Introduction. The acquired muscle
paralysis associated with modern
critical care can be of neurogenic or myogenic origin, yet the distinction between these origins is hampered by the precision of current diagnostic methods. This has resulted in the pooling of all acquired muscle
paralyses, independent of their origin, into the term Intensive Care Unit Acquired
Muscle Weakness (ICUAW). This is unfortunate since the acquired neuropathy (
critical illness polyneuropathy, CIP) has a slower recovery than the
myopathy (
critical illness myopathy, CIM);
therapies need to target underlying mechanisms and every patient deserves as accurate a diagnosis as possible. This study aims at evaluating different diagnostic methods in the diagnosis of CIP and CIM in
critically ill, immobilized and mechanically ventilated intensive care unit (ICU) patients. Methods. ICU patients with acquired
quadriplegia in response to
critical care were included in the study. A total of 142 patients were examined with routine electrophysiological methods, together with biochemical analyses of
myosin:actin (M:A) ratios of muscle biopsies. In addition, comparisons of evoked electromyographic (EMG) responses in direct vs. indirect muscle stimulation and histopathological analyses of muscle biopsies were performed in a subset of the patients. Results. ICU patients with
quadriplegia were stratified into five groups based on the hallmark of CIM, i.e., preferential
myosin loss (
myosin:actin ratio, M:A) and classified as severe (M:A < 0.5; n = 12), moderate (0.5 ≤ M:A < 1; n = 40), mildly moderate (1 ≤ M:A < 1.5; n = 49), mild (1.5 ≤ M:A < 1.7; n = 24) and normal (1.7 ≤ M:A; n = 19). Identical M:A ratios were obtained in the small (4-15 mg) muscle samples, using a disposable semiautomatic microbiopsy needle instrument, and the larger (>80 mg) samples, obtained with a conchotome instrument. Compound muscle action potential (CMAP) duration was increased and amplitude decreased in patients with preferential
myosin loss, but deviations from this relationship were observed in numerous patients, resulting in only weak correlations between CMAP properties and M:A. Advanced electrophysiological methods measuring refractoriness and comparing CMAP amplitude after indirect nerve vs. direct muscle stimulation are time consuming and did not increase precision compared with conventional electrophysiological measurements in the diagnosis of CIM. Low CMAP amplitude upon indirect vs. direct stimulation strongly suggest a neurogenic lesion, i.e., CIP, but this was rarely observed among the patients in this study. Histopathological diagnosis of CIM/CIP based on
enzyme histochemical mATPase stainings were hampered by poor quantitative precision of
myosin loss and the impact of pathological findings unrelated to acute
quadriplegia. Conclusion. Conventional electrophysiological methods are valuable in identifying the peripheral origin of
quadriplegia in ICU patients, but do not reliably separate between neurogenic vs. myogenic origins of
paralysis. The hallmark of CIM, preferential
myosin loss, can be reliably evaluated in the small samples obtained with the microbiopsy instrument. The major advantage of this method is that it is less invasive than conventional muscle biopsies, reducing the risk of
bleeding in ICU patients, who are frequently receiving
anticoagulant treatment, and it can be repeated multiple times during follow up for monitoring purposes.