Schwannoma

Schwannomas are benign, encapsulated tumours of Schwann cells that grow eccentrically from a nerve, displacing rather than infiltrating its fascicles. Vestibular schwannomas (acoustic neuromas), arising from the vestibular nerves in the internal auditory canal (IAC), account for most intracranial schwannomas; trigeminal, facial, jugular-foramen and hypoglossal schwannomas are much less common^[1]. Bilateral vestibular schwannomas are diagnostic of NF2-related schwannomatosis.

In the Samii series of 1000 vestibular schwannomas, the presenting symptoms were hearing loss in nearly all patients, followed by tinnitus, vestibular symptoms, and, with larger tumours, trigeminal symptoms, facial weakness, ataxia and hydrocephalus^[2]. Sudden sensorineural hearing loss can be the first sign. Trigeminal schwannomas present with facial numbness or pain; jugular-foramen schwannomas with hearing loss or lower cranial-nerve deficits.

Contrast-enhanced MRI shows an enhancing mass centred on the IAC, with an “ice-cream cone” shape when it extends into the CPA; it forms an acute angle with the petrous bone and widens the IAC, distinguishing it from meningioma. Cystic change is common in large tumours. Audiometry (pure-tone and speech discrimination) is essential to counsel about hearing preservation. Tumours are often graded by the Koos classification (1 intracanalicular; 2 CPA without brainstem contact; 3 brainstem contact; 4 brainstem compression).

• Observation with serial MRI: for small, non-growing tumours, especially in older patients; many tumours do not grow.
• Stereotactic radiosurgery: for small to medium tumours (usually under about 3 cm) that grow, with high tumour control.
• Microsurgery: for large tumours with brainstem compression, cystic tumours, younger patients, and when tumour removal is the goal.

With modern microsurgery and continuous facial-nerve monitoring, the facial nerve is anatomically preserved in the large majority of patients, and the rate of good facial function is inversely related to tumour size^[3]. Facial function is reported on the House–Brackmann scale from grade I (normal) to VI (total paralysis)^[4]. A planned near-total or subtotal resection, with radiosurgery for growing residual tumour, improves facial-nerve outcomes in large tumours.

• Retrosigmoid: the most versatile; suits tumours of any size and allows hearing preservation^[5].
• Translabyrinthine: for large tumours or when hearing is already poor; identifies the facial nerve at the fundus without cerebellar retraction^[6].
• Middle fossa: for small intracanalicular tumours with good hearing^[7].

• Use facial-nerve electromyography and stimulation throughout, and brainstem auditory evoked potentials or direct cochlear nerve monitoring when hearing is preserved.
• Debulk the tumour internally before dissecting the capsule; stay in the arachnoid plane.
• Identify the facial nerve both at the brainstem and at the fundus of the IAC, and dissect toward the area where the nerve is most splayed, usually the porus.
• Drill the posterior wall of the IAC without opening the labyrinth when hearing is to be preserved; seal opened air cells with bone wax to prevent CSF leak.

Tumour control after gross-total resection is very high. Complications include facial weakness, hearing loss, CSF leak, headache, and, rarely, cerebellar or brainstem injury. Hearing preservation is possible in selected small tumours but declines over time with all treatments.