Astigmatism - My Child Needs Glasses?

In order to see things clearly, the front part of the eye (the cornea and lens) must focus a tiny picture of the object being looked at onto the back of the eye (the retina).  This tiny picture inside the eye is called an “image.”  If the image is not focused precisely on the retina, then the object is seen as a blur.   The lens in a pair of glasses (or a contact lens) puts the image into focus on the retina when the eye’s natural focusing system is not sufficient to do the job.

1 = Cornea.  The clear tissue that makes up the front of the eye.   Normally clear, like glass, the cornea does 2/3 of the focusing work.

2 = Iris.  The colored part at the front of the eye; some are brown and some are blue.  The iris regulates the amount of light entering the eye.

3 = Natural Lens.  The lens does 1/3 of the focusing work. Until the age of 40 years, or so, the lens changes shape put things into focus.    Typically clear at birth, with age the lens becomes cloudy (opaque) – this is referred to as a  “cataract.”  

4 = Retina.  The lining of the inside of the eyeball.  The retina converts images (tiny pictures of real objects) into chemical, and then electrical, signals, which are then sent to the brain.  The brain processes these signals and makes sense of it all – vision occurs in the brain.

Astigmatism: “Two Focal Points”

If the front of the eye (cornea) were perfectly round (spherical) then all of the light rays that constitute an image would pass through cornea equally and be put into focus onto a single spot of the retina.   However, human body parts are not “perfect,” so the cornea is never perfectly round.  This deviation from perfect roundness is microscopic, but it is enough so that light rays passing through one area in the cornea are focused differently from light rays that pass through other areas.  When one image is focused into two spots, this is called astigmatism.  Most people have some degree of astigmatism – it is only a problem when the degree of astigmatism is sufficient to blur the vision.

To correct astigmatism, both of the focal points need to be put into a single focal point on the retina.  To achieve this, an artificial lens is placed in front of the eye (glasses or a contact lens) to shift the two images onto the same single spot of retina.       

Anisometropic astigmatism.
(One eye has more astigmatism)
When each eye has a different degree of astigmatism, this is termed anisometropic astigmatism (aniso is Greek for “different” and metropia means “focusing error”).  The problem with anisometropia is that one eye focuses pretty well and the other eye does not focus as well. When one eye sends a better image to the brain than the other eye, the brain does not use the out of focus eye as much.  In anisometropic astigmatism, the more astigmatic eye is out of focus – this is a problem during visual development.   When a child younger than 8 years sends her brain a blurred image from one eye, the brain does not learn how to see well with that eye – this is called amblyopia.  If amblyopia is not corrected by the age of 8 years (or so), then the vision will always be less in that eye, even with glasses – the vision loss is permanent and irreversible because the brain is finished with “learning” to use the eye by that age.   In fact, it is increasingly difficult to reverse amblyopia as a child ages – it is much easier (and more effective) to reverse anisometropic amblyopia in a 3 year old than in a 7 year old.