The presence of normal sensory nerve action potentials in the face of small compound muscle action potentials suggests that critical illness myopathy is present,5,10,12- 14but small or absent sensory nerve action potentials,4- 6,9,12,15indicative of the neuropathy, do not exclude the latter diagnosis. Slowed nerve conduction or conduction blocks are not consistent with either critical illness neuropathy or myopathy. Small, brief, polyphasic motor unit potentials with a good interference pattern despite muscle weakness are an indication that a myopathic process is present.4,5,9- 11,13- 15These motor unit potential changes might also occur with delayed neuromuscular junction blockade from the use of nondepolarizing blocking agents and require routine 2-Hz nerve stimulation studies as part of the electrophysiological evaluation. Inexcitability of muscle to direct electrical stimulation has recently been suggested as an additional criterion.14Fibrillations on needle electromyography may be absent or plentiful, and creatine kinase levels are often normal, making these features of little value in differentiating the myopathy from the neuropathy. Muscle biopsy remains the ultimate diagnostic study.