I read with interest David Lacomis and colleagues' editorial 4 on critical illness myopathy. I must take issue with their comments regarding the role of histopathological proof and the role of muscle biopsy.
The causes for persistent weakness in a critically ill patient are multiple. The so-called "critical illness" myopathy most probably represents a spectrum of illnesses and not one entity. This continuum includes severe disuse (type II) atrophy, cancer-related necrotizing myopathy (paraneoplastic or those related to immunotherapy for malignant disorders), 3,5 and the spectrum of myopathies with heavyfilament (myosin) loss, from the focal loss of myosin seen in postparalysis paralysis syndrome 2,6 to a more severe, rapidly evolving myopathy with diffuse myosin loss. 1 Electrophysiologic studies may differentiate a neuropathy from a myopathy and may show muscle fiber irritability. More often, the electrophysiologic studies show a combination of neuropathic and myopathic features or are technically limited because of the problems inherent in performing electrodiagnostic studies in the intensive care units. Muscle enzyme elevation is not useful and may not always be seen. Muscle membrane inexcitability 7 is not specific to any one subset of this spectrum. Thus, an exact diagnosis cannot be ascertained without an open muscle biopsy. Muscle biopsy results impact treatment, and open muscle biopsy should therefore remain a mandatory part of the investigation of persistent weakness in critically ill patients.