given the complexities of ICU-acquired weakness, a standard definition and list of nosologic features are both elusive and difficult to apply readily in clinical practice. For instance, electromyography and nerve-conduction studies are difficult to perform in many critically ill patients. The former requires that patients are awake and able to contract their muscles voluntarily; patients in the ICU are often not able to do so. Nerve-conduction studies can be confounded by problems such as tissue edema. There is controversy regarding the need — outside of research settings — to establish the diagnosis of ICU-acquired weakness on the basis of formal electrophysiological criteria.16,17 The nosologic scheme proposed by Stevens and colleagues18 is a reasonable classification. These investigators suggest that the term “ICU-acquired weakness” be applied in cases in which a patient is noted to have clinically detected weakness with no plausible cause other than critical illness.
ICU-acquired weakness, with documented polyneuropathy, myopathy, or both, can be subclassified. Critical illness polyneuropathy refers to ICU-acquired weakness with electrophysiological evidence of an axonal polyneuropathy. Critical illness myopathy refers to ICU-acquired weakness with myopathy that is documented electrophysiologically or histologically. Critical illness neuromyopathy refers to electrophysiological or histologic findings of both critical illness polyneuropathy and critical illness myopathy.18
The diagnosis of ICU-acquired weakness is made with the use of the Medical Research Council (MRC) scale for grading the strength of various muscle groups in the upper and lower extremities. The scale ranges from 0 to 5, with higher scores indicating greater muscle strength19; a combined score of less than 48 is diagnostic of ICU-acquired weakness.11 Patients with ICU-acquired weakness according to the MRC examination should undergo serial evaluations, and if persistent deficits are noted, electrophysiological studies, muscle biopsy, or both are warranted. Patients with persistent coma after discontinuation of sedation should undergo studies of the central nervous system such as cranial computed tomography or magnetic resonance imaging. If such studies are normal, electrophysiological studies, a muscle biopsy, or both should be performed. The MRC scale has important limitations, such as poor discrimination and a potential ceiling effect. Better bedside tools are needed to more precisely identify the presence of ICU-acquired weakness. Of course, weakness can have a broad differential diagnosis. A detailed review of specific diseases leading to weakness in patients in the ICU is beyond the scope of this article. Table 2TABLE 2
Diseases and Syndromes Causing Weakness in Patients in the ICU.
summarizes common diseases that cause weakness in these patients, and Figure 1FIGURE 1
Diagnostic Algorithm for the Evaluation of Weakness in Critically Ill Patients.
shows a diagnostic algorithm for the evaluation of weakness in critically ill patients.