Understanding Astigmatism and other refractive Errors

Undoubtedly the most misunderstood refractive error that has patients confused. Many times I have heard patients say " I have an astigmatism in this eye." as if it was growing there. Or they have "a stigma". Stigma originally means a sign, point or branding mark such as a badge of disgrace. However that has nothing to do with vision. They may have a stigma but thats a different issue altogether.

Astigmatism refers to a refractive error of the eye where the power of the eye is different in different meridians of the eye. The largest difference in the power is 90 degrees apart usually. If not the astigmatism is called irregular. Usually the vertical meridian has more power. This condition is known as "with the rule astigmatism". If the horizontal meridian has more power it is known as against the rule astigmatism.

This variation in power within an eye (astigmatism) can vary in magnitude and direction. Most people have about a half diopter of variation within the eye. A diopter is an optical unit of measure. Greater than one diopter becomes significant with regards to contact lens fitting. At this point one must consider using a special contact lens to correct the refractive error. Most soft lens manufacturers stop making astigmatism lens above 2.25 diopters of variation. Those patients with 3 diopters of astigmatism experience so much magnification distortion of their spectacle lenses that they have great difficulty adjusting to their glasses.

Myopia is a refractive error iwhere the refracting power of the eye is too strong for it's length. This is due to the antero-posterior diameter of the eye being too large or an excess corneal refracting power. In myopia rays of light coming from distant objects are focused in the vitreous cavity in front of the retina of the eye.

The enlargement of the eye causes the retina to be stretched and thinned. As a result , myopes are more susceptable to retinal detachments. One third of all non-traumatic retinal detachments occur in myopes.

Correction is through glasses or contact lenses with a minus lens which diverges the incoming rays of light pushing the focus back on to the retina.

All lenses have a magnification effect and with minus lenses there is a minification effect through the lens. This effect becomes substantial at -4 and above.

Hyperopia is a type of refractive error where the eye possesses insufficient refracting power. This is usually due to a short antero-posterior diameter of the eye (axial hyperopia). In Hyperopia distant rays of light converge to a point behind the eye.

The hyperopic eye is typically small in all meridians. The cornea is small and the anterior chamber shallow. Anatomically the hyperopic eye is predisposed to narrow angle glaucoma.

This refractive error is corrected with a plus lens either with glasses or contact lenses, that converges the incoming rays of light to a focus on the retina. These plus lenses cause magnification of objects seen through them. This effect becomes significant at +4.00 and above.

Our eyes have the ability to focus from distant objects to near. This capability is called accommodation. This change in focusing power is achieved through the crystalline lens within the eye. By changing it's shape, steepening it's curvature, the lens can accommodate or focus. In the very young eye the lens possesses as much as 18 diopters of amplitude of accommodation. By age ten the amplitude drops to 14 diopters. This trend of losing focusing power continues with age and typically becomes an issue at age 43. The age related loss of focusing power is called presbyopia. At age 43 and older we prescribe reading glasses and bifocals to supplement the power naturally lost over the years and enable the patient to see at nearpoint..

I hope that this brief discussion has helped you to have a general understanding of refractive errors. I don't want you to be stigmatized in not knowing. It is very challenging prescribing for the infinite number of possible combinations of refractive errors. For the optometrist it is always about how to prescribe without making a spectacle of one's self.