In addition to the losses of perception of form, movement, and color, lesions of the visual system may also give rise to a variety of positive sensory visual experiences. The simplest of these are called phosphenes, that is, flashes of light and colored spots in the absence of luminous stimuli. Mechanical pressure on the normal eyeball may induce them at the retinal level, as every child discovers. Or they may occur with disease of the visual system at many different sites. As mentioned earlier, elderly patients commonly complain of flashes of light in the peripheral field of one eye, most evident in the dark (Moore lightning streaks); these are related to vitreous tags that rest on the retinal equator, and may be quite benign, or may be residual evidence of retinal detachment. Autoimmune and cancer associated retinopathy is frequently associated with photopsias prior to visual loss. Retinal toxicity from digitalis causes chromatopsia with a characteristic “yellowish vision” and may also cause photopsias. In patients with migraine, cortical spreading depression through the occipital lobe gives rise to the bright zigzag lines of a fortification spectrum. Stimulation of the cortical visual pathways accounts for stereotyped, repetitive simple or unformed visual hallucinations in epilepsy. Formed or complex visual hallucinations (of people, animals, landscapes) are observed in a variety of conditions, notably in old age when vision fails (Charles Bonnet syndrome, discussed in “Visual Hallucinations” in Chap. 21), with infarcts of the occipitoparietal or occipitotemporal regions (release hallucinations), in the withdrawal state following chronic intoxication with alcohol and other sedative-hypnotic drugs (see Chap. 41), and with lesions in the diencephalon (“peduncular hallucinosis”). These disorders are also discussed in Chap. 21.
Occasionally, patients with a parietal lesion may perceive transposition of images, flipped across the vertical or horizontal axes (visual allesthesia), or a visual image may persist for minutes to hours or reappear episodically, after the exciting stimulus has been removed (palinopsia mentioned earlier). Polyopia, the perception of multiple images when a single stimulus is presented, is said to be associated predominantly with right occipital lesions and can occur with either eye. Usually there is one primary and a number of secondary images, and their relationships may be constant or changing. Bender and Krieger, who described several such patients, attributed the polyopia to unstable fixation. Oscillopsia, or illusory movement of the environment, is a perception caused by nystagmus and occurs mainly with lesions of the labyrinthine-vestibular apparatus; it is described with disorders of ocular movement. In patients describing intermittent monocular oscillopsia, myokymia of one superior oblique muscle should be considered (see “Fourth Nerve Palsy” in Chap. 13).
Chapter 21 further discusses the clinical effects and syndromes that result from occipital lobe lesions.
Amblyopia Because of Early Life Strabismus (Amblyopia Ex Anopsia)
As noted in the introductory part of this chapter, the generic term “amblyopia” has been adopted for a special circumstance in which a normal eye fails to acquire its potential visual acuity because images are not properly projected onto the fovea during a formative period of cerebral development. It is a disorder, as in the quip by van Noorden and Campos, “in which the patient sees nothing and the doctor sees nothing,” meaning that the pupillary response and fundus are observed to be normal despite the visual deficit. The degradation of vision and disuse of the fovea may be the result of a number of processes, including misaligned ocular axes (strabismus), reduced monocular inputs (deprivation, from ptosis or cataract), or unequal refractive errors (anisometropia; discussed in Chap. 13). The period of risk is during the first 7 years but is greatest in the earlier part of this epoch, and the visual loss may still be rectifiable beyond the time.