Anatomy & Refractive States of the Eye
Emmatropia and ametropia

In your practice you may have come across the relatively young hyperope of say under 35 years of age with a mild prescription of about three quarters to one diopter of plus power. This hyperope, even though he has a prescription, claims he is capable of seeing quite clearly at all distances without any corrective lenses. And in fact he can. The reason for this is since he is relatively young, there is still an ample supply of accommodative power available within the crystalline lens. Or, stated another way, there is reserve plus power available for use inside his eye. And, since hyperopia is corrected with plus powered lenses, he is capable of supplying his own power as needed when gazing at any distance. As he gets older, however, and the crystalline lens begins to lose its elasticity, the amount of reserve accommodation will gradually diminish and he will eventually require corrective lenses.

On the other hand, when the myope is gazing at infinity without corrective lenses, the image will be blurred, and there is nothing that patient can do to bring it into focus. So when even the low minus patient of say minus a half diopter of power says they absolutely must have their glasses because they can’t function properly without them, they are in fact telling the truth. Their world will simply be fuzzy. And if they attempt to accommodate for distance viewing like the hyperope, it will only add plus power only making matters worse.

Page 11 illustrates the various astigmatisms which are correctable with spectacle lenses. All of the astigmatic conditions shown are forms of what are called regular astigmatism as compared to irregular astigmatism. Regular astigmatism can be corrected with spectacle lenses while irregular astigmatism cannot. Regular astigmatism is a refractive condition where light comes to two points of focus. Irregular astigmatism is a refractive condition caused by a cornea which is damaged and irregular so that rays of light come to many points of focus and this condition is not correctable with spectacle lenses.

There are basically three categories of regular astigmatism: simple astigmatism, compound astigmatism, and mixed astigmatism. In each of these cases light comes to two points of focus. In simple astigmatism one of these points of focus always falls on the retina. Page 11 shows an example of simple myopic astigmatism. In this case one point of focus is on the retina, and the other point is located in front of the retina. The next example is simple hyperopic astigmatism. Again this shows one point of focus on the retina, but the other is located behind the retina.

In any kind of compound or mixed astigmatism both points of focus are located at some point off the retina. Referring again to page 11 and the example of compound myopic astigmatism, notice how both focal points fall in front of the retina, while In compound hyperopic astigmatism both points of focus are located behind the retina.

The example of mixed astigmatism shows one point of light coming to focus in front of the retina, while the other point is focused behind the retina. The lens used to correct this type of prescription will contain both plus and minus power.

Some additional terminology related to refractive error

Anisometropia is a condition in which the refractive error of one eye differs significantly from that of the other and where both eyes have the same sign. That is, both eyes are myopic or both eyes are hyperopic.

Antimetropia is the refractive condition in which one eye is myopic (nearsighted) and the other eye is hyperopic (farsighted). “Anti” meaning opposite, or opposite signs. Antimetropia should not be confused with mixed astigmatism in which there is a plus and minus correction within each eye. Plus lenses will magnify an image while minus lenses will minify causing an image to appear smaller than the actual object. The greater the power of the lens, the greater the magnification or minification. So in the case of antimetropia if one lens is plus, and the other lens is minus, the images received by the brain could differ in size significantly. In a like manner both lenses could be either plus or minus but with significantly different degrees of power such as in anisometropia, resulting in differing image sizes.

Aniseikonia, a form of double vision, may occur when the image seen by one eye differs so much in size or shape from the image seen by the other that the two images cannot be fused by the brain into a single impression. One method of treating aniseikonia is through the use of contact lenses. Another is by using iseikonic lenses. Iseikonic lenses are spectacle lenses specially designed to manipulate image size in those cases when dissimilar sizes makes it difficult or impossible for fusion to occur.

Amblyopia, is a loss of vision without any apparent disease of the eye. It is also referred to as “lazy eye.” It sometimes happens that young children with diplopia, will suppress the image in one of the eyes to avoid double vision. The suppressed eye will eventually lose its ability to see if the condition is not noticed and treated in time. A common method of treatment is to patch or occlude the good eye, forcing the patient to use the amblyopic eye.

The terms OD, OS, and OU, are abbreviations for oculus dexter, oculus sinister, and oculus uterque. You will often see these on prescription forms and they mean right eye, left eye, and each eye respectively.