Electrophysiology of Neuromuscular Disorders in Critical Illness

Neuromuscular disorders-predominantly critical illness myopathy (CIM), critical illness polyneuropathy (CIP), and critical illness neuromyopathy-commonly occur in intensive care unit (ICU) patients. Given the limitations of the neurologic examination in the setting of ICU-acquired weakness, electrophysiology is very helpful in localizing the cause of weakness and defining its etiology. In CIM, motor unit potentials have typical myopathic features, but motor unit potential activation may be limited. In addition, there are features of sarcolemmal inexcitability reflected in low motor responses that may have prolonged durations. In CIP, the findings are usually those of a recent onset generalized axonal sensorimotor polyneuropathy; however, it is essential to note that sensory conductions may be prone to artifacts and other confounders in the ICU. Critical illness neuromyopathy has features of both CIM and CIP. For research studies and in some clinical situations, direct muscle stimulation may be used to help differentiate CIP and CIM. Histopathologic studies may be useful in confirming CIM and identifying other neuromuscular causes of weakness. In addition, identifying evolving reductions in fibular/peroneal motor amplitudes in ICU patients can help predict developing neuromuscular weakness. Knowledge of the various neuromuscular disorders that can occur in critically ill patients, their risk factors, and associated electrodiagnostic findings enables a rational approach to diagnosing the causes of neuromuscular weakness in ICU patients.