The Neurosurgical Atlas by Aaron Cohen-Gadol, M.D. Brainstem AVMs Operative Anatomy The brainstem consists of the midbrain, pons, medulla, and superior, middle, and inferior cerebellar peduncles. It is separated from the cerebellum by the cerebellomesencephalic, cerebellopontine, and cerebellomedullary fissures. The posterior circulation includes the relevant vascular pedicles for brainstem arteriovenous malformations (AVMs). The basilar artery serves as the major vascular supply for the brainstem, with three main branches that include the superior cerebellar artery (SCA), the anterior inferior cerebellar artery (AICA), and the posterior inferior cerebellar artery (PICA). The SCA is comprised of four segments: 1. Anterior pontomesencephalic segment (S1): from the SCA origin to the anterolateral brainstem coursing beneath cranial nerve (CN) III 2. Lateral pontomesencephalic segment (S2): from the anterolateral brainstem caudally to the trigeminal root until it enters the cerebellomesencephalic fissure 3. Cerebellomesencephalic segment (S3): within the cerebellomesencephalic fissure and eventually accompanying CN IV through hairpin curves and reaching the tentorial edge 4. Cortical segment (S4): emerges from the cerebellomesencephalic fissure and supplies the tentorial surface of the cerebellum Figure 1: The relevant segmental anatomy of the SCA is shown (images courtesy of AL Rhoton, Jr). The AICA is also comprised of four segments: 1. Anterior pontine segment (A1): from the origin of the AICA, proximal to CN VI root exit zone, and ending inferiorly along the long axis of the inferior olive 2. Lateral pontine segment (A2): from the anterolateral margin of the pons through the cerebellopontine angle (CPA) up to the flocculus, with terminal branches including: the labyrinthine artery, recurrent perforating artery, and subarcuate artery 3. Flocculopeduncular segment (A3): from the flocculus superiorly to the cerebellopontine fissure, commonly with rostral and caudal trunks surrounding CNs VII/VIII 4. Cortical segment (A4): distal to the cerebellopontine fissure to supply the cerebellar petrosal surface Figure 2: The relevant segmental anatomy of the AICA is shown (images courtesy of AL Rhoton, Jr). The PICA is divided into five segments: 1. Anterior medullary segment (P1): the origin of the PICA courses anterior to the medulla and passes along the hypoglossal rootlets, terminating at the medial border of the inferior olive 2. Lateral medullary segment (P2): this short segment has its origin at the most prominent point of the inferior olive to the rootlets of CNs IX/X/XI at the lateral edge of the olive 3. Tonsillomedullary segment (P3): descends from the lateral edge of the olive to the inferior pole of the cerebellar tonsil and reverses rostrally along the medial tonsil to its midpoint (the infratonsillar/caudal loop) 4. Telovelotonsillar segment (P4): ascends from the midpoint of the cerebellar tonsil toward the roof of the fourth ventricle and turns caudally, coursing posteriorly toward the tonsillobiventral fissure (the supratonsillar/rostral loop). This branch perfuses the choroid plexus of the fourth ventricle and the inferior medullary velum. 5. Cortical segment (P5): its origin emerges from the tonsillobiventral fissure with two medial and lateral trunks for the vermis and tonsils/cerebellar hemisphere, respectively Figure 3: The relevant segmental anatomy of the PICA is shown (images courtesy of AL Rhoton, Jr). The segmental anatomy of the brainstem, with the corresponding cerebellar peduncle, major fissure, and arterial feeder, allows for a discussion of the anatomy based on three neurovascular complexes: 1. Upper complex: midbrain, cerebellomesencephalic fissure, superior cerebellar peduncle, tentorial cerebellar surface, the SCA, and CNs III, IV, and V 2. Middle complex: pons, cerebellopontine fissure, middle cerebellar peduncle, cerebellar petrosal surface, the AICA, and CNs VI, VII, and VIII 3. Inferior complex: medulla, cerebellomedullary fissure, inferior cerebellar peduncle, suboccipital cerebellar surface, the PICA, and CNs IX, X, XI, and XII These divisions also have evolutionary and developmental significance and relationships. Designation of brainstem veins is not as straightforward as the arterial anatomy because venous appellation is based on the direction of venous drainage (longitudinal or transverse) and the adjacent segment and surface of the brainstem. The longitudinal veins and the posterior communicating, tectal, and peduncular veins drain into the basal vein of Rosenthal en route to the vein of Galen. Anterior brainstem veins, which are most often transverse, drain into the superior petrosal and inferior petrosal veins en route to the petrosal sinuses. The complexity of the neurovascular anatomy of the brainstem demands that the surgeon has an excellent intraoperative anatomic awareness. The combination of precise knowledge of the anatomy, specific brainstem landmarks, and three-dimensional conceptualization of their relationship is the mainstay for any surgical approach to the brainstem, particularly for AVMs. The close proximity of the major motor, sensory, and autonomic tracts, in addition to the cranial nerves and their corresponding nuclei, highlights the importance of identifying the landmark anatomic points reliably. Following is a brief discussion of the pertinent landmarks for a rapid review: CN III exits from the anterior midline in the pontomesencephalic fissure and courses between the PCA and SCA. CN IV emerges from the dorsal brainstem surface in the cerebellomesencephalic fissure below the inferior colliculus, and wraps around the midbrain adjacent and inferior to the tentorial edge. CN V emerges from the posterolateral aspect of the pons, coursing anteriorly through the cerebellopontine angle cistern. CN VI exits anteriorly in the pontomesencephalic fissure rostral to the anterolateral sulcus near the origin of the AICA and courses in the prepontine cistern to the Dorello’s canal. CNs VII and VIII emerge posterolateral to the pontomesencephalic fissure and course laterally in the cerebellopontine angle cistern toward the internal acoustic meatus. CNs IX, X, and XI arise from the posterolateral sulcus of the medulla and course laterally to the jugular foramen within the cerebellomedullary cistern. CN XII emerges from the anterolateral sulcus of the medulla into the premedullary cistern and courses laterally within the cerebellomedullary cistern and the hypoglossal canal. The fourth ventricular floor has a configuration that resembles a rhomboid containing pontine, junctional, and medullary regions. These regions are detailed below. Figure 4: The surface anatomy of the fourth ventricle and its landmarks are shown. Please refer to the text for more details. The fourth ventricular floor contains the following anatomic landmarks: Pontine part is triangular, with the apex at the cerebral aqueduct. Its lateral limbs are the medial borders of the superior cerebellar peduncles, and the base is an imaginary line connecting the lower margin of the cerebellar peduncles. Junctional part is the strip between the lower margin of the cerebellar peduncles and the tela choroidea attachment to the teniae just below the lateral recess. Medullary part is also triangular shaped and is limited laterally by the teniae and the obex at the caudal tip, anterior to the foramen of Magendie. Median sulcus divides the fourth ventricular floor into rostral-to- caudal symmetrical halves. Sulcus limitans is another parallel longitudinal sulcus lateral to the median sulcus bilaterally. Median eminence is the raised strip along the fourth ventricular floor between the median sulcus and sulcus limitans containing the facial colliculus (a rounded prominence overlies the nucleus of the abducens nerve and the ascending section of the root of the facial nerve). Three triangular areas overlie the hypoglossal nucleus, vagus nucleus, and area postrema. Locus ceruleus is a bluish gray area (pigmented due to melanin granules) located at the rostral end of each sulcus limitans in the lateral margin of the fourth ventricular floor. Vestibular area is located lateral to the sulcus limitans along the fourth ventricular floor, and just deep to this region is the vestibular nuclei. Auditory tubercle is a prominence in the lateral part of the vestibular area formed by the underlying dorsal cochlear nucleus and the cochlear part of the vestibulocochlear nerve. Figure 5: The basic anatomy of the brainstem and associated vasculature is shown (image courtesy of AL Rhoton, Jr). BRAINSTEM ARTERIOVENOUS MALFORMATION SUBTYPES Brainstem AVMs are rare and comprise approximately 5% of all brain AVMs. According to the location and eloquence of the brainstem, all brainstem AVMs are associated with moderate to high surgical risk. The intrinsically exquisite eloquence of the brainstem demands no transgression of the brainstem parenchyma or pia during resection of the lesion. The pial dissection may be strictly limited by “in situ” occlusion. The arterial feeders are reliably recognized and disconnected, after which the draining vein turns dark blue. Subsequently, the vein(s) is disconnected, but the nidus is left in the brainstem and is considered nonfunctional. Some surgeons advocate the “pial resection technique.” In other words, only the extraparenchymal portion of the AVM and its associated feeders and draining veins are resected, and the intrinsic portions are left untouched. Brainstem AVMs may have epipial, pial, or subpial (parenchymal)