4. Coma. When the reticular formation is affected bilaterally in the medial pontine tegmentum, coma results. Sensory and cerebellar abnormalities are absent or slight because the infarct usually affects the midline and paramedian structures in the basis pontis, sparing the spinothalamic tracts and the cerebellum. Collateral circulation is mainly through the circumferential vessels, which course around the lateral portions of the brainstem to nourish the lateral base, tegmentum, and cerebellum. The cerebellar hemispheres are mostly nourished by the posterior inferior cerebellar artery that originates before the basilar artery, and the superior cerebellar artery (SCA), which is preserved when the basilar artery clot does not extend to the distal basilar artery.

TOP-OF-THE-BASILAR ARTERY EMBOLISM

Occlusion of the distal basilar artery is most often caused by embolism from the heart or the proximal vertebral artery system. Emboli small enough to pass through the vertebral arteries seldom lodge in the proximal basilar artery, a vessel larger than each intracranial vertebral artery, but reach the distal basilar artery or its terminal branches. The distal basilar artery supplies the midbrain and diencephalon through small vessels that pierce the posterior perforated substance. The findings in patients with top-of-the-basilar embolism include

1. Pupillary abnormalities. The lesion often interrupts the afferent reflex arc by interfering with fibers going toward the Edinger-Westphal nucleus. The third-nerve nucleus can also be involved, as well as the rostral descending sympathetic system. The pupils are usually abnormal and can be small, midposition, or dilated, depending on the level and extent of the lesion. Decreased pupillary reactivity and eccentricity or an oval shape of the pupil is also found.

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