent just in low light conditions when we want to look at a distant object. It may affect any people (myopic or not), mainly young people, and even if daytime vision is normal. Its value is usually -0.50 diopter, but may reach even -1.00 diopter in extreme cases, and in myopic people may increase the refraction at the same proportion.

This myopia is due to a disruption in the “accommodation process” (briefly, accommodation is the ability of our eyes in order to focus on different distances – I will write more about it in another later post -). When we look at distant object, accommodation does not work, so, it must be totally relaxed; but for that to happen, we need to see all details of the object so a clear image is achieved in the retina. If the light decreases, it is more complicated to see those details; so accommodation, in an attempt to clear the image, starts to work and focuses on an intermediate distance that it is not really where the object is.
Besides this, when the light decreases, the pupil is dilated (increases its size) in order to allow more light to get into the eye. But when the pupil diameter increases, the “depth of focus” (**) decreases and spherical aberrations of the visual system increase.
Because of all the previous, this nocturnal myopia is more noticed when we drive at night, since we need to see well, and we feel some insecurity behind the wheel because of this reduced vision that this myopia causes.

A little myopia of just -0.50 diopter is equivalent to losing around 15% of visual acuity, which is enough to make nocturnal driving difficult.

In the case of a emmetropic person (without refraction on daytime conditions) or a myopic person, both may solve the problem by wearing eyeglasses that correct this myopia in those conditions; in the case of the low hyperopic person, sometimes, she may remove her eyeglasses in order to drive at night and feel better; but in many occasions, the performance of some visual exercises focused to use that accommodation correctly, avoids the use of those “extra-glasses”.

Here you have an interesting article, that I encourage you to read (though it is written in Spanish) “The drivers visual acuity is reduced 70% at night”


PSEUDOMYOPIA – During many years, the progression of myopia has been researched, but nowadays there are few specific findings; as the factors are not very clear, neither what happens in the eye when the myopia increases.

Some people that say when a myopic person performs near work for a long time and a prolonged accommodation is made, a strain is carried out in the vitreous camera (9), that causes an increase of the axial length of the eye; this is why the image is not focused on the retina, but in front of it and this causes an increase of myopia.

In the other hand, others say this progression is due to a “ciliary spasm” because of a prolonged accommodation: the tonicity of ciliary muscle increases until that in a certain moment, this muscle can not get totally relaxed in order to allow seeing at distance. It is like when you take a very heavy object, and keep it strongly with your arms during some specific time, although that involves a great effort. When you let the object go, you are not capable of stretching your arms, they are rigid in that position; it is what you call “the muscle is spasmodic”. That is why, when myopia appears, or initially increases, the patient usually complains that distance vision is blurred after working at near a prolonged time, but after some minutes the vision is clear. This is called “pseudo-myopia” or “false myopia”; a real myopia does not allow that after any short time, distance vision improves. When this pseudo-myopia begins to become evident, the degree is only -0.50 to -1 diopter, this is the reason why if we can not avoid it, the best thing would be for you to receive visual therapy in order to relax that muscle, and teach your eyes to work at near without the effort that favors the increase of myopia. Thereby, we avoid that myopia to be irreversible and lastly, that an increase of the eye axial length is made, leaving myopia already structured.


MAGNA or PATHOLOGICAL MYOPIA – They are very high myopias (more than -10.00 diopters). These people should encourage periodic eye checks, since they may suffer complications as cataracts, glaucoma, retinal detachment (sudden and abundant floaters) or vitreous detachment, or macula pathologies.



(*) Factors that cause myopia

If you have always asked what you could do in order for you and your children to avoid having myopia, or even to avoid that the myopia that you already have, increases, I am so sorry tell you it is very difficult to achieve it :-(
This is because the factors that cause the myopia and its progression are nowadays a very controversial subject and have carried out much research, without finding out exactly which is the main factor: whether the hereditary character, the environment factors or both of them.

We are more worried about the progression of myopia than of any another refractive error, because, as I wrote before, although only a 2% of the children that start school suffer from it, this percentage increases very much during school stage, and also its progression is very fast during the later years. In the case that the myopia appears in adult stage, the progression is slower, but unlike other ametropias, once the myopia begins, the progression is sure. This might mean that exists an environment influence in this progression.



(**) DEPTH OF FOCUS: Until now, I had explained that in order to see an object clear, its image had to be just in the retina (fovea), but that is not totally correct. A small MARGIN is accepted, in front and behind of the retina, where the image of the object that we see, can be located, without losing clarity; this is called “depth of focus”.

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Refractive disorders: Myopia or Nearsightedness (1). Vision and Symptoms

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As I wrote two posts ago, myopia is the best known refractive disorder worldwide. And that’s why, I have many things to tell you about this, so, I will need more than one post…

Myopic person’s vision

The typical complaint in myopia is “BLURRED VISION WHEN TRYING TO VIEW DISTANT OBJECTS”.

DISTANT vision:

Therefore, a myopic person placed next to this lamppost in Gran Via at Madrid, looking at distance, she will be able to focus (see clear) this nearby lamppost, but as her eyes look further, she will see in a blurred way more and more. Of course, it will depend on the degree of myopia or quantity of myopia that she does not wear (assessed in DIOPTERS –D-); a myopic person with a little refraction, will be able to see clear further than other that has more myopia.

CLOSE vision:

As with distance vision, this also depends on the degree of the eye refraction:

- If it is compensated (she wears corrective lenses -eyeglasses or contact lenses-), she sees well, but, in the case of a low degree myopia (lower than -3.00D), she will be doing an unnecessary effort with them. In the case of a medium degree myopia (between -3.00 and -6.00D) or a high degree myopia (higher than -6.00D), the optical effect of the lens will decrease the size of the letters very much, and she will need to bring the text very close.

- If it is not compensated, a person with low degree myopia will read without any problem and much more relaxed. In the case of a young person with a medium degree myopia, she will have to do much more effort than any person, but she will be able to do it; and an adult person with medium degree myopia or any person with a high degree myopia, she will see it probably blurred.


Appearance of myopic eye

- The nearsighted eye is usually longer than normal, because of the eye axial length is greater than normal (> 24 milimeters).

- In the other hand, the power of its lenses (cornea -1- and crystalline lens -8-) is higher than normal (cornea: >43.05 diopters and lens: 19.22 diopters), being its surfaces more curved.

According to these two features (and/or others), the image is not focused on the retina, but in a point in front of it, so that the image of the distant object (placed at 20 feet -6 meters-), is seen blurred.



Prevalence

Some data:
- In Spain, as well as in USA and UK, up to 25% of the population is myopic.
- In some Asian areas, such as China or Malaysia, up to 41% of the adult population is myopic to -1.00 dioptre and up to 80% to -0.50 dioptre.
- In Australia, the overall prevalence of myopia (worse than −0.50 dioptres) has been estimated to be 77%.
- However, in countries such as Brazil and India, the prevalence is estimated at 10% or lower.

In general, approximately a quarter of the adult population from Western Countries and 80% of children during school-age years, have myopia.


Behavior of a myopic people (symptoms)

Without wearing her refraction correction:

- She sees blurred at distance.

- A myopic person sees better at distance when squeezing the eye lids making a narrow opening.
- She can not see at distance but she does see very well at near: It is difficult for her, for instance, seeing the chalkboard at school or seeing a sign while she drives; however, she sees very well a book or the directions for using a medicine.

- She sits very close to watch television, computer or chalkboard.

- She brings the writing, reading, painting, drawing, etc., very close to her eyes; it is like if she were eating the paper or what she is looking at, mainly if the myopia is high.

- If the myopia is low, she can work at near in a more comfortable way and for more number of hours without getting tired.

That’s why, they prefer activities as reading, drawing or handicrafts, that is, any activity at near.


- They are shier and more absent-minded, since they are very “focal”, that is, they are not very aware about what surrounds them; they are able to concentrate very much and to be many hours performing a task, so that the world can fall surrounding them and they do not realize, neither whether it gets dark or not, nor whether anyone has arrived at home or not, nor whether it is already the dinner time,… This ability to concentrate (that others, as a hyperopic people, envy), acts against themselves, because, it favors the increase of the myopia, since they do not rest even to have a glass of water. This ability allows them to be good workers and to have a good school/labor performance.

- Related to all previous, they have a reduced peripheral awareness, due to as the “excessive” concentration, as well as to the close working distance that they use sometimes without realizing.
For instance, when they are reading a book, they bring it closer more and more, because, this way, the letters seem bigger. But, in one specific moment, the eye movements are not enough and they need to move their head to be able to read a whole line. If they try to get away from the text, the letters “seem smaller” (therefore, more uncomfortable for them), but the visual field increases and they are able to read faster, because they can cover a whole line just by having a look.

- Sometimes they suffer headaches (mainly in forehead and temples) or eyestrain, and sometimes, they might even squint noticeably when they are physically tired.



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Extracts of the Presentation to the International Society for Geographic and Epidemiologic Ophthalmology by David Dunaway and Ian Berger (InFOCUS Center for Primary Eye Care Development).


[…] Estimates of the number of people worldwide with refractive error range from about 800 million to 2.3 billion. No prevalence data are available from the World Health Organization or from any other source relating to very large populations such as groups of nations, or whole countries. […]

[…] As for the proportion of an entire population with refractive error, Daniel Etyale of the W.H.O. reported at a special session on refractive error at an International Agency for Prevention of Blindness meeting in 2001, that 5 -15 percent of children are considered to have refractive errors, the majority of which are uncorrected, and that there is currently a need for population based studies to ascertain these figures. […]

[…] Although sufficient data have not been collected to accurately predict prevalence per diopter of correction for most populations, general prevalence information on the three types of refractive errors and of presbyopia can provide an orientation of what to expect.
- Hyperopia, for example, is usually present at birth, except in premature infants. Hyperopia decreases in magnitude through age 4 years, and the prevalence of hyperopia less than +1.25 D (an amount usually difficult to compensate accomodatively) is 4-7 percent between ages 5-20 years, remains constant through early middle age, then increases in populations aged 45 or more. […] Hyperopia also occurs in patients with poorly controlled diabetes and in patients who've had cataract surgery.
- Myopia prevalence varies with age, race and sex, increasing at least through adolescence, and is present in 1 per cent of children at age 5 years, increasing to 8 per cent at age 10 years and about 15 per cent at 15 years. Myopia occurs slightly more frequently in females than in males.
- The prevalence of astigmatism may be as high as 70 per cent, if all amounts are included, but is considered to decrease to 3 percent if the extent is limited to 1.25 diopters or more. […] If astigmatism is greater than 1 diopter, it usually develops before age 2 years.
- Not a refractive error, presbyopia is due to a loss of accommodative ability. It usually starts between ages 38-45 years and the prevalence is 100 percent by age 55. Early onset is associated with hyperopia. […]

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Refractive visual Disorders. Some clarifications.

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As I wrote in the previous post, when we assess somebody’s Visual Acuity and the result obtained is not 20/20, it may be, among other reasons, because of the presence of some “ametropia” also called “refractive eye disorder” or “refractive error”.


But before explaining what these words mean, first I will explain what Ocular Refraction is: it is a physical phenomenon where the light rays from the object that we look at and from the whole visual field surrounds it, when crossing certain optical structures of the eye, change their path to focus on the fovea (13) and certain points of the retina. There, these light stimuli are converted into nervous stimuli which are sent to the brain (occipital lobe) in order to convert the information from the both eyes into simple and clear image of the target and all that surrounds it.

This change of the light path is mainly due to two structures which act as lens into the eye: the cornea (1) and the crystalline lens (8) . And to a lesser extent: the aqueous humor (4) (in the anterior and posterior chamber) and the vitreous body (9) (inside of the vitreous chamber), which also influence the “dioptric power” of the eye.

Therefore, when the image of an object, placed at 20 feet (6 meters), is focused on the retina of an eye, it shows that this eye is emmetrope and its Visual Acuity (if it does not have any pathologic disorder that hampers it) will be 20/20 or better. That is, this eye does not suffer any refractive eye disorder.


But if the two following conditions are fulfilled:
- its Visual Acuity is lower than 20/20
- and it improves with the help of eyeglasses, contact lenses or another optical option,
then we can catalog this eye disorder as “Refractive Error” or “Ametropia”. This disorder is due to an upset in the power of either one of or both lenses (surfaces more curved in the case of the myopia and more flat in the case of the hypermetropia), or a change of the eye axial length (the eye is too short in the case of the myopia and too long in the case of the hypermetropia):

Grosso modo, I will now explain the refractive eye disorders, but in later posts I will explain each one of them in detail.

MYOPIA: It is the refractive error that is better known, worldwide. This prevents the distant objects to be clearly seen. In this case, optically, the light rays from the distant object (placed at 20 feet -6 meters-), are focused in a point in front of the retina, so the image is blurred.

HYPERMETROPIA or HYPEROPIA: This refractive error prevents the nearby objects, and sometimes the distant ones too, to be clearly seen. In this case, optically, the light rays from this object are focused in a point behind of the retina; so, as with the myopic eye, the hypermetropic one also sees the image blurred.
But the difference with the myopic eye is that this defect of vision is more unnoticed, and it is a defect no so well-known by people, because of the modifications that the crystalline lens can perform of its curvature, so this lens can compensate part or all of this hypermetropia ( if this is not very high) so it remains latent.
It is the most common refractive error in the newborn, because when the baby is born, the eye is not completely developed and as the body grows, so does the eyes. So, a baby with a small hypermetropia may turn into an emmetrope when she grows.

ASTIGMATISM: In the previous refractive errors, the cornea is a spherical structure (as if it was an inflated basketball that we have cut in half). However, in this defect of vision the cornea can be seen as an American football also cut in half.
Therefore, each curve in the cornea has a different dioptric power (I will better explain this concept later) and consequently, the light rays that cross each curvature suffer a different change of direction, and causing a focusing of the image on different points regarding to the retina, causing not only a blurred image but also a drop-shadow effect in the surroundings of the distant and/or nearby images. Because of this reason, the astigmatism can co-exist with myopia or hypermetropia in the same eye, but these last errors can not co-exist in the same eye at the same time.


All these refractive eye disorders cause a “faulty vision”, but in these cases this is only due to a wrong Visual Acuity. That’s why, VISION and VISUAL ACUITY are not the same concept, because we can have a value 20/20 of Visual Acuity and not being able to read the registration number of the car in front of us at 2 meters (6 feet) or keep on reading for 1 hour. Visual Acuity is only one of the different features of vision.

These “ametropias” are primary; secondarily, other refractive eye disorders may also arise:

ANISOMETROPIA: It is very strange to find a person with the same quantity of ametropia in both eyes (because we do not have two identical eyes), but anisometropia is the condition in which this difference is so big, that it is difficult that the information from the images from each eye that arrive to the brain, can be fused (I will explain this better) and can create a simple clear image.

AMBLYOPIA: It more well-known as “LAZY EYE”. This may be related to the previous disorder. I am sure some time you had seen a child wearing a patch at school or in the street, since this is one of the many ways to treat this problem (I will explain this one too).
In this case, the difference of eyeglass prescription and the functional features of one eye may be very different from the other, so one of them develops better. In this case the early diagnosis and treatment is VERY IMPORTANT.

PRESBYOPIA: Sooner or later everybody will “suffer” from this ametropia, even the person that think that, while being young, has got the best vision of the world… Sorry
This is just because of the natural aging process of human body. As time goes by we keep on losing our forces and things we could perform some years ago, now we are not capable of doing them; in vision the same fact happens too. Inside the eye, the muscle that controls the change of focus for different distances, by modifying the curvature of crystalline lens, “is more tired” too and keeps on losing flexibility, force and speed of response. As time goes by we have more difficulty for focusing more quickly and we can not see small details of nearby objects. The arms seem to stretch more and more and we would wish we’d have them longer in order to read a medicine directions for use or just a newspaper. In this case, the problem we have I resides in nearby tasks.


In the other hand, there are other disorders that also cause reduced Visual Acuity, but the difference is that this value can not be improved with any optical option, they are not refractive eye disorders but pathological disorders: Cataract, Macular Degeneration, Glaucoma ... In these cases the required treatment can not be offered by the optometrist (medicines and surgeries).


Besides there are defects of vision that are independent of the Visual Acuity value, such as, for instance: a reduced visual field, a disorder of color vision, a problem in order to focus in an efficient way when we read a book, or to work with both eyes as a team, or to discriminate shapes, or to remember what we see, or to copy something we see, or to follow a straight line… Therefore, these are not disorders of the Visual Acuity.

ILYD

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