Acoustic Schwannoma and Trigeminal Neurilemmoma

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Pathogenesis and Pathophysiology. Acoustic neuromas, found incidentally on imaging performed for other reasons or due to new symptoms as described below, have a variable course. Less than 10 percent have a familial association. There appears to be no association with unilateral acoustic neuromas or with either form of neurofibromatosis. Some investigators have tried to examine the growth rates of these tumors by performing serial neuro-imaging of the tumors to gauge their growth. Some suggest that the female predominance of these tumors indicates a sex hormonal influence.

Epidemiology and Risk Factors. Neuromas (schwannoma and neurilemmoma) are benign tumors that usually involve the sensory and motor cranial nerves, most commonly the eighth, fifth, and seventh cranial nerves. Neuromas represent about 10 percent of all intracranial tumors, the majority being acoustic neuromas. They present most frequently in the fourth and fifth decades of life, predominantly among women. Except for bilateral acoustic neuromas, which are pathognomonic for neurofibromatosis II (see subsequent discussion), they are usually unilateral.

Clinical Features and Associated Disorders. Acoustic neuromas are considered slow growing, but they grow in a physiologically eloquent area, the cerebellopontine angle, thus leading to subtle complaints that may evolve slowly over many years. Symptoms may progress slowly, although sudden presentations can be seen. Patients usually present with hearing loss (especially sound discrimination), tinnitus, loss of balance, nystagmus, loss of facial sensation, or loss of function of the facial muscles or the muscles of mastication. Pain is uncommon but may sometimes mimic trigeminal neuralgia. Cerebellar findings such as ataxia and gait abnormalities may be noted when extremely large tumors have extended into the cerebellum. In patients with trigeminal pain multiple sclerosis may be diagnosed initially; only with further hearing loss and a lack of involvement of other areas of the nervous system will re-evaluation be conducted. Meningiomas are occasionally confused with neuromas. Rarely, cholesteatomas or choroid plexus papillomas are identified at surgery.

Evaluation. Sometimes early neuroradiological examination may fail to detect these tumors. Patients with increasing symptoms that are not amenable to medical treatment may require repeat studies. MRI with gadolinium enhancement remains the imaging modality of choice because it permits better definition of soft tissue involvement and identification of alternative diagnoses. Neuromas most often demonstrate bright enhancement with contrast agents. CT is performed to evaluate bony erosion of the skull. All patients should undergo audiometry and brain stem auditory evoked potentials at presentation to define pretreatment abnormalities.

Management. Surgical removal remains the mainstay of treatment. Recent reports have focused on the use of stereotactic irradiation or gamma knife surgery. No large series are available to compare their efficacy with that of surgery alone. After surgery patients are at risk of hearing loss and facial nerve paralysis. In patients with smaller tumors, hearing can sometimes be preserved. Radiotherapy has been shown to delay tumor regrowth after subtotal resection but carries a risk of radiation damage to other cranial nerves and brain stem vascular events. Chemotherapy has so far not been sufficiently explored.

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