It is only during a systematic workup for generalized weakness and ventilatory failure in the critically ill patient that a diagnosis of CIM can be reached. A combination of electrophysiologic studies and histopathologic findings is required. Criteria for the diagnosis of CIM include sensory nerve action potential amplitudes >80% of the lower limit of normal; needle electromyogram (EMG) with short-duration, low-amplitude motor unit potentials with early or normal full recruitment (with or without fibrillation potentials); absence of a decremental response on repetitive nerve stimulation; muscle biopsy findings of myopathy with myosin loss; and compound muscle action potential amplitudes <80% of the lower limit of normal in two or more nerves without conduction block. Elevated serum creatinine kinase and demonstration of muscle inexcitability are also diagnostic features when taken together with the other findings. Further identification of the subtype of CIM (thick filament myosin loss,
rhabdomyolysis, necrotizing myopathy of intensive care, or cachectic myopathy) may aid in prognostication, since only necrotizing myopathy of intensive care is associated with a poor prognosis for return of muscle strength.