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 peripheral motor nerves. CMUA has been reported in association with acquired neuropathies such as chronic inflammatory demyelinating polyneuropathy. The precise mechanism responsible for the excess muscle activity is not defined, but the activity is believed to originate in the peripheral nerves, perhaps at sites of focal demyelination. We describe a case of an acquired, demyelinating neuropathy associated with distal motor conduction block in which CMUA was observed in muscles innervated by blocked axons. Despite the prolonged disease duration of nearly 40 years, marked clinical and electrophysiological improvement as well as resolution of the CMUA were observed following immunosuppressive therapy. A relationship between the chronic motor conduction block and the excess muscle activity is postulated.