Abnormalities of Refraction and Accommodation

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Chapter 612 Abnormalities of Refraction and Accommodation

Scott E. Olitsky, Denise Hug, Laura S. Plummer, Merrill Stass-Isern

Emmetropia is the state in which parallel rays of light come to focus on the retina with the eye at rest (nonaccommodating). Although such an ideal optical state is common, the opposite condition, ametropia, often occurs. Three principal types of ametropia exist: hyperopia (farsightedness), myopia (nearsightedness), and astigmatism. The majority of children are physiologically hyperopic at birth, but a significant number, especially those born prematurely, are myopic and often have some degree of astigmatism. With growth, the refractive state tends to change and should be evaluated periodically.

Measurement of the refractive state of the eye (refraction) can be accomplished both objectively and subjectively. The objective method involves directing a beam of light from a retinoscope onto a patient’s retina. Based on the way the light behaves with movement of the retinoscope and manipulation with lenses of various strengths held in front of the eye, a precise refraction can be performed. An objective refraction can be carried out at any age because it requires no response from the patient. In infants and children, it is generally more accurate to perform a refraction after instilling eyedrops that produce mydriasis (dilatation of the pupil) and cycloplegia (paralysis of accommodation); those used most commonly are tropicamide (Mydriacyl), cyclopentolate (Cyclogyl), and atropine sulfate.

A subjective refraction involves placing lenses in front of the eye and having the patient report which lenses provide the clearest image of the letters on a chart. This method depends on a patient’s ability to discriminate and communicate, but it can be used for some children and can be helpful in determining the best refractive correction for children who are developmentally capable.

Hyperopia

If parallel rays of light come to focus posterior to the retina with the eye in a state of rest, hyperopia or farsightedness exists. This can result because the anteroposterior diameter of the eye is too short or the refractive power of the cornea or lens is less than normal.

In hyperopia, accommodation is used to bring objects into focus for both far and near gaze. If the accommodative effort required is not too great, the child has clear vision and is comfortable with both distant and close work. In high degrees of hyperopia requiring greater accommodative effort, vision may be blurred, and the child might complain of eyestrain, headaches, or fatigue. Squinting, eye rubbing, and lack of interest in reading are common manifestations. If the induced discomfort is great enough, a child might not make an effort to see well and can develop bilateral amblyopia (ametropic amblyopia). Esotropia may also be associated (see the discussion of convergent strabismus, accommodative esotropia in Chapter 615).

Convex lenses (spectacles or contact lenses) of sufficient strength to provide clear vision and comfort are prescribed when indicated. Even children who have high degrees of hyperopia but who have good vision will happily wear glasses because they provide comfort by eliminating the excessive accommodation required to see well. Preverbal children should also be given glasses for high levels of hyperopia to prevent the development of esotropia or amblyopia. Children with normal levels of hyperopia do not require correction in the majority of cases.

Myopia

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