TOPICAL DIAGNOSIS of RETROCHIASMAL VISUAL FIELD DEFECTS with Few Exceptions, Unilateral Lesions of the Visual Sen- Pias Caused by Brain Tumors (206–208)
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516 CLINICAL NEURO-OPHTHALMOLOGY betes insipidus and hypothalamic hypopituitarism; prepuber- tal children may also suffer both growth retardation and de- layed sexual development; and young children and infants may have severe failure to thrive. ‘‘CHIASMAL SYNDROME’’ OF CUSHING In 1930, Cushing described the importance of optic atro- phy and bitemporal field defects in adults as indicative of tumor when the sella turcica seemed normal in the plain lateral skull x-ray (153). This syndrome is most often pro- duced by suprasellar meningiomas, aneurysms, and cranio- pharyngiomas. This notable contribution was once of great value in eliminating, or at least reducing, misdiagnosis of sinusitis and retrobulbar optic neuritis as common causes for slowly progressive bilateral loss of vision. Optic atrophy in such cases may be very slight, or the optic discs may Figure 12.13. Seesaw nystagmus. As the right eye elevates, it also intorts. remain normal in appearance for months despite pronounced At the same time, the left eye depresses and extorts. The right eye then loss of vision and extensive defects in the visual fields. The depresses and extorts, as the left eye elevates and intorts. bitemporal field defects in some cases are extremely mild and detected only by careful kinetic perimetry or automated static perimetry. Finally, the normalcy of the sella turcica The unusual phenomenon of seesaw nystagmus may occur may only be relative. Since Cushing (153) described this in patients with tumors of the diencephalon and chiasmal syndrome, there have been substantial advances in neuroim- regions (195–198). This condition is characterized by syn- aging. With the almost universal availability of computed chronous alternating elevation and intorsion of one eye and tomography (CT) scanning and magnetic resonance (MR) depression and extorsion of the opposite eye (Fig. 12.13). imaging, most patients with progressive visual loss no longer The cause of seesaw nystagmus is unknown, but it may be undergo plain skull x-rays. Cushing’s syndrome of the chi- related to damage to structures near the interstitial nucleus asm is thus more important for its historical significance of Cajal, but not the nucleus itself (199–201). Rarely, seesaw than for its place in current neuro-ophthalmologic diagnosis. nystagmus occurs in the absence of a brain stem or dience- Nevertheless, there are still some areas in the world in which phalic lesion (202), for instance in congenital absence of the skull radiographs are used as the sole or initial step in assess- chiasm (203). ing patients with an optic chiasmal syndrome. In such cases, Lesions causing a chiasmal syndrome may arise from or the triad that represents Cushing’s syndrome remains a use- extend to the hypothalamus. Such patients may develop dia- ful clinical entity. TOPICAL DIAGNOSIS OF RETROCHIASMAL VISUAL FIELD DEFECTS With few exceptions, unilateral lesions of the visual sen- pias caused by brain tumors (206–208). When the cerebral sory pathway posterior to the optic chiasm—affecting the tumor is removed or the visual system is decompressed, im- optic tract, lateral geniculate body, optic radiation, or striate provement first occurs in the central region and continues cortex—produce homonymous visual field defects without toward the periphery. Bender and Kanzer (205) also ob- loss of visual acuity. When such defects are complete, they served that defects for colored objects invariably appeared do not, in themselves, permit localization. In such instances, before disturbances for either form or for black-and-white the clinician must rely on other symptoms and signs of neu- objects. This observation may constitute another valid reason rologic disease or on neuroimaging to define both the area for including colored stimuli or objects in examination of of damage and the etiology of the lesion (204). Nevertheless, visual fields. This should be done not only when testing for some general comments about retrochiasmal field defects evidence of optic nerve disease but also when testing for are worth noting before launching into a comprehensive dis- chiasmal and retrochiasmal lesions, particularly when no de- cussion of the clinical syndromes associated with specific fect has been found using standard kinetic perimetry with regions of the retrochiasmal visual sensory system. either white light or white test objects. Static perimetry Homonymous defects in the visual fields characteristi- seems to be as sensitive as manual testing using colored cally develop slowly if they are caused by intracranial tumors objects (especially red) for detecting subtle visual field de- and rapidly when they originate from vascular processes fects (see Chapter 2). such as hemorrhage or infarction. This applies wherever the The onset of homonymous hemianopias arising as the re- lesion may be situated. sult of vascular lesions is sudden. Such defects include com- In the instance of homonymous hemianopia arising from plete homonymous quadrantanopias and hemianopias, in- cerebral tumor, a paper by Bender and Kanzer (205) is valu- complete homonymous quadrantanopias and hemianopias able. They noted progression of the field defects from the with varying degrees of congruity, and homonymous para- periphery of the field to its center in homonymous hemiano- central scotomas. When and if improvement occurs, the cen- TOPICAL DIAGNOSIS OF CHIASMAL AND RETROCHIASMAL DISORDERS 517 tral field resolves first, and such improvement may be fol- with the greatest number in the sixth decade. The visual field lowed by gradual enlargement of the peripheral fields if they defects were not necessarily isolated, and the location and have been affected. properties of the underlying lesions were confirmed by sur- Younge (209) described midline tilting between seeing gery or CT scanning in about half the cases. Fifty-one per- and nonseeing areas in the perimetric evaluation of homony- cent of patients had occipital lobe pathology, 29% had le- mous hemianopic field defects. A deviation from the abso- sions of the optic radiation, and 21% had lesions in the region lute vertical meridian is often found when careful perimetry of the optic tract and lateral geniculate. Vascular causes, is performed in such patients. Stone et al. (210,211) and including infarction, hemorrhage, and arteriovenous malfor- others (212–214) found that in nonhuman primates, a certain mations, were responsible for 71% of cases and mass lesions, percentage of retinal ganglion cells nasal to the vertical mid- including tumors and abscesses, for another 19%. Other line bisecting the fovea have axons that do not cross at the etiologies included trauma, inflammation, degeneration, and optic chiasm, and a certain percentage of ganglion cells tem- presumed demyelinating disease. Molia et al. (219) reported poral to the same vertical midline have axons that do cross the development of homonymous hemianopic field defects in the chiasm. It is not surprising, then, that there is some in two children with intracranial shunts. In both cases the variation from a perfectly vertical separation of seeing and field defect was caused by displacement of the shunt cath- nonseeing field. Such a variation is rarely more than a few eter. degrees (see below). Given the preceding, it should be self-evident that neu- In 1962 Smith reported on the causes of homonymous roimaging (preferably MR imaging) should be the first diag- hemianopia in 100 patients (215). Forty-three patients had nostic procedure used to determine the location and etiology proven etiologies, with the location of the lesion being in of a homonymous hemianopia, following a complete history the occipital lobe in 12 patients, parietal lobe in 17 patients, and examination. and temporal lobe in 14 patients. Fifty-seven patients had Special consideration is given here to a syndrome origi- lesions in which the location was assumed on the basis of nally emphasized by Hoyt et al. (220) and later by Bajandas clinical evidence alone. Of these patients, 27 had lesions et al. (221): homonymous hemioptic (hemianopic) hypopla- presumed to be in the occipital lobe, 16 had lesions presumed sia. In this disorder, patients with cerebral hemiatrophy, pre- to be in the parietal lobe, and 10 had lesions presumed to sumably from fetal vascular insufficiency or trauma, suffer be in the temporal lobe. In addition, three patients had lesions from mental retardation, a seizure disorder, congenital hemi- presumed to be in the optic tract, and one patient had a plegia, and an ipsilateral complete homonymous hemia- presumed lateral geniculate lesion. Thus, almost 40% of pa- nopia. Such patients have classic hemianopic ophthalmo- tients with homonymous hemianopias had proven or pre- scopic findings in the ocular fundi, with the eye ipsilateral sumed occipital lobe lesions, whereas 33% had parietal lobe to the hemispheric defect showing a slightly small optic disc lesions and only 24% had temporal lobe lesions. Optic tract with mild temporal pallor and impressive loss of the nerve and lateral geniculate lesions were rare. Smith found only a fibers subserving ganglion cells temporal to the fovea. The 42% incidence of vascular etiology in his series, compared superior and inferior arcuate bundles in such eyes are thus with a 38% incidence of intracranial tumor. poorly visible. In the eye contralateral to the hemispheric Trobe et al. (216) reviewed 104 cases of isolated homony- defect, the optic disc is often small and oval, with a horizon- mous hemianopia and found, as might be expected, that the tal band of atrophy and loss of nerve fibers from ganglion vast majority (86%) were caused by vascular disease in the cells nasal to the fovea, as occurs in patients with bitemporal territory of the posterior cerebral artery. Another three pa- hemianopia from lesions of the optic chiasm. Whether the tients were believed to have suffered infarctions in the terri- ophthalmoscopic and histologic changes observed in these tory of the middle cerebral artery, bringing the total percent- patients occur from retrograde degeneration after embryonic age of isolated homonymous hemianopias thought to be damage to the optic tract or from transsynaptic degeneration caused by infarction to 89%.