T75. The relationship between Motor Unit (MU) behaviour and nerve conduction velocity follow cooling at the elbow: An exploration of conduction dependent MUs rate block

Abstract

Nerve conduction velocity is a common assessment of peripheral demyelinating conditions used to locate the lesions and assess severity. Considering measurements involve low frequency or single stimuli, changes due to continuous activity would remain out of their scope. Accordingly, slowing of conduction velocity is not thought to result in weakness, but may account for loss of deep tendon reflexes leading to neural transmission failure. It has previously been shown that a decrease in temperature of 1 °C can cause a 2.4 m/s reduction in nerve conduction velocity, however little is known on the neuromuscular effects, specifically the behavior of individual Motor Units (MUs). This study explored the relationship between the MU behavior and motor-sensory nerve conduction velocities following the application of a standard therapeutic cooling technique at the elbow. Eleven healthy individuals, aged 20–49, were tested using two non-invasive Nerve Conduction Studies (NCS); compound muscle action potentials of the First Dorsal Interosseous (FDI) muscle and sensory nerve action potentials, and a non-invasive investigation of the firing of MU using surface EMG decomposition (dEMG, Delsys Inc.) of FDI. Testing was performed prior to cooling, immediately after cooling and after 15 min of rewarming. Cooling was performed for 15 min using crushed ice and water to a skin temperature between 10 and 15 °C. Repeated measures ANOVAs with post hoc pairwise comparisons showed significant reductions in the MU firing between pre-cooling and post-cooling and between pre-cooling and re-warming (p = 0.013, p = 0.045) respectively. Similar patterns were seen in both the Motor and Sensory NCS with significant differences between pre-cooling and post-cooling, pre-cooling and re-warming, and between post-cooling and 15 min of re-warming in the Sensory assessment, although the latter is of questionable clinical importance (p < 0.01), Fig. 1. This showed a 22% delay in the Sensory and 26% delay in the Motor nerve conduction, which caused a corresponding 12% mean MUs firing rate decay. Further analysis separating MUs in tertiles revealed higher firing MUs were significantly reduced after cooling, leaving lower firing MUs unchanged (Table 1). There appears to be a clear relationship between the motor and sensory NCS and the MU firing rate. In contrast to high stimulation frequencies used clinically or activity dependent conduction block, this study shows conduction dependent MUs rate block (CDRB) verifying motor deficit whilst only nerve conduction slowing is evident, and revealing the first recruited MUs are affected more than the later recruited ones. This highlights the potential for dEMG as a novel neurological assessment, which could be used when conduction velocity is difficult to measure and motor control is affected, especially early in the clinical state before axonal degeneration is evident, which is still challenging for routine neurophysiological methods.