An altitudinal defect is one that is confined by a horizontal border and crosses the vertical meridian. Homonymous altitudinal hemianopia is usually caused by lesions of both occipital lobes below or above the calcarine sulcus, and rarely to a lesion of the optic chiasm or nerves. The most common cause of this rare phenomenon is still occlusion of both posterior cerebral arteries. Herniation of the occipital lobe over the tentorial margin can produce a homonymous superior altitudinal defect by selectively compressing the inferior branches of the posterior cerebral arteries. A monocular altitudinal hemianopia, by contrast, is almost invariably an optic neuropathy, usually the nonarteritic anterior ischemic type that arises from occlusion of the posterior ciliary vessels.
In certain instances of homonymous hemianopia, the patient is capable of some visual perception in the hemianopic fields, a circumstance that permits the study of the vulnerability of different visual functions. For example, colored targets may be detected in the hemianopic fields, whereas achromatic ones cannot. The tendency for patients with occipital lesions to have greater sensitivity for kinetic stimuli than for static ones was described by Riddoch in 1917. But even in seemingly complete hemianopic defects, in which the patient consciously admits to being blind, it has been shown that he may still react to visual stimuli when forced-choice techniques are used. Blythe and coworkers found that 20 percent of their patients with no ability to discriminate patterns in the hemianopic field nonetheless could still reach accurately and look at a moving light in the “blind” field. This type of residual visual function has been called “blindsight” by Weiskrantz and colleagues. These residual visual functions are generally attributed to the preserved function of retinocollicular or geniculoprestriate cortical connections, but in some cases, they may be a result of sparing of small islands of calcarine neurons.
In yet other instances of complete homonymous hemianopia, the patient may be little disabled by visual field loss (Benton et al; Meienberg). This is because of preservation of vision in a small part of the visual field known as the monocular temporal crescent, which is the peripheral portion of the visual field, between 60 and 90 degrees from the fixation point, that is represented in the most anterior part of the visual striate cortex. In particular, the temporal crescent is sensitive to moving stimuli, allowing the patient to avoid collisions with people and objects.
Blindness in the Hysterical or Malingering Patient
Hysterical, or psychogenic blindness, is described in Chap. 47, along with other features of hysteria, but a few comments are in order here. Feigned or hysterical visual loss is usually detected by attending to the patient’s activities when he thinks he is unobserved, and it can be confirmed by a number of simple tests. Complete feigned blindness is disproved by observing the normal ocular jerk movements in response to a rotating optokinetic drum or strip, or by noting that the patient’s eyes follow their own image in a mirror that is moved in front of them. The hysterical nature of total monocular blindness is apparent from the presence of a normal direct pupillary response to light. An optokinetic response in the nonseeing eye (with the good eye covered) is an even more convincing test. The visual evoked potential from the allegedly blind eye is also normal. Hysterical monocular loss may also be revealed by the use of red-green glasses and an acuity chart with red and green letters, where each eye can only see letters with the color of its lens. The patient cannot tell which letters should be visible to them, and the intact acuity in the involved eye is soon exposed. Hysterical homonymous hemianopia is rare and is displayed mostly by practiced malingerers; all manner of field defects are common in this population (Keane). The uniformly constricted tubular field defect of hysteria, which does not expand as the testing distance is increased, has already been mentioned. Starand spiral-shaped visual fields results obtained by manual kinetic (Goldmann) perimetry are also indicative of psychogenic visual loss.