Surgical Anatomy of Cerebellopontine Angle1

The cerebellopontine angle (CPA) is the angular space located between the superior and inferior limbs of the cerebellopontine fissure formed by the petrosal cerebellar surface folding around the pons and middle cerebellar peduncle. The cerebellopontine fissure is open medially and has superior and inferior limbs that meet at a lateral apex. Broadly, however, the CPA is constituted by the brainstem, cerebellar peduncles, fissures between the cerebellum and brainstem, cerebellar surfaces, and is traversed by various cranial nerves (CN IV to XII) and the cerebellar arteries (Fig. 1.1). It is essential to comprehend the relationships of various structures that inhabit the CPA and their individual variations. It is not the purpose of this book to present the detailed review of the surgical anatomy of the CPA but we present a brief introduction required for surgical procedures and suggest excellent literature by various stalwarts for further reading (prominent among them being wonderfully illustrated articles from Rhoton et al.).^1–8

To better understand the intricacies of the posterior cranial fossa, it is divided into three zones (Figs 1.2 and 1.3) based on the relationship of the three cerebellar arteries to its adjacent structures: superior zone includes the superior cerebellar artery (SCA) and its relations, middle zone includes the anteroinferior cerebellar artery (AICA) and its relations, and inferior zone includes the posteroinferior cerebellar artery (PICA) and its relations. Each of the three zones has its anatomical landmarks in the form of an artery (one of SCA, AICA or PICA), part of the brainstem (either midbrain, pons or medulla), the cerebellar surface (either tentorial, petrosal or suboccipital), the fissures between the brainstem and the cerebellum (cerebellomesencephalic, cerebellopontine or cerebellomedullary), cerebellar peduncles that connect the brainstem to the cerebellum (superior, middle or inferior) and associated cranial nerves (IV to XII).

The superior zone includes the SCA, midbrain, cerebellomesencephalic fissure, superior cerebellar peduncle, tentorial surface of the cerebellum, and the oculomotor, trochlear and trigeminal nerves. The SCA arises from the basilar artery in front of the midbrain, passes below the oculomotor and trochlear nerves and above the trigeminal nerve to reach the cerebellomesencephalic fissure, where it runs on the superior cerebellar peduncle and terminates by supplying the tentorial surface of the cerebellum. The SCA divides at a variable level of its path between the basilar artery and cerebellomesencephalic fissure into two branches, rostral and caudal. The trigeminal root joins the brainstem about halfway between the lower and upper borders of the pons. In its intradural course, the trigeminal nerve uniformly runs obliquely upwards from the lateral part of the pons toward the petrous apex. It exits the posterior fossa to enter the middle cranial fossa by passing forward beneath the tentorial attachment to enter the Meckel's cave. This nerve has two roots: the wide sensory root or “portio major” located laterally and the 3narrow motor root or “portio minor” located at the medial edge of the sensory root. The superior petrosal vein or Dandy's vein is formed by the anastomosis of various veins draining the cerebellum and brainstem. It drains into the superior petrosal sinus posterior to the trigeminal nerve as one or more venous pedicles. The superior petrosal vein is sometimes a barrier to accessing the trigeminal nerve during surgery for trigeminal neuralgia (Fig. 1.4).

The middle zone includes the AICA, pons, middle cerebellar peduncle, cerebellopontine fissure, petrosal surface of the cerebellum, and the abducens, facial and vestibulocochlear nerves. The AICA arises at the pontine level, usually from the basilar artery, occasionally from the vertebral artery, courses in relation to the abducens, facial and vestibulocochlear nerves to reach the surface of the middle cerebellar peduncle, where it courses along the cerebellopontine fissure and terminates by supplying the petrosal surface of the cerebellum. In 40% of cases, the AICA forms a vascular loop running toward the porus acousticus. The AICA usually passes inferior to the VII–VIII complex, but it can also run superior to them or between the VII and VIII. The branches of the AICA near the VII–VIII complex are the labyrinthine artery, perforating arteries to the brainstem and “subarcuate” artery.

The anteriorly placed facial nerve (VII) is hidden by the vestibulocochlear nerve in the posterior approaches to the CPA. The vestibulocochlear nerve is separated by a furrow, the cleavage plane between two components first described by Bremond. Between the facial nerve and the auditory nerve travels the nerve of Wrisberg (nervus intermedius). The three nerves are included in the same arachnoid cover. The cisternal route has a length of 8–12 mm. The flocculus is the lobe of the cerebellum that hides the zone of emergence of the acousticofacial nerve bundle during the posterior surgical approach.

The lower zone includes the PICA, medulla, inferior cerebellar peduncle, cerebellomedullary fissure, suboccipital surface of the cerebellum, and the glossopharyngeal, vagus, spinal accessory and hypoglossal nerves. The PICA arises from the vertebral artery at the medullary level, encircles the medulla, passing in relation to the glossopharyngeal, vagus, accessory and hypoglossal nerves to reach the surface of the inferior cerebellar peduncle, where it dips into the cerebellomedullary fissure and terminates by supplying the suboccipital surface of the cerebellum. The “glossopharyngeal nerve” arises as one or rarely two rootlets from the upper medulla, posterior to the upper one-third of the olive, just caudal to the origin of the facial nerve. It courses ventral to the choroid plexus protruding from the foramen of Luschka on its way to the jugular foramen. The “vagus nerve” arises inferior to the glossopharyngeal nerve as a line of tightly packed rootlets along a line 2–5.5 mm in length posterior to the superior one-third of the olive. The vagus is composed of multiple combinations of large and small rootlets that pass ventral to the choroid plexus protruding from the foramen Luschka on its way to the anteromedial part of the pars venosa. The “spinal accessory nerve” arises as a widely separated series of rootlets that originate from the medulla at the level of the lower two-thirds of olive and from the upper cervical cord. The cranial rootlets of the spinal accessory nerve arise as a line of rootlets just caudal to the vagal fibers. The spinal accessory fibers pass superolaterally from their origin to reach the jugular foramen.

The 30° angled endoscope is a valuable and indispensable optical device to visualize the CPA and it permits us to observe the blind spots in the CPA by “looking around the corner” giving a panoramic view of its contents (Figs 1.5 and 1.6). Through retrosigmoid approach using the endoscope, the acousticofacial nerve bundle is the reference level, crossing the middle of the CPA, giving distinct separate anatomical endoscopic surgically assisted areas (Figs 1.7 to 1.12). O'Donoghue and O'Flynn⁷ divided the CPA area into four levels on the basis of neuroendoscopic inspection.5

Level 1 contains the trigeminal and abducens nerves, Meckel's cave, Dandy's vein and the SCA (Fig. 1.8).

Level 2 contains the acousticofacial bundle traversing the CPA from the pontomedullary sulcus to the ICA, the length varying between 10 and 15 mm (average 13 mm). The facial nerve can be seen entering the anterosuperior compartment of the IAM as it passes laterally (Fig. 1.7). The facial nerve is separated laterally from the superior vestibular nerve in the fundus of the IAM and the AICA is entwined within the acousticofacial bundle. The AICA arises at the pontine level, usually from the basilar artery, occasionally from vertebral artery, courses in relation to the facial and vestibulocochlear nerves to reach the petrosal surface of the cerebellum (Fig. 1.10). In 40% of cases, the AICA forms a vascular loop running toward the porus acousticus. The AICA usually passes inferior to the VII–VIII complex, but it can also run superior to them or between the VII and VIII (Fig. 1.11).

The dominant anatomic features at this level are the lower cranial nerves as they pass laterally from the medulla to the jugular foramen (Fig. 1.9). The lower cranial nerves along with root entry/exit zone of the facial and vestibulocochlear nerves are intimately related to the loop of the PICA (Fig. 1.12).

An inferior extension of the CPA in which the lower medulla and spinal cord are readily apparent. The accessory nerve exits through the foramen magnum and the rootlets of the hypoglossal nerve run horizontally to gain entry to the hypoglossal foramen.6

The PICA can be seen intimately related to the rootlets of the lower cranial nerves.

The internal auditory meatus (IAM) is a bony canal that forms a transition zone before the vestibulocochlear nerve enters the respective inner ear structures and transmits the facial nerve from CPA through the temporal bone to the face (Figs 1.13 to 1.17).7

It also carries the labyrinthine branch of AICA that acts as an end artery for cochlea and 8labyrinth. Its medial end toward the CPA is known as porus acousticus and is located on the posterior surface of the temporal bone. Its lateral end is known as fundus and it forms the medial end of bony labyrinth. The neurovascular contents of IAM are surrounded by the dura mater and arachnoid wrapping up to the fundus. The falciform (transverse) crest, a transverse ridge of bone, divides the lamina cribrosa into upper and lower compartments and provides an attachment site for the dura. The upper compartment is divided into anterior (containing the facial nerve) and posterior (containing the superior vestibular nerve) quadrants by the vertical crest (also known as Bill's bar). In the lower compartment, the cochlear nerve courses in the anterior quadrant, while the inferior vestibular nerve lies in the posterior quadrant. The average width of IAM is less than 4 mm (around 3.65 mm) and the average length of the IAM is around 8 mm with minor individual variations (Fig. 1.17). These dimensions are important when drilling the IAM blindly through the retrosigmoid approach as there are no landmarks to guide the drilling to avoid injury to bony labyrinth. The posterior semicircular canal and crus commune are intimately related to the IAM when drilling it through the retrosigmoid approach.