Porth's Essentials of Pathophysiology, 4e

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

C h a p t e r 3 8

Levator palpebrae superioris

Inferior oblique

Superior oblique

Medial rectus

Temporalis muscle

Superior rectus

Lateral rectus

Medial rectus

Superior oblique

Inferior rectus

Superior rectus

Stump of levator palpebrae

Inferior rectus

Lateral rectus

Optic nerve

Inferior oblique

FIGURE 38-15. Extraocular muscles of the right eye.

half-fields in the two eyes is called a heteronymous loss, and the abnormality is called heteronymous hemianopia. Destruction of one or both lateral halves of the chiasm is common with multiple aneurysms of the circle of Willis (see Chapter 37). In this condition, the function of one or both temporal retinas is lost, and the nasal fields of one or both eyes are lost. The loss of the temporal fields (nasal retina) of both eyes is called bitemporal heteronymous anopia. With both eyes open, the person with bilateral defects still has the full binocular visual field. Loss of the optic tract, LGN, full optic radiation, or complete visual cortex on one side results in loss of the corresponding visual half-fields in each eye. Homonymous means “the same” for both eyes. In left- side lesions, the right visual field is lost for each eye and is called complete right homonymous hemianopia. Partial injury to the left optic tract, LGN, or optic radiation can result in the loss of a quarter of the visual field in both eyes. This is called homonymous quadrantanopia, and depending on the lesion it can involve the upper (superior) or lower (inferior) fields. The LGN, optic radiation, and visual cortex all receive their major blood supply from the posterior cerebral artery; unilateral occlusion of this artery results in complete loss of the opposite field (i.e., homonymous hemianopia). Bilateral occlusion of these arteries results in total cortical blindness. For complete function of the eyes, it is necessary that the two eyes point toward the same fixation point and that the retinal and central nervous system (CNS) visual acuity mechanisms are functional. Despite slight variations in the view of the external world for each eye, it is important that these two images become fused, which is a forebrain function. Binocular fusion is controlled by ocular reflex mechanisms that adjust the orientation of each eye to produce a single image. If these reflexes fail, diplopia or double vision occurs. The Extraocular Eye Muscles and Disorders of Eye Movement

Binocular vision depends on three pairs of extraocular muscles—the medial and lateral recti, the superior and inferior recti, and the superior and inferior obliques 16 (Fig. 38-15). Each of the three sets of muscles in each eye is reciprocally innervated so that one muscle relaxes when the other contracts. Reciprocal contraction of the medial and lateral recti moves the eye from side to side (adduction and abduction); the superior and inferior recti move the eye up and down (elevation and depression). The oblique muscles rotate (intorsion and extorsion) the eye around its optic axis. A seventh muscle, the levator palpebrae superioris, elevates the upper eye lid. The extraocular muscles are innervated by three cranial nerves. The trochlear nerve (CN IV) innervates the supe- rior oblique, the abducens nerve (CN VI) innervates the lateral rectus, and the oculomotor nerve (CN III) inner- vates the remaining four muscles. Table 38-1 describes the function and innervation of the extraocular muscles. Normal vision depends on the coordinated action of the entire visual system and a number of central control systems. It is through these mechanisms that an object is simultaneously imaged on the fovea of both eyes and perceived as a single image. Strabismus and amblyopia are two disorders that affect this highly integrated system.  Strabismus Strabismus, or squint, refers to any abnormality of eye coordination or alignment that results in loss of binocular vision. Strabismus affects approximately 4% of children younger than 6 years of age. 39–41 Because 30% to 50% of these children sustain permanent secondary loss of vision, or amblyopia (to be discussed), early diagnosis and treatment are essential. Strabismus may be divided into two forms: paralytic, in which there is weakness or paralysis of one or more of the extraocular muscles; and nonparalytic, in which there is no primary muscle impairment. In terms of