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Tumors of the Peripheral Nerves: Neurofibromatoses The neurofibromatoses are a group of three genetically distinct but related disorders of the nervous system that cause tumors to grow around the nerves. Tumors begin in the cells that make up the myelin sheath, a thin membrane that envelops and protects nerve fibers, and often spread into adjacent areas. The type of tumor that develops depends on its location in the body and the kind of cells involved. The most common tumors are neurofibromas, which develop in the tissue surrounding peripheral nerves. Most tumors are non-cancerous, although occasionally they become cancerous over time.
An estimated 100,000 Americans have a neurofibromatosis (the singular form of neurofibromatoses) disorder, which occurs in both sexes and in all races and ethnic groups. Scientists have classified the disorders as neurofibromatosis type 1 (NF1), neurofibromatosis type 2 (NF2), and a type that was once considered to be a variation of NF2 but is now called schwannomatosis. NF1 NF2 Schwannomatosis This is the most common neurofibromatosis, occurring in 1 in 3,000 to 4,000 individuals in the United States . Although many affected people inherit the disorder, between 30 to 50 percent of new cases occur because of a spontaneous genetic mutation from unknown causes. Once this mutation has taken place, the mutant gene can be passed on to succeeding generations.
Many children with NF1 have larger than normal head circumference and are shorter than average. Hydrocephalus, the abnormal buildup of fluid in the brain, is a possible complication of the disorder. Headache and epilepsy are also more likely in individuals with NF1 than in the normal population. Cardiovascular complications are associated with NF1, including congenital heart defects, high blood pressure (hypertension), and constricted, blocked, or damaged blood vessels (vasculopathy). Children with NF1 may have poor linguistic and visual-spatial skills, and perform less well on academic achievement tests, including those that measure reading, spelling, and math skills. Learning disabilities, such as attention deficit hyperactivity disorder (ADHD), are common in children with NF1. This rare disorder affects about 1 in 40,000 people. NF2 is characterized by slow-growing tumors on the eighth cranial nerve. This nerve has two branches: the acoustic branch helps people hear by transmitting sound sensations to the brain; the vestibular branch helps people maintain their balance. The tumors of NF2, called vestibular schwannomas because of their location and the types of cells that compose them (Schwann cells, which form the myelin sheath around nerves), press against and sometimes even damage the nerves they surround.
Signs of NF2 may be present in childhood but are so subtle that they can be overlooked, especially in children who don’t have a family history of the disorder. Typically, symptoms of NF2 are noticed between 18 and 22 years of age. The most frequent first symptom is hearing loss or ringing in the ears (tinnitus). Less often, the first visit to a doctor will be because of disturbances in balance, vision impairment (such as vision loss from cataracts), weakness in an arm or leg, seizures, or skin tumors. This is a newly recognized neurofibromatosis that is genetically and clinically distinct from NF1 and NF2. Like NF2 it occurs rarely. Inherited forms of the disorder account for only 15 percent of all cases. Researchers still don’t fully understand what causes the tumors and the intense pain that are characteristics of the disorder.
The distinguishing feature of schwannomatosis is the development of multiple schwannomas everywhere in the body except on the vestibular nerve. The dominant symptom is excruciatingly intense pain, which develops when a schwannoma enlarges, compresses nerves, or presses on adjacent tissue. Some people experience additional neurological symptoms, such as numbness, tingling, or weakness in the fingers and toes. Patients with schwannomatosis never have neurofibromas. Source: U.S. Department of Health and Human Services, National Institutes of Health, National Institute of Neurological Disorders and Stroke. www.ninds.nih.gov