New index for distinguishing between critical illness myopathy and neuromyopathy: CMAP amplitude/duration index

Background/Aim: Critical illness-associated weakness (CIAW) is a common neuromuscular syndrome that may manifest as critical illness myopathy (CIM), critical illness polyneuropathy (CIP), or an overlap syndrome known as critical illness polyneuromyopathy (CIPM). Distinguishing between these subtypes can be challenging due to technical issues, patient-related factors such as insufficient cooperation with needle electromyography (EMG) and edema, and the need for sophisticated, time-consuming electrophysiological methods and invasive procedures such as biopsy. This study aims to contribute to the electrophysiological distinction between CIM and CIPM. Materials and methods: A new index was calculated based on distal compound muscle action potential (CMAP) amplitude and negative peak duration values. Comparative and ROC analyses were performed on the parameters of patients with CIM and CIPM, as well as between patient groups and controls.Results: The median and ulnar CMAP index cut-off values for distinguishing between CIM and CIPM were determined to be 0.35 and 0.51, respectively. Values below these cut-off points support a diagnosis of CIPM, while higher values indicate CIM. The best parameter for distinguishing both CIM and CIPM patients from healthy controls was the peroneal CMAP index, with a cut-off value of 0.45.Conclusion: The CMAP index can be easily calculated from CMAP obtained during routine nerve conduction studies. This index may serve as a practical and guiding method for differentiating between CIM and CIPM, contributing to the electrophysiological diagnosis in critically ill patients, particularly those with unreliable sensory nerve action potentials and needle EMG examinations.