In this group of tumors, symptoms and signs of general cerebral impairment and increased pressure occur late or not at all. Instead, special syndromes referable to particular intracranial loci arise and progress slowly. One arrives at the correct diagnosis by localizing the lesion accurately from the neurologic findings and by reasoning that the etiology must be neoplastic because of an afebrile and steadily progressive nature. Investigation by CT, MRI, and other special studies will usually confirm the clinical impression.
The tumors that most often produce these unique intracranial syndromes are the ones listed above as well as a number of other erosive tumors at the base of the skull. The aforementioned medulloblastoma, hemangioblastoma, and ependymoma of the fourth ventricle may have a similar regional clinical signature.
This tumor was first described as a pathologic entity by Sandifort in 1777, diagnosed clinically by Oppenheim in 1890, and recognized as a surgically treatable disease in the early 1900s. Cushing’s monograph (1917) was a milestone, and the papers of House and Hitselberger and of Ojemann and colleagues provide valuable descriptions of from the era before modern imaging.
Approximately 3,000 new cases of acoustic neuroma are diagnosed each year in the United States (incidence rate of 1 per 100,000 per year). The tumor occurs occasionally as part of von Recklinghausen neurofibromatosis, in which case it takes one of two forms. In classic von Recklinghausen disease (peripheral or type 1 neurofibromatosis), a schwannoma may sporadically involve the eighth nerve, usually in adult life, but it may involve any other cranial (particularly trigeminal) or spinal nerve root. Rarely, if ever, do bilateral acoustic neuromas occur in this form of the disease. However, bilateral acoustic neuromas are the hallmark of the genetically distinct neurofibromatosis type 2 (NF2), in which they practically always occur before the age of 21 and show a strong (autosomal dominant) heredity (Fig. 30-19). Schwannomas are distinguished from neurofibromas (composed of both Schwann cells and fibroblasts) found in peripheral nerves of type 1 von Recklinghausen disease. A small percentage of neurofibromas become malignant, a phenomenon that is highly unusual in schwannomas.
In this context, a rare form of familial schwannomatosis should be mentioned, characterized by multiple schwannomas without vestibular tumors, which maps genetically to chromosome 22 but is distinct from NF2. The primary gene defect in this familial schwannomatosis has not been defined (MacCollin et al), although mutations in the SMARCB1 gene on chromosome 22 locus have been identified in some patients.
The typical vestibular schwannoma in adults presents as a solitary tumor. Being a schwannoma, it originates in nerve. The examination of small tumors reveals that they practically always arise from the vestibular rather than the cochlear division of the eighth nerve, just within the internal auditory canal (Fig. 30-19). As the eighth nerve schwannoma grows, it extends into the posterior fossa to occupy the angle between the cerebellum and pons (cerebellopontine angle). In this lateral position, it is so situated as to compress the seventh, fifth, and less often the ninth and tenth cranial nerves, which are implicated in various combinations. Later it displaces and compresses the pons and lateral medulla and obstructs the CSF circulation; very rarely, it is a source of subarachnoid hemorrhage.