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

In myopia, parallel rays of light come to focus anterior to the retina. This can result because the anteroposterior diameter of the eye is too long or the refractive power of the cornea or lens is greater than normal. The principal symptom is blurred vision for distant objects. The far point of clear vision varies inversely with the degree of myopia; as the myopia increases, the far point of clear vision moves closer to the eye. With myopia of 1 diopter, for example, the far point of clear focus is 1 m from the eye; with myopia of 3 diopters, the far point of clear vision is only image m from the eye. Thus, myopic children tend to hold objects and reading matter close, prefer to be close to the blackboard, and may be uninterested in distant activities. Squinting is common because the visual acuity is improved when the lid aperture is reduced, also known as the pinhole effect.

Myopia is uncommon in infants and preschool-aged children. It is more common in infants with a history of retinopathy of prematurity. A hereditary tendency to myopia is also observed, and children of myopic parents should be examined at an early age. The incidence of myopia increases during the school years, especially during the preteen and teen years. The degree of myopia also increases with age during the growing years.

Concave lenses (spectacles or contact lenses) of appropriate strength to provide clear vision and comfort are prescribed. Changes are usually needed periodically, sometimes in every year or 2, sometimes every few months. Excessive accommodation during near work has been considered by some to lead to progression of myopia. Based on this philosophy, some practitioners advocate the use of cycloplegic agents, bifocals, intentional undercorrection of myopic refractive errors, or mandatory removal of myopic glasses for near work in an effort to retard the progression of myopia. The value of such treatment has not been scientifically proved.

Excimer laser correction for myopia has been approved for adults since 1995. The LASIK (laser-assisted in situ keratomileusis) procedure uses either a microkeratome or a femtosecond laser that produces an epithelial-stromal flap, permitting the underlying corneal tissue to be ablated to correct vision; the flap then re-covers the area of the cornea. Correction of vision is usually excellent and stable over time. Risks are greatest with high degrees of myopia (>10 diopters) and include starbursts, halos, and distorted images or multiple images (usually at night). Refractive surgery is currently not approved for pediatric patients but its use is being studied, especially for the treatment of some forms of amblyopia.

In most cases, myopia is not a result of pathologic alteration of the eye and is referred to as simple or physiologic myopia. Some children have pathologic myopia, a rare condition caused by a pathologically abnormal axial length of the eye; this is usually associated with thinning of the sclera, choroid, and retina and often with some degree of uncorrectable visual impairment. Tears or breaks in the retina can occur as it becomes increasingly thin, leading to the development of retinal detachments. Myopia can also occur as a result of other ocular abnormalities, such as keratoconus, ectopia lentis, congenital stationary night blindness, and glaucoma. Myopia is also a major feature of Stickler syndrome.

Astigmatism

In astigmatism, the refractive power of the various meridians of the eye differs. Most cases are caused by irregularity in the curvature of the cornea; some astigmatism results from changes in the lens. Mild degrees of astigmatism are common and might produce no symptoms. With greater degrees, there may be distortion of vision. To achieve a clearer image, a person with astigmatism uses accommodation or squints to obtain a pinhole effect. Symptoms include eyestrain, headache, and fatigue. Cylindric or spherocylindric lenses are used to provide optical correction when indicated. Glasses may be needed constantly or only part time, depending on the degree of astigmatism and the severity of the attendant symptoms. In some cases, contact lenses are used.

Infants and children with corneal irregularity resulting from injury, periorbital and eyelid hemangiomas, and ptosis are at increased risk of astigmatism and attendant amblyopia.

Anisometropia

When the refractive state of one eye is significantly different from the refractive state of the other eye, anisometropia exists. If the condition remains uncorrected, one eye might always be out of focus, leading to the development of amblyopia. Early detection and correction are essential if normal visual development in both eyes is to be achieved.

Accommodation

During accommodation, the ciliary muscle contracts, the suspensory fibers of the lens relax, and the lens assumes a more rounded shape to bring rays of light into focus on the retina. The amplitude of accommodation is greatest during childhood and gradually diminishes with age. The physiologic decrease in accommodative ability that occurs with age is called presbyopia.

Disorders of accommodation in children are relatively rare. Premature presbyopia is occasionally encountered in young children. The most common cause of paralysis of accommodation in children is intentional or inadvertent use of cycloplegic substances, topically or systemically; included are all the anticholinergic drugs and poisons, as well as plants and plant substances having these effects. Neurogenic causes of accommodative paralysis include lesions affecting the oculomotor nerve (cranial nerve III) in any part of its course.

Differential diagnosis includes tumors, degenerative diseases, vascular lesions, trauma, and infectious diseases. Systemic disorders that can cause impairment of accommodation include botulism, diphtheria, Wilson disease, diabetes mellitus, and syphilis. Adie tonic pupil can also lead to a deficiency of accommodation after some viral illnesses (Chapter 614). An apparent defect in accommodation may be psychogenic in origin; it is common for a child to feign inability to read when it can be demonstrated that visual acuity and ability to focus are normal.

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