Bilateral lesions of the occipital lobes, if extensive, cause “cortical blindness” that is essentially bilateral homonymous hemianopia, sometimes accompanied by unformed visual hallucinations. The pupillary reflexes are preserved and the optic discs appear normal. Sometimes the patient is unaware of being blind and denies the problem even when it is pointed out to him (Anton syndrome). More frequently, the lesions are incomplete, and a sector of the vision, usually on one side, remains intact. When the visual remnant is small, vision may seemingly fluctuate from moment to moment as the patient attempts to capture the image in the island of intact vision, in which case hysteria may be incorrectly inferred. In bilateral lesions confined to the occipital poles, there may be a loss of central vision only (homonymous central scotomas). With more anteriorly placed lesions of the occipital pole, there may be homonymous paracentral scotomas, or the occipital poles may be spared, leaving the patient with only central vision (bilateral central, or macular sparing). Horizontal or altitudinal field homonymous defects are usually a result of similar restricted lesions affecting the upper or lower banks of the calcarine sulci (essentially, quadrantanopia). The Balint syndrome (see Chap. 21) is an effect of bilateral occipitoparietal border-zone lesions.
With bilateral lesions that involve the inferomedial portions of the temporal lobes, including the hippocampi and their associated structures, the impairment of memory may be severe, causing the Korsakoff amnesic state. In several of our patients, a solely left-sided infarction of the inferomedial temporal lobe impaired retentive memory. Bilateral mesiotemporal-occipital lesions also cause a lack of recognition of faces (prosopagnosia). These and other effects of temporal and occipital lesions are discussed in Chaps. 12 and 22.
The vertebral arteries are the chief arteries of the medulla; each supplies the lower three-fourths of the pyramid, the medial lemniscus, all or nearly all of the retroolivary (lateral medullary) region, the restiform body, and the posteroinferior part of the cerebellar hemisphere through the posterior inferior cerebellar arteries (Figs. 33-2 and 33-12). The relative sizes of the vertebral arteries vary considerably and in approximately 10 percent of cases, one vessel is so small that the other is essentially the only artery of supply to the brainstem. The dominant vertebral artery can usually be identified by the convexity of the basilar artery, which swings in a direction away from the dominant vertebral artery. This may be helpful to the clinician in identifying that the dominant vertebral artery is occluded. If there is no collateral flow from the carotid system via the circle of Willis, occlusion of one functional vertebral artery is equivalent to occlusion of the basilar artery (see below). The posteroinferior cerebellar artery (PICA) is usually a branch of the vertebral artery but can have a common origin and form a loop with the anteroinferior cerebellar artery (AICA) from the basilar artery. It is necessary to keep these anatomic variations in mind in considering the effects of vertebral artery occlusion.
The vertebral artery is assigned four numerical segments for convenience of explication. They are: V1, from the origin to the first entry into the cervical transverse foramen (usually C6 as noted); V2, from the transverse foramen to the uppermost foramen (at C1); V3 from this site to the dural penetration at the foramen magnum; and V4 from the dural entry to the junction with the opposite vertebral artery and the origin of the basilar artery.