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A vestibular schwannoma is a benign primary intracranial tumor of the myelin-forming cells of the vestibulocochlear nerve (8th cranial nerve). A type of schwannoma, this tumor arises from the Schwann cells responsible for the myelin sheath that helps keep peripheral nerves insulated. Approximately 3,000 cases are diagnosed each year in the United States. Most recent publications suggest that the incidence of acoustic neuromas is rising because of advances in MRI scanning. Studies in Denmark published in 2004 showed an annual incidence of 17.4 per million. Most cases are diagnosed in people between the ages of 30 and 60, and men and women appear to be affected equally. It is also commonly called an acoustic neuroma, although this term is technically a misnomer: the tumor rarely arises from the acoustic division of the vestibulocochlear nerve, and it is really a schwannoma rather than a tumor of actual nerve tissue (the literal meaning of the somewhat ambiguous term "neuroma"). ==Signs and symptoms== Acoustic neuromas normally develop gradually over a period of years, roughly 1–2 mm each year. They expand in size at their site of origin and when large, can displace normal brain tissue. The brain is not invaded by the tumor, but the tumor pushes the brain as it enlarges. Vital functions to sustain life can be threatened when large tumors cause severe pressure on the brainstem and cerebellum. Tumors are typically described as small (less than 1.5 cm), medium (1.5 cm to 2.5 cm) or large (more than 2.5 cm). Early symptoms are easily overlooked, sometimes mistaken for the normal changes of aging or attributed to noise exposure earlier in life, often delaying diagnosis. The first symptom in 90% of those with an acoustic neuroma is unilateral hearing loss (a reduction in hearing in one ear), often accompanied by ringing in the ear called tinnitus. The loss of hearing is usually subtle and worsens slowly, although occasionally a sudden loss of hearing can occur. There may be a feeling of fullness in the affected ear. Since the balance portion of the eighth nerve is where the tumor arises, unsteadiness and balance problems or even vertigo (the feeling like the world is spinning), may occur during the growth of the tumor. The remainder of the balance system sometimes compensates for this loss, and, in some cases, no imbalance will be noticed. Larger tumors can press on the trigeminal nerve, causing facial numbness and tingling - constantly or intermittently. Although the facial nerve may be compressed by the tumor, it is unusual for patients to experience weakness or paralysis of the face from an acoustic neuroma, although this may occasionally occur. Tumor related increase of intracranial pressure may cause headaches, clumsy gait and mental confusion. This can be a life-threatening complication requiring urgent treatment. Large tumors that compress the adjacent brainstem may affect other local cranial nerves. Paradoxically, the 7th cranial nerves are rarely involved pre-operatively; involvement of the trigeminal nerve (CN V) may lead to loss of sensation in the involved side's face and mouth. The glossopharyngeal and vagus nerves are uncommonly involved, but their involvement may lead to altered gag or swallowing reflexes. Larger tumors may lead to increased intracranial pressure, with its associated symptoms such as headache, vomiting, and altered consciousness. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Vestibular schwannoma」の詳細全文を読む スポンサード リンク
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