Refractive Error


Glasses may be prescribed to help a child see better, to help straighten a child's eyes, to help relieve amblyopia ("Lazy Eye"), or for any combination of these three purposes. In order to understand the use of glasses it is necessary to understand the various problems which an eye can encounter during its process of bringing incoming light rays to a focus.

As light enters the eye, it is bent (refracted) by the cornea and lens. The purpose of this refraction is to bring the rays of light to a sharp focus on the retina, the film-like area of the back of the eye which converts the light energy into electrical energy for transmission of the information back to the brain. In many cases, however, the eye is not properly constituted to bring an image into a sharp focus on the retina; this inability to bring about a proper focus on the retina is called a "refractive error". Refractive errors are the most frequent cause for the prescription of corrective lenses, usually glasses initially. There are three major types of refractive error: hyperopia (farsightedness) myopia (nearsightedness), and astigmatism. Astigmatism may exist alone, or, as is most frequently the case, it may be combined with either nearsightedness or farsightedness.

Myopia, or nearsightedness, means that the patient is able to see near objects more clearly than far ones. In myopia, the eyeball can be considered to have grown too long for its focusing power; thus, the image of an object is brought to a focus before its light rays reach the retina, the rays diverging again by the time they finally reach the retina resulting in a blurred image on the retina. In most cases, simple myopia is not present at birth, but comes on some years later, as the child grows; nevertheless, the tendency to become myopic is present at birth as an inherited, genetic trait, which may be passed on to future generations. In general, myopia increases during the first several years that it is present; after those several years it generally stabilizes, and the glasses prescription remains approximately the same for many years.

Astigmatism results when the cornea is slightly irregular in shape, so that objects at all distances are blurred and distorted. The irregularity in the shape of the cornea or lens is quite minimal in most cases and is not visible to an outside observer, except by special optical means. Astigmatism may be thought of as being like the view one gets through a window made of irregular glass.

Astigmatism may improve or worsen throughout childhood. Hyperopia, or farsightedness, on the other hand, usually does not cause any blurred vision during childhood. It may, however, cause strabismus. In hyperopia, the eyeball can be considered to be too short. Light rays from objects would be expected to come to a focus behind the retina, so that the image falling on the retina, being not yet completely focused, would be blurred. However, the child can easily and automatically change the shape of his or her lens to focus these rays properly on the retina: this process is called accommodation and it requires unconscious effort by the visual system. During accommodation the eyes normally turn inward, or converge. In some cases this effort is too great for the system, and causes the eyes turn inward excessively. Thus, although most children with small or moderate degrees of hyperopia do not need any glasses or other optical correction to see clearly, those children who develop esotropia, or crossed eyes, do require those glasses to keep their eyes straight. In these cases, the glasses must be worn full time, since the eyes will begin to cross the moment the glasses are removed.

In cases of severe hyperopia, glasses must be worn even if the eyes are straight without them. This is because the visual system is unable to accommodate sufficiently to overcome the large degree of farsightedness and bring a clear image into focus on the retina. As an adult ages, the ability to accommodate decreases progressively, so that a person over 45 years of age is usually unable to focus on near objects; this condition is called presbyopia, and is not the same as hyperopia, although the two conditions are often confused by lay people. Presbyopia is appropriately treated with reading glasses, sometimes in the form of bifocals, to help the adult focus on near objects; reading glasses are usually not needed during childhood since children possess a great deal of reserve accommodation with which to focus on near objects. Nevertheless, some unscrupulous practitioners may try to sell reading glasses to parents of some children, especially those children with reading problems; these glasses are usually of no value.

Hyperopia generally decreases as a child grows. Thus, many children with small degrees of farsightedness who are wearing glasses to keep their eyes straight may be able to stop wearing glasses when they reach adolescence. This is usually not the case for those children who are wearing glasses for large degrees of hyperopia.

Glasses may also be necessary when each of the two eyes has a different refractive error (Anisometrodia). This condition results in a inability to use the two eyes together, and may cause a lazy eye (Amblyopia) or severe visual discomfort (Asthenopia).

Glasses or, later, contact lenses, which may frequently be used interchangeably with them, are thus used for many purposes in visual disorders; it is important to understand their use in each individual case.