The objective of the prospective study was to analyze electrophysiological abnormalities in critically ill patients and their correlation with histopathological abnormalities and to test thus the possibility of electrophysiological differentiationof critical illnes polyneurophathy and myopathy (CIPM). Agroup of 79 critically ill patients was followed up for one month by clinical and electrophysiological methods. The investigation was completed by 48 patients. Electrophysiological abnormalities suggesting a newly developed CIPM were found during the 5th week in 27 patients, in 9 patients they were present already during the baseline examination during the first week. The dominating electrophysiological abnormalities (with the exception of one patient) were a reduction of the totalmuscular action potentials(CMAP) or abnormal spontaneous activity suggesting motor axonopathy or myopathy. In 11 patients with this abnormality muscle biopsy was performed which proved signs of myogenetic affection in all instances (in 7 in combination with neurogenetic affection). Reduced CMAP during direct muscle stimulation was found in 7 patients with histological signs of myopathy. Electrophysiological signs of sensitive neuropathy were present in 14 patients, only in one patient isolated sensitive neuropathy. The investigation confirmed the high prevalence of CIMP in critically ill patients (56 %) with a dominating motor affection of the type of axonal neuropathy and myopathy, usually combined. Electrophysiological differentiation of both types of affections is not reliable. The most valuable electrophysiological parameter is the CMAP amplitude which makes early detection and the follow up of the grade and dynamics of the affection possible.

        Key words: critical illness polyneuropathy, critical illness myopathy, direct muscular stimulation, electrodiagnosis, muscle biopsy