Vascular Supply Of The Midbrain

The blood supply to the midbrain is via branches of the basilar artery, which bifurcates at the level of the third cranial nerves into a left and right posterior cerebral artery (PCA). The initial portion of the PCA, the P1 segment, is proximal to the posterior communicating artery, whereas the P2 segment is distal (Fig. 9G). In as many as 10% of patients, a fetal circulation persists and the basilar communicating segment (i.e., the mesencephalic artery) is hypoplastic. In this situation the PCA is derived primarily from the internal carotid artery via a large posterior communicating artery.

In the caudal midbrain (Fig. 9), the blood supply to the ventral portion is from the basilar artery, whereas the dorsolateral midbrain is perfused via the superior cerebellar artery. At more rostral levels, the dorsal midbrain is supplied by the basilar artery and direct branches from the proximal PCAs. The vessels arising from the initial portion of the PCAs can be variable but include paramedian mesencephalic arteries and peduncular penetrating branches, which arise from the PCAs after the posterior communicating arteries join to form the P2 segment of the PCAs (see Figs. 9B and 9C).

Midbrain infarction may accompany thalamic infarction. Branches to the mesencephalon may arise either directly from the basilar artery or as a common trunk with the thalamic paramedian vessels. It is the occlusion of these paramedian thalamic-subthalamic perforating vessels that can often infarct both rostral interstitial nuclei of the medial longitudinal fasciculus and produce a complete paralysis of both upward and downward vertical and torsional eye movements. The thalamogeniculate arteries originate further posterior along the PCAs just at or after the connection with the posterior communicating artery, and they supply the posterior lateral thalamus, including the lateral geni-culate, the pulvinar, portions of the hippocampal gyrus, the dentate fascia, and the dorsolateral nuclei of the thalamus. Infarctions of the midbrain are distinguished from the meso-diencephalic junction by third nerve involvement.

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