Background: Critical illness myopathy (CIM) is the most common cause of weakness in patients on the Intensive Care Unit (ICU), yet its pathophysiology is not well understood.
Aims: The aim of our study was to see if jitter values from direct muscle stimulation in patients with CIM are increased compared to healthy controls. If jitter is increased, our secondary aim was to determine whether this was dependant on the length of the muscle fibre studied.
Methods: 10 healthy volunteers and 10 patients with diagnosis of critical illness myopathy were recruited. Direct muscle stimulation studies of the right tibialis anterior were performed using standardized protocols. Muscle fibre conduction velocities (MFCV) were measured. Jitter studies were performed with the distance between the stimulating and recording electrode set at a fixed distance of 30 and 50mm.
Results: The mean MFCV were 4.2±0.6m/s and 4.1±0.5m/s at 30mms and 50mms for controls. In patients this was slowed to 3.0±0.7m/s and 3.0±0.8m/s respectively. The mean jitter values at 30mm were 2.9±1.6µs in controls and 9.5±5.7µs in patients. The corresponding values at 50mm were 3.4±1.3µs for controls and15.1±8.5µs for patients. The jitter values at 30 and 50mms were significantly different in patients and not for controls.
Conclusion: Higher jitter values among patients with CIM show increased muscle firing variability and this appears to be due to changing muscle fibre conduction velocities. Sodium channel is the main instigator of the muscle fibre action potential propagation. These results provide further evidence for their dysfunction. These findings broaden our understanding of pathophysiology of CIM.