Deep Bypasses to the Distal Posterior Circulation

Abstract

The subtemporal approach for a superficial temporal artery-to-superior cerebellar artery bypass requires significant superior retraction that can injure the temporal lobe, compromise veins, and cause edema postoperatively. In contrast, the pretemporal approach requires posterolateral retraction that seems to be less injurious to the temporal lobe and better tolerated clinically. We hypothesized that the pretemporal approach provides ample exposure, more gentle retraction, and better clinical results than the subtemporal approach. Standard orbitozygomatic-pterional and subtemporal approaches were performed on both sides of 4 formalin-fixed cadaver heads for morphometric measurements. Temporal lobe retraction was quantified for each approach in terms of brain shift and retraction pressure by using both sides of 3 fresh, unfixed cadaver heads. Similar morphometric measurements were made in 14 patients in whom bypasses to the distal posterior circulation were performed. The effect of temporal lobe retraction was assessed with edema volumes on postoperative computed tomography scans. In cadaver heads and in patients, the pretemporal approach optimized exposure of the P2A segment of the posterior cerebral artery (PCA) and the subtemporal approach optimized exposure of the lateral pontomesencephalic segment of the superior cerebellar artery (SCA). Working depths and lengths of exposed artery were similar with these 2 approaches, but the PCA was a larger recipient than the SCA. Brain shift was 42% less with pretemporal than with subtemporal retraction, and retraction pressure was 43% less with pretemporal than with subtemporal retraction. The volume of temporal lobe edema was 56% less in patients with bypasses performed with the pretemporal approach as compared with the subtemporal approach. Pretemporal exposure of the PCA is equivalent to subtemporal exposure of the SCA, but the pretemporal approach is facilitated by a larger recipient artery. Posterolateral temporal lobe retraction associated with the pretemporal approach is gentler than superior retraction with the subtemporal approach. These results validate our preference for the pretemporal approach over the subtemporal approach when performing deep bypasses to the posterior circulation.