Vestibular Schwannomas

Histologically, vestibular schwannomas appear quite similar to peripheral schwannomas. On a microscopic level, there are zones of dense and sparse cellularity, identified as Antoni A and B areas, respectively. They are typically benign, with malignant transformation a very rare occurrence.

Clinical Manifestations. Clinical signs and symptoms result from cranial nerve involvement or mass effect on the cerebellum and other posterior fossa structures. Symptom onset is usually insidious, given the slow rate of growth of these tumors. Almost all patients will present with hearing loss and tinnitus secondary to cochlear nerve involvement, although patients may not necessarily be aware of their deficits. In a study of 1000 patients, 95% of patients were found to have hearing loss, but only two thirds of patients recognized their limitations. Rarely, patients will have the acute onset of hearing loss secondary to compression of the vascular supply to the auditory nerve. More than half of patients will also have involvement of the vestibular nerve with complaints of gait unsteadiness. Although true vertigo is uncommon, given the slow onset of symptoms, patients may complain of nonspecific dizziness. A minority of patients will have trigeminal involvement that can produce symptoms ranging from facial numbness to trigeminal neuralgia. Involvement of the seventh cranial nerve is rare but can occur with the onset of facial weakness, hemifacial spasm, or taste disturbance. Finally, mass effect of the tumor can cause various types of neurologic dysfunction. Compression of the cerebellum can result in ataxia, and disruption of the lower cranial nerves can cause dysarthria, dysphagia, and aspiration. Involvement of the brainstem can lead to hydrocephalus, coma, and even death.

Diagnostic Studies. When suspicion for a vestibular schwannoma is raised, pure tone and speech audiometry should be performed as an initial screening test. Typically, results will reveal asymmetric hearing loss, especially with high frequencies. Speech discrimination is reduced in the affected ear and is usually out of proportion to the measured hearing loss. Another screening measure used is brainstem-evoked response, which detects a delay on the affected side. However, it had a relatively low sensitivity and specificity compared with magnetic resonance images (MRIs), especially with smaller lesions. Thus MRI has becomes the diagnostic modality of choice when suspicion for a vestibular schwannoma is raised. Specifying fine cuts (3-mm slices or less) through the internal auditory canal (IAC) may increase the sensitivity and specificity even further. Computed tomography (CT) scans with bone windows are also useful because the extent of tumor growth in the IAC has prognostic significance. It should be noted that the diagnosis of a vestibular schwannoma is based on the clinical history (asymmetric hearing loss) in conjunction with audiometry and imaging because they constitute the majority of posterior fossa lesions that behave in this manner. These lesions are rarely biopsied. Nonetheless, other considerations include meningiomas, schwannomas of other cranial nerves, hemangiomas, gliomas, metastatic tumor, aneurysms, and arachnoid cysts.

Only gold members can continue reading. Log In or Register to continue