The usual cause of cortical blindness is occlusion of the posterior cerebral arteries (most often embolic) or the equivalent, occlusion of the distal basilar artery. The above-mentioned macular sparing may leave the patient with an island of barely serviceable central vision. The infarct may also involve the mediotemporal regions or thalami, which share the posterior cerebral artery supply, with a resulting Korsakoff amnesic defect and a variety of other neurologic deficits referable to the high midbrain and diencephalon (drowsiness, akinetic mutism as described in Chap. 16).
The main characteristic of this disorder is the denial of blindness by a patient who obviously cannot see. These patients act as though they could see, and in attempting to walk, collide with objects, even to the point of injury. They may offer excuses for the difficulties—“I lost my glasses,” “The light is dim”—or may only evince indifference to loss of sight. The lesions in cases of negation of blindness extend beyond the striate cortex to involve the visual association areas.
Rarely, the opposite condition arises: a patient is able to see small objects but claims to be blind. This individual walks about avoiding obstacles, picks up crumbs or pills from the table, and catches a small ball thrown from a distance. This simulates the condition of hysterical blindness (see further on).
These may present as distortions of form, size, movement, or color. In a group of 83 patients with visual perceptual abnormalities, Hécaen found that 71 fell under one of four headings: deformation of the image, change in size, illusion of movement, or a combination of all three. Illusions of these types have been reported with lesions confined to the occipital lobes but are more frequently caused by shared occipitoparietal or occipitotemporal lesions; consequently, they are also considered in earlier sections of this chapter as well as in Chap. 12. The right hemisphere appears to be involved more often than the left. Illusions of movement occur more frequently with posterior temporal lesions or seizures, polyopia (one object appearing as two or more objects) more frequently with occipital lesions (it also occurs in hysteria), and palinopsia (perseveration of visual images, as in the frames of a celluloid film) with both posterior parietal and occipital lesions. Visual field defects are present in many of the cases. In all these conditions, the anatomic correlates are imprecise.
It is likely that an element of cortical vestibular disorder underlies the metamorphosis of parietooccipital lesions. The vestibular and proprioceptive systems are represented in the parietal lobes of each side and the lesions there are probably responsible for misperceptions of movement and spatial relations. The illusion of tilting of the environment or upside-down vision is known to occur with parietooccipital lesions, but occurs more often with abnormalities of the vestibular system.
These phenomena may be elementary or complex, and both types have sensory as well as cognitive aspects. Elementary (or unformed) hallucinations include flashes of light, colors, luminous points, stars, multiple lights (like candles), and geometric forms (circles, squares, and hexagons). They may be stationary or moving (zigzag, oscillations, vibrations, or pulsations). They are much the same as the effects that Penfield and Erickson obtained by stimulating the calcarine cortex in a conscious patient. Complex (formed) hallucinations include objects, persons, or animals and infrequently, more complete scenes that are indicative of lesions in the visual association areas or their connections with the temporal lobes. They may be of natural size, Lilliputian, or grossly enlarged. With hemianopia, they appear in the defective field or move from the intact field toward the hemianopic one. The patient may realize that the hallucinations are false experiences or may be convinced of their reality. Because the patient’s response is usually in accord with the nature of the hallucination, he may react with fear to a threatening vision or casually if its content is benign.