Retrosigmoid (Suboccipital)

The retrosigmoid approach is a lateral suboccipital craniotomy immediately behind the sigmoid sinus and below the transverse sinus. It opens the cerebellopontine angle from the tentorium to the foramen magnum and allows drilling of the posterior wall of the internal auditory canal. It is the standard route for vestibular schwannomas of all sizes when hearing preservation is possible, for CPA meningiomas and epidermoids, and for microvascular decompression in trigeminal neuralgia and hemifacial spasm^[1,2,3].

Rhoton described the CPA in terms of three neurovascular complexes: an upper complex (superior cerebellar artery, midbrain, trigeminal nerve and tentorial surface of the cerebellum), a middle complex (anterior inferior cerebellar artery, pons, facial and vestibulocochlear nerves and the petrosal surface) and a lower complex (posterior inferior cerebellar artery, medulla and the glossopharyngeal, vagus, accessory and hypoglossal nerves)^[1]. The asterion overlies the transverse–sigmoid junction in most patients, but imaging-based navigation is more reliable. The petrosal vein (superior petrosal venous complex) drains into the superior petrosal sinus and lies in the path to the trigeminal nerve.

• Park-bench or lateral position, with the head flexed and rotated so the mastoid is the highest point; avoid compressing the jugular veins.
• Curvilinear incision behind the mastoid; expose the asterion and the mastoid emissary vein.
• Craniotomy exposing the edges of the transverse and sigmoid sinuses; wax any opened mastoid air cells.
• Open the dura in a C shape based on the sigmoid sinus.
• Release CSF from the cerebellomedullary or lateral cerebellopontine cistern to relax the cerebellum before any retraction.
• For tumours, debulk internally, then dissect the capsule from the nerves; drill the posterior wall of the IAC when the tumour extends into it.

Facial-nerve electromyography is standard in CPA surgery; brainstem auditory evoked potentials and direct cochlear nerve action potentials are used when hearing is to be preserved, and lower cranial-nerve monitoring is used for lesions extending to the jugular foramen. With modern monitoring, anatomical preservation of the facial nerve is achieved in the great majority of vestibular schwannoma resections^[2].

• CSF leak through mastoid air cells or the IAC: wax the bone carefully and close the dura watertight.
• Facial and cochlear nerve injury: early identification and continuous monitoring.
• Cerebellar contusion or swelling: release CSF and avoid fixed retraction.
• Venous sinus injury or thrombosis.
• Postoperative headache, which is reduced by replacing the bone flap rather than leaving a craniectomy.