Refractive disorders: Astigmatism.(5) Solutions

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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.

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Refractive disorders: Astigmatism. (4) Symptoms.

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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.

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Refractive disorders: Astigmatism. (3) Different features

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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.

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Refractive disorders: Astigmatism.(2) Appearance

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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.


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Refractive disorders: Astigmatism.(1) Vision

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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:

1. 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.
   
   
   
   
2. 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.

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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.

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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