A, The main trunk of the SCA bifurcates above the trigeminal nerve into a rostral and caudal trunk. The main trunk passes below the trochlear nerve and tentorial edge at the anterolateral brainstem, but distally the rostral trunk passes above and the caudal trunk below the trochlear nerve and tentorial edge. B, View after removing the tentorial edge. The most common compression of the trigeminal nerve in trigeminal neuralgia is by the SCA at the junction of the main with the rostral and caudal trunks, which in this case is located above the trigeminal nerve. Both trunks dip into the cerebellomesencephalic fissure before reaching the tentorial surface. C, This superior petrosal vein has multiple tributaries that have become entwined with the branches of the SCA. These veins often need to be coagulated and divided in reaching the trigeminal nerve. The SCA could be obliterated in coagulating the tributaries of the superior petrosal vein unless care is taken to carefully separate the arterial trunks from the venous tributaries. D, This SCA has a duplicate origin in which both the rostral and caudal trunks arise directly from the basilar artery. Both trunks, at the anterolateral brainstem, pass below the tentorial edge and trochlear nerve and above the trigeminal nerve. At the posterolateral margin of the brainstem, the rostral trunk loops above the level of the trochlear nerve and tentorial edge. The caudal trunk rests against the posterior trigeminal root as the nerve passes below the anterior edge of the tentorium to enter Meckel’s cave. E, Another SCA. The main trunk passes above the trigeminal nerve before bifurcating into rostral and caudal trunks. The main trunk courses below the trochlear nerve, but the rostral trunk loops upward medial to the nerve. The caudal trunk divides into a large hemispheric branch that supplies the tentorial surface and a marginal branch, which supplies some of the upper part of the petrosal surface. F, Another SCA. The artery bifurcates below the oculomotor nerve. Both trunks pass below the trochlear nerve at the anterolateral margin of the brainstem and above the trochlear nerve distally at the entrance into the cerebellomesencephalic fissure. (Images courtesy of AL Rhoton, Jr.)