Pathogenesis and Pathophysiology. Both types of neurofibroma are associated with a gene localized to chromosome 17q11.2 for NFI y and to chromosome 22 for NFII. The NFI gene encodes a protein termed neurofibromin that is involved in regulation of the RAS protein.
Epidemiology and Risk Factors. Neurofibromatosis (NFI), also known as von Recklinghausen's disease, comprises a constellation of tumors of the CNS. NFII is defined by bilateral acoustic neuromas. Patients with NFI have an increased risk of astrocytic tumors including optic pathway glioma and brain stem gliomas. Patients with NFII are at risk of ocular abnormalities, other cranial nerve schwannomas, and meningiomas. Spinal cord gliomas are seen more commonly in those with NFII. NFI has a frequency of about 1 in 4000, and the frequency of NFII is 1 in 40,000. Both are inherited as autosomal dominant traits, with NFI having incomplete penetrance (see Chapter 32 ).
Clinical Features and Associated Disorders. In NFII the bilateral acoustic neuromas usually become evident at the same time. Presenting complaints include tinnitus or hearing loss. Most patients have a first-degree relative who is similarly afflicted. Patients with NFII are also at risk of meningioma, gliomas, ependymoma, and schwannoma. Symptoms are the same as those discussed under the respective tumors.
Differential Diagnosis and Evaluation. Patients with NFI must be closely examined for any history consistent with visual disturbances or other visual abnormalities or signs of nerve root impingement, including bowel and bladder dysfunction or constipation. Patients may show
evidence of peripheral nerve dysfunction caused by impingement by a neurofibroma. Only when the clinical findings suggest changes requiring intervention should tests be performed. Yearly MRI scans of the brain and spinal cord looking for tumor are not indicated. Some clinicians advocate yearly screening of children with visual-evoked potentials for early detection of optic gliomas. '55! Patients and their spouses need genetic counseling for family planning purposes y and extensive social work intervention to help with issues related to the disease and the possibility of tumor generation.
Patients with NFII should be closely observed for any changes in auditory function, including hearing loss and tinnitus. During the neurological examination, particular attention should be focused on other cranial nerve abnormalities. Physicians should monitor the patient for any evidence of vertigo or difficulty with balance, possibly with yearly brain stem auditory evoked potentials. Formal audiometry on a yearly basis is a reasonable alternative as well; yearly MRI scanning is not generally indicated but may be repeated serially to allow a gauge of tumor enlargement.
Management. Patients with NFI should be monitored closely for development of treatable abnormalities as previously described. Optic gliomas can be treated with surgical removal when feasible or with radiotherapy. '571 Malignant degeneration of a schwannoma or a neurofibroma (neurofibrosarcoma) occurs in 5 percent of patients. Patients with NFII require intervention to remove the acoustic neuromas as described previously. In these patients, referral to a center that has facilities to perform a cochlear implant should be considered because these patients are at great risk of bilateral hearing loss. Patients receiving radiotherapy, especially children, are at risk for further neurobehavioral deterioration and endocrine abnormalities.
Prognosis and Future Perspectives. Further investigation into the genotype and how it relates to the variable phenotypic expression and manifestation of tumors remains to be conducted. A better understanding of the mechanism of action of the genes responsible for these diseases and the way in which malignant degeneration occurs may advance treatment of patients not only with NF but with gliomas as well. In utero diagnosis remains a possibility. Genetic analysis of families and in family planning should be thoroughly explored.
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