A, Both SCAs arise as duplicate arteries at the midbrain level and accompany the basal vein around the brainstem to enter the cerebellomesencephalic fissure. They pass below the oculomotor and trochlear nerves and above the trigeminal nerves. The SCA trunks are intertwined with the trochlear nerve on the posterolateral brainstem. B, The level of the brainstem section has been extended downward to the pons. The rostral and caudal trunks of the duplicate SCAs arise directly from the side of the basilar artery and pass laterally above the trigeminal nerve. C, The brainstem section has been extended downward to the midpons. The trigeminal, oculomotor, and trochlear nerves have been divided so that the brainstem could be reflected backward to expose the AICA and the facial and vestibulocochlear nerves. Both AICAs pass below the abducens nerves and loop laterally toward the internal acoustic meatus. The left PICA loops upward in front of the pons between the facial and vestibulocochlear nerves and the AICA before turning downward to encircle the medulla. D, Enlarged view. The right AICA loops laterally into the porus of the internal acoustic meatus, as occurs in approximately half of cases. The AICA has a premeatal segment that passes toward the meatus, a meatal segment that loops into the porus in about half of cerebellopontine angles, and a postmeatal segment that loops back to the brainstem. The vestibulocochlear nerve has been retracted to expose the nervus intermedius, which arises at the brainstem along the anterior surface of the vestibulocochlear nerve, has a free segment in the cerebellopontine angle, and joins the facial nerve as it proceeds laterally toward the meatus. The AICA gives rise to a recurrent perforating branch to the brainstem. (Images courtesy of AL Rhoton, Jr.)