P 45. Different K+-release in distal myopathy and motor neuropathy during non-ischemic exercise

Abstract

Background. In myotonic dystrophy, a significantly increased potassium release upon exercise was previously described. However, it remained unclear so far whether this is a specific feature of myotonic dystrophies or a consequence of distal muscular weakness in general. Methods. We performed non-ischemic forearm exercise test (NIFET) and measured venous K + concentration at rest and at three different force levels (20–30%, 50–60%, 70–80%) related to maximal contraction force (MCF) in patients with different myopathies ( n = 7), motor neuropathies ( n = 8) with distal phenotype and healthy volunteers ( n = 12). The specific K + production was defined as venous K+ increase related to workload. Results. Maximal contraction force, workload and venous K + increase were lower at all force levels in both myopathy and neuropathy groups compared to the control group. With increasing workload, the control group showed a quasi-linear increase in K + release, resulting in a nearly constant specific K + production within the three force levels ( p = 0.752, Kruskal-Wallis-Test). In contrast, in myopathies a higher specific K + production was measured at low force levels (20-30% MCF) in comparison to higher force levels ( p = 0.015, Kruskal-Wallis test). In neuropathies, the lowest specific K + production was obtained at 20–30% MCF. At this force level, the specific K + production was significantly higher in myopathies compared to neuropathies (Fig. 1; p = 0.005, Mann-Whitney U Test). At 50–60% and 70–80% MCF, the specific K + values increasingly converged and did not significantly differ between the three groups (Fig. 1; p = 0.09 and p = 0.37, Kruskal-Wallis test). Conclusion. In our study, increased K + efflux related to workload occurred in patients with myogenic in comparison to neurogenic distal muscular weakness at low force levels. High specific K + production does not seem to be an exclusive feature of myotonic dystrophies but of myogenic muscular weakness in general. Our results point to a different regulation of K+ balance in neurogenic and myogenic muscular weakness possibly due to a different recruitment behaviour of motor units and the action potential frequency of motor neurons in neurogenic and myogenic processes. Fig. 1.