Schwannomas account for 6% to 8% of all primary brain tumors, with the majority occurring in adulthood (Newton 1994; Propp et al 2006). The great majority of schwannomas are sporadic and unrelated to either neurofibromatosis type 1 or 2. Over 85% to 90% of intracranial schwannomas affect the vestibular portion of cranial nerve VIII in the cerebellopontine angle (Miller 1988; Jackler and Pitts 1990; Macfarlane and King 1995). Recent estimates suggest an incidence of vestibular schwannomas of 0.6 to 0.8 per 100,000 person-years (Propp et al 2006). Overall, the incidence appears to be increasing in recent decades. The second most common site for these tumors is the trigeminal nerve, which is affected by 0.8% to 8% of all intracranial schwannomas (McCormick et al 1988; Miller 1988; Samii et al 1995; Strauss and Post 1995). Tumors of the facial nerve are rare, accounting for 0.5% to 1.9% of intracranial schwannomas (Miller 1988; Rocchi et al 1991; Strauss and Post 1995). Although extremely rare, sporadic schwannomas can develop from cranial nerves III, IV, VI, IX, X, XI, and XII (Miller 1988; Mehta et al 1990; Sweasey et al 1991; Tung et al 1991; Jackowski et al 1994; Strauss and Post 1995; Sarma et al 2002). In many clinical series spinal schwannomas and neurofibromas are combined; the exact incidence of sporadic spinal schwannomas is unclear (Halliday et al 1991). Estimates of their incidence range from 10% to 30% of all primary spinal neoplasms (Halliday et al 1991; Newton et al 1995; Seppala et al 1995b).
Sporadic neurofibromas of the intracranial cavity typically arise from cranial nerves, and are extremely rare in patients without neurofibromatosis type 1 or 2. The vast majority of these tumors develop from spinal nerves, nerve roots, or peripheral nerves (Seppala et al 1995a). Exact incidence figures are unavailable for sporadic neurofibromas in these locations.