The patient may be placed in semi-sitting position (preference at University of Tübingen) (Fig. 9.2), dorsal decubitus or “mastoid position” (preference at the Neurological Institute of Curitiba) (Fig. 9.3), or “park-bench position .” The anesthesiologist should check the lateral neck on both sides for jugular vein compression to avoid air embolism. In dorsal decubitus the head is rotated about 30° to the opposite side with light lateral extension. A pillow is placed under the shoulder to avoid excessive rotation of the head and compression of vertebral artery at the cranio-cervical junction. The opposite IJV must be checked to assure that it is not compressed (Fig. 9.4).

In semi-sitting position the legs are placed above the level of the heart with the knees slightly flexed (Fig. 9.5). The head is rotated 30° toward the affected side, hyperextended, and flexed. Bilateral SEPs are continuously monitored during positioning. In order to check the occurrence of air embolus, transesophageal echocardiography or precordial doppler is used. Neurophysiological monitoring of cranial nerves V, VI, VII, VIII, IX, X, XI, and XII is performed (Figs. 9.6, 9.7, and 9.8).

Skin incision (straight or slightly curved) starts in the retromastoid region about 5 cm behind the external auditory canal and extends 2 cm behind the mastoid tip, ending in the upper neck (Fig. 9.9). The inferior extension of the skin incision depends on the tumor extension. Scalp is elevated with the periosteum. Fascia and muscles are cut straight down exposing the occipital bone, the asterium, and the retromastoid region and held with a self-retaining retractor (Fig. 9.10). With the aid of neuronavigation a burr hole is performed at the transversus and sigmoid sinuses junction (usually at the asterion ) (Fig. 9.11). A 4 cm diameter craniotomy is cut (Fig. 9.12). With a high-speed diamond drill the transverse and sigmoid sinuses are exposed. Part of squamous occipital bone is removed with rongeurs until the jugular bulb. Craniectomy is also an option in cases of dura adherence to the occipital bone (elderly patients). The mastoid emissary vein is coagulated and cut. In semi-sitting position care should be taken to avoid air embolism when the sinuses and the emissary veins are exposed. Jugular vein compression in the neck or Valsalva maneuver helps to identify venous bleeding. Small sigmoid sinus lacerations are repaired either with sutures or small pieces of muscle fascia and fibrin glue (Fig. 9.13). Care should be taken to not occlude the sinus by packing lacerations with hemostatic material or muscle. The opened mastoid cells are packed with bone wax or muscle pieces and fibrin glue. Mannitol is administered intravenously before opening of the dura mater. Under the magnification of the operation microscope, the dura mater is incised in a C-shaped fashion, parallel to sigmoid sinus (Fig. 9.14). The lateral aspect of cerebellum is slightly retracted and the cerebellomedullary cistern is opened relaxing the posterior fossa. This maneuver is carried out with cotton strips or a rubber strip to protect the cerebellum against lacerations. Brain retractor is used to protect rather than retract the cerebellum. The lower cranial nerves are mostly involved or infiltrated by the tumor. The VI, VII, and VIII cranial nerves, the PICA, the vertebral artery, and the brainstem are usually attached to the tumor capsule (Fig. 9.15). In cases of meningiomas, paragangliomas, and aggressive tumors, the cranial nerves IX, X, and XI may be embedded or even infiltrated. The posterior wall or the jugular foramen is exposed (Fig. 9.16). A C-shaped dura incision based on the posterior lip of the jugular foramen is carried out and the foramen is opened with high-speed diamond drill (Fig. 9.17). Very careful dissection of the nerves within the jugular foramen is performed, under monitoring, to avoid damage to these structures and to the jugular bulb. The use of endoscope is helpful to dissect the tumor from cranial nerves within the jugular foramen (Fig. 9.18). If these nerves are infiltrated and the patient preoperatively has no deficit, radical resection is not attempted. In these cases, benign lesions are treated postoperatively with radiosurgery only if growth of the tumor rest is observed. Solid tumors are initially intracapsular enucleated. Gently dissection of the tumor capsule shows the arachnoid interface between the tumor and the surrounding structures. After coagulation of the dural attachments, intracapsular piecemeal resection is carried out. In contrast to solid tumors, cystic schwannomas have strong adherence to cranial nerves and the arachnoid interface should be first identified and dissected, avoiding opening the cyst [9–11]. Meticulous sharp dissection of the arachnoid plane followed by evacuation of the cyst will permit removal of the cyst wall from the surrounding structures (Fig. 9.19). After tumor removal, watertight continuous suture of dura mater is performed. The bone flap is replaced and fixed with mini-plates or cranioplasty with methyl methacrylate is performed. The wound is sutured in usual fashion and no drain is used.