Infiltration of an optic nerve may occur in sarcoidosis (see Fig. 31-4, bottom panel), granulomatosis with polyangiitis (formerly Wegener granulomatosis), and with certain neoplasms, notably leukemia and lymphoma. Systemic lupus erythematosus, diabetes, sarcoidosis, neurosyphilis, and AIDS also rarely give rise to optic neuropathies. The condition called “orbital pseudotumor,” essentially an inflammatory condition of all the orbital contents, can include visual loss from optic neuropathy, and is discussed in Chap. 13.
Lesions of the Chiasm, Optic Tract, and Geniculocalcarine Pathway
Hemianopia describes visual loss in one-half of the visual field for each eye. Bitemporal hemianopia is heteronymous, because the field deficit is on opposite sides of the vertical meridian for each eye. Bitemporal hemianopia is caused by a lesion that disrupts the decussating fibers of the optic chiasm and is caused most often by the suprasellar extension of a tumor of the pituitary gland (see
Fig. 12-2C). It may also be the result, at this same site, of a craniopharyngioma, Rathke cleft cyst, a saccular aneurysm or dolichoectatic artery of the anterior circle of Willis, and a meningioma of the tuberculum sellae; less often, it may be a result of sarcoidosis, metastatic carcinoma, ectopic pinealoma or dysgerminoma, Hand-Schüller-Christian disease, opticochiasmic arachnoiditis, or hydrocephalus with dilatation and downward herniation of the anterior part of the third ventricle (Corbett). In some instances a tumor pushing upward presses the medial parts of the optic nerves, just anterior to the chiasm, against the anterior cerebral arteries.
Variations in the pattern of visual loss from chiasmal lesions are frequent, in part accounted for by the location of the chiasm in an individual patient. A “postfixed” chiasm is located more posteriorly, and a compressive lesion arising from the pituitary sella is more likely to present with deficits relating to optic neuropathy. In contrast, a prefixed chiasm is located more anteriorly, so that a compressive lesion from the pituitary sella is more likely to compress the optic tract rather than the optic chiasm. As noted previously, the visual field pattern created by a lesion of the optic nerve as it joins the chiasm typically includes a scotomatous defect on the affected side coupled with a contralateral superior quadrantanopia (a combination referred to as a “junctional field defect”). The latter is caused by interruption of nasal retinal fibers from the contralateral optic nerve. This was originally attributed to fibers from the inferonasal retina projecting into the base of the affected optic nerve before looping back to decussate through the chiasm; however, there is now evidence against the normal existence of this structure and discussed in the reference by Horton.