Porth's Essentials of Pathophysiology, 4e

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Disorders of Special Sensory Function: Vision, Hearing, and Vestibular Function

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overactivity or underactivity of the extraocular muscles in some fields of gaze. The disorder may be nonaccommodative, accommodative, or a combination of the two. Infantile esotropia is the most common cause of nonaccommodative strabismus. It occurs in the first 6 months of life, with large-angle deviations, in otherwise developmentally and neurologically normal infants. Eye movements are full, and the child often uses each eye independently to alter fixation (cross-fixation). The cause of the disorder is unclear. Research suggests that idiopathic strabismus may have a genetic basis, since siblings often present with similar disorders. Accommodative strabismus is caused by disorders such as uncorrected hyperopia of a significant degree, in which the esotropia occurs with accommodation that is undertaken to focus clearly. Onset of this type of esotropia characteristically occurs between 18 months and 4 years of age because accommodation is not well developed until that time. The disorder most often is monocular but may be alternating. Paralytic Strabismus. Paralytic strabismus results from paresis (i.e., weakness) or plegia (i.e., paralysis) of one or more of the extraocular muscles. When the normal eye fixates, the affected eye is in the position of primary deviation. In the case of esotropia, there is weakness of one of the lateral rectus muscles. When the affected eye fixates, the unaffected eye is in a position of secondary deviation. 39 Paralytic strabismus is uncommon in children but accounts for nearly all cases of adult strabismus; it can be caused by infiltrative processes (e.g., Graves disease; see Chapter 32), myasthenia gravis, stroke, and direct optical trauma. 38 In infants, paralytic strabismus can be caused by birth injuries affecting the extraocular muscles or the cranial nerves supplying these muscles. In general, binocular vision can be maintained when paralytic strabismus is corrected. Treatment. Treatment of strabismus is directed toward the development of normal visual acuity, correction of the deviation, and superimposition of the retinal images to provide binocular vision. Early and adequate treatment is crucial because a delay in or lack of treatment can lead to amblyopia and permanent loss of vision. Treatment includes both surgical and nonsurgical methods. Infantile esotropia is usually treated surgically by weakening the medial rectus muscle on each eye while the infant is under general anesthesia. Recurrences are common with infantile esotropia, and multiple surgeries are often required. Nonsurgical treatment includes glasses, occlusive patching, and eye exercises (i.e., pleoptics). Glasses are often used in the treatment of accommodative esotropia that occurs with hypermetropia. Because accommodation is linkedwith convergence, focusingdrives the eyes inward, producing esotropia. In infants and toddlers, intermittent exotropia is commonly treated with patching for 1 to 2 hours daily for several months. The use of over-minus glasses stimulates accommodative convergences, which contracts the exotropic drift. Vision therapy involves

exercises to stimulate convergence and techniques to train the visual system to recognize the suppressed images. Surgical treatment of intermittent exotropia is indicted when conservative methods fail to correct the deviation. Early treatment of children with intermittent exotropia is not as crucial as it is for those with constant deviations because stereopsis can still develop. A relatively new form of treatment involves the injection of botulinum toxin type A (Botox) into the extraocular muscle to produce paralysis of that extraocular muscle. 39 Paralysis of the muscle shifts the eye into the field of action of the antagonist muscle. During the time the eye is deviated, the paralyzed muscle is stretched, whereas the antagonistic muscle is contracted. Usually two or more injections of the drug are necessary to obtain a lasting effect.  Amblyopia Amblyopia, sometimes called lazy eye, describes a decrease in visual acuity resulting fromabnormal visual development in infancy or early childhood. 42–44 It is the most common cause of monocular visual impairment, affecting 1% to 4% of the population. With early detection and treatment, most cases of amblyopia are reversible and the most severe forms of the condition can be prevented. Normal development of the thalamic and cortical circuitry necessary for binocular visual perception requires simultaneous binocular use of each fovea during a critical period early in life (0 to 5 years). Amblyopia can result from visual deprivation (e.g., cataracts, ptosis) or abnormal binocular interactions (e.g., strabismus, anisometropia) during visual immaturity. In infants with unilateral cataracts that are dense, central, and larger than 2 mm in diameter, this period is before 2 months of age. 15,21 In conditions causing abnormal binocular interactions, one image is suppressed to provide clearer vision. In esotropia, vision of the deviated eye is suppressed to prevent diplopia. A similar situation exists in anisometropia, in which the refractive indexes of the two eyes are different. Although the eyes are correctly aligned, they are unable to focus together, and the image of one eye is suppressed. The reversibility of amblyopia depends on the matu- rity of the visual system at the time of onset and the duration of the abnormal experience. Occasionally in strabismus, some persons alternate eye fixation and do not experience significant amblyopia or diplopia. With late adolescent or adult onset, this habit pattern must be unlearned after correction. Amblyopia is remark- ably responsive to treatment if the treatment is initiated early in life; thus, all infants and young children should be evaluated for visual conditions that could lead to amblyopia. The American Academy of Pediatrics in association with the American Association of Certified Orthoptists, American Association of Pediatric Ophthalmology and Strabismus, and American Academy of Ophthalmology recommends that all newborn infants be examined in the nursery for structural abnormalities and have a red