Conduction block and continuous motor unit activity in chronic acquired demyelinating polyneuropathy

The term continuous motor unit activity (CMUA) may be used to refer to the involuntary, sustained activity of motor units caused by hyperactivity of p eripheral motor nerves. CMUA has been reported in association with acquired neuropathies such as chronic inflammatory demyelinating polyneuropathy. Th e precise mechanism responsible for the excess muscle activity is not defin ed, but the activity is believed to originate in the peripheral nerves, per haps at sites of focal demyelination. We describe a case of an acquired, de myelinating neuropathy associated with distal motor conduction block in whi ch CMUA was observed in muscles innervated by blocked axons. Despite the pr olonged disease duration of nearly 40 years, marked clinical and electrophy siological improvement as well as resolution of the CMUA were observed foll owing immunosuppressive therapy. A relationship between the chronic motor c onduction block and the excess muscle activity is postulated, (C) 1999 John Wiley & Sons, Inc.