Prolonged compound muscle action potential duration in critical illness myopathy: report of nine cases.

The objective of this study was to determine the diagnostic use of compound muscle action potential duration in patients with critical illness myopathy. Accurate diagnosis is important because the muscles recover once the offending agents are withdrawn. We retrospectively reviewed 9 cases seen at our institution between 1999 and 2003 in which critical illness myopathy was diagnosed on the basis of clinical and electrophysiological evaluations. All the patients had weakness, difficulty weaning from mechanical ventilation, sepsis, and exposure to intravenous corticosteroids in the intensive-care unit. Seventy-five percent of tested motor nerves had responses. Amplitude was decreased in 100% of ulnar and peroneal nerves, and 71% of median and tibial nerves. Duration was prolonged in 100% of responsive motor nerves. Prolonged compound muscle action potential duration was the most characteristic electrophysiological finding of critical illness myopathy. Its recognition as part of a quick and simple bedside diagnostic test makes it preferable to other tests.