Electrophysiologic techniques in critical illness-associated weakness

Neuromuscular disorders are increasingly recognized in the critically ill but conventional electrodiagnostic techniques often provide non-specific results or are hampered by local conditions that prevent adequate disease classification. Muscle fiber inexcitability is a common phenomenon in critical illness myopathy possibly secondary to disordered sodium channel fast inactivation and associated with loss of myosin staining. Direct muscle stimulation techniques, measuring evoked response amplitudes and comparison of nerve and muscle stimulated responses, are recognized methods of demonstrating this phenomenon. Other measures studied in this population include increased compound motor action potential duration, motor unit number estimates and mean step area of individual motor unit potentials during motor unit number estimate studies. An electrophysiologic approach to the study of patients with critical illness associated weakness is proposed.