Cerebellopontine angle

Surgical surface anatomy

The relationship between the cranial sutures and venous sinuses can be used in surgical planning. Surface relationships have been described to identify the junction of the transverse and sigmoid sinuses. A line is drawn from the root of the zygoma to the inion: this line defines the transverse sinus. A vertical line drawn from the mastoid groove to meet this line identifies the point overlying the junction between the transverse and sigmoid sinuses, as the sigmoid sinus then curves caudally ( Fig. 22.1 ). A burr-hole placed just inferior and posterior to this point should expose the sinus junction safely while minimizing the risk of damage to the sinuses. The asterion corresponds to the meeting point of the lambdoid (occipitoparietal), occipitomastoid and parietomastoid sutures ( Ch. 14 ). This craniometric point has often been used to identify the junction of the transverse and sigmoid sinuses, which is the key to this approach. However, this point may be difficult to distinguish in some adults and has a variable relationship to the sinuses.

Clinical anatomy

The standard position in which patients are placed for access to the CPA is the lateral decubitus position, with the head secured with three-point fixation, although some surgeons use supine variations. In this position, the neck is flexed and rotated approximately 10° away from the affected side. For decompression of the trigeminal nerve, the vertex is kept parallel with the floor; for an approach to the facial or lower cranial nerves, the vertex is tilted down 10–15° to improve exposure of the proximal aspect of the facial nerve. The shoulder is taped down and out of the way but care should be taken not to apply excessive force to avoid stretching and damaging the brachial plexus. These manœuvres allow optimum positioning for surgical access.

The above landmarks are identified and a curved incision, with the concave side facing the ear, is made, one-third above and two-thirds below the junction of the transverse and sigmoid sinuses. There has been a recent trend to perform a craniotomy rather than a craniectomy to expose the dura of the posterior fossa. This is due to a perceived difference in the rate of postoperative cerebrospinal fluid (CSF) leak and morbidity; however, there may be an increased risk of damage to the sinuses. With either technique, it is essential to identify the junction of the transverse and sigmoid sinuses to allow adequate surgical exposure. However, a large exposure is not required and a 2 cm craniotomy is typically sufficient. Bleeding may be encountered from the mastoid emissary vein. It is important to note that this is a poor landmark for the sigmoid sinus due to its variable location and course. Care should be taken to wax any bleeding thoroughly from the bone, and particularly from the mastoid air cells, should they be entered. An incision is made in the dura mater and extended. The dura adjacent to the junct­ion of the sigmoid and transverse sinuses must be clearly identified.

CSF should be drained to reduce the pressure in the posterior fossa and relax the cerebellum to help minimize retraction injury. This is best achieved by opening the arachnoid mater caudally under the microscope with the aid of gentle retraction to enter the cisterna magna. Time spent draining CSF at this juncture will be rewarded with much-improved exposure. A lumbar drain is not necessary. Opening of the cerebellopontine and cerebellomedullary cisterns enables identification of neurovascular structures.

Once CSF drainage has been undertaken, the superolateral angle of the posterior fossa can now be entered. It is essential for the angle between the tentorium and the dura overlying the petrous temporal bone to be visualized ( Fig. 22.2 ). A retractor is first placed horizontally over the cortical surface to identify the junction before gradually being positioned more vertically with the aid of the microscope to achieve deeper exposure. Bridging veins may be present and care should be taken. Placing a retractor at the angle or more superiorly, and slowly moving more laterally rather than directly retracting laterally, reduces the risk of injury to the facial and vestibulocochlear nerve complex, which is very susceptible to retraction injury.

The superior petrosal venous complex is usually encountered (see Fig. 22.2 ). This is the main draining vein of the anterolateral posterior fossa structures. Care should be taken not to place this vein under tension, as avulsion from the superior petrosal sinus may result. It may be necessary to sacrifice this vein to gain adequate exposure, particularly of the trigeminal nerve and the superior cerebellar artery. Venous infarction is a rare occurrence. Once adequate retraction has been achieved, the facial and vestibulocochlear nerves are usually seen first, with the trigeminal nerve located anteriorly and rostrally.

It is important to recognize the relationships of the cranial nerves to the arteries in the cerebellopontine region. These will be considered in three neurovascular complexes. The superior neurovascular complex is related to the superior cerebellar artery (SCA) and the trigeminal nerve. The middle neurovascular complex is related to the anterior inferior cerebellar artery (AICA) and the facial and vestibulocochlear nerves. The inferior neurovascular complex is related to the posterior inferior cerebellar artery (PICA) and the glossopharyngeal, vagus, spinal access­ory and hypoglossal nerves.

Each is associated with a separate neurovascular compression syndrome, which can be treated by microvascular decompression. The most common of these is trigeminal neuralgia, followed by hemifacial spasm and glossopharyngeal neuralgia.