Forty-seven cases of Charcot-Marie-Tooth peripheral neuropathy were seen in 18 families within a defined area, with a disease prevalence of 1 in 16 400. Maximum motor nerve conduction velocity (MNCV) measurement divided off two types of neuropathy (MNCV less than 30 ms-1 and greater than 40 ms-1), but did not distinguish clinically affected from normal in families whose probands had median nerve MNCV greater than 40 ms-1. In the neuronal type of neuropathy ((MNCV greater than 40 ms-1) two genotypes were seen, autosomal dominant (ADN) and autosomal recessive (ARN). Most cases with the demyelinating type (MNCV less than 30 ms-1) had an autosomal dominant genotype (ADD) but one family had possible X linked recessive inheritance (XRD). In one autosomal dominant family a father and son had different electrophysiological types of neuropathy. Peroneal muscle weakness was progressive with age in the ADD genotype and certain patterns of phenotypic features were associated with the major genotypes. Age of onset was not found to be reliable in distinguishing genotypes. Care is needed when counselling isolated male cases because of asymptomatic affected females in the autosomal dominant genotypes, and the possibility of ill defined X linked forms.
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