Optic Chiasm

A chiasmal process should be considered in patients with temporal field defects that respect the vertical meridian in either or both eyes, or in patients with visual loss of any type accompanied by an endocrinopathy. The most common field defect is a bitemporal hemianopsia, although the pattern of visual loss may vary depending both on the chiasm's position and the exact nature and location of the offending process (see Fig. 8-3 (Figure Not Available) ). When a sellar mass is the cause, prefixed chiasms or more posteriorly situated lesions predispose to optic tract syndromes or central hemianopic scotomas. Postfixed chiasms or more anteriorly situated lesions are more likely to appear as an optic neuropathy or junctional scotoma with involvement of the ipsilateral optic nerve and Wilbrand's knee.

Patients with a bitemporal hemianopia are often without visual complaints unless visual acuity is abnormal, and such defects may not be apparent until the patient reads only the nasal half of the acuity chart. Some patients may complain of double vision because they may be unable to align the noncorresponding nasal fields of each eye (hemifield slide phenomenon). Asymmetrical lesions may produce an ipsilateral afferent pupillary defect, and color vision may be abnormal only in the defective field. Congenital or chronic processes may lead to optic atrophy, but rarely does a lesion compressing the optic chiasm lead to optic disc swelling without concomitant compression of the third ventricle. Rarely patients present with asymmetrical shimmering nystagmus, torticollis, and head bobbing (mimicking spasmus nutans) or seesaw nystagmus.

Chiasmal syndromes are most commonly caused by sellar and suprasellar compressive masses. Such lesions are suggested by historical evidence of pituitary or hypothalamic dysfunction, and the differential diagnosis depends largely on the age of the patient. In the pediatric population, chiasmal-hypothalamic gliomas y and craniopharyngiomas y are most likely, whereas in middle-age to elderly-age patients, pituitary adenomas, y internal carotid aneurysms, craniopharyngiomas, and meningiomas should be considered. Compressive lesions usually produce insidious visual loss, and medical or surgical decompression may provide partial or complete visual recovery, especially in patients without evidence of optic atrophy. Rapid onset of visual loss and hypopituitarism suggests pituitary apoplexy. y , y Rarely, chiasmal syndromes can be congenital as in septo-optic dysplasia (de Morsier's syndrome), characterized by hypoplasia of the optic nerves and chiasm, a hypoplastic or absent septum pellucidum, and pituitary ectopia.

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