Sarcolemmal depolarization in sporadic inclusion body myositis assessed with muscle velocity recovery cycles.

OBJECTIVE

To study patients with sporadic inclusion body myositis (sIBM) with muscle velocity recovery cycles (MVRC) to assess muscle membrane excitability, pathophysiological mechanisms and potential biomarkers of this disorder.

METHODS

MVRC were recorded from 20 individuals with sIBM from tibialis anterior (TA) and rectus femoris (RF) muscles. Excitability parameters were compared with MVRC data obtained from 22 normal controls >50 years.

RESULTS

Muscle relative refractory period was prolonged in both TA (6.4 ms vs 4.4 ms, P < 0.001) and RF (7.1 ms vs 3.9 ms, P < 0.001) of sIBM affected muscle when compared to controls. Early supernormality was reduced in both TA (3.6% vs 8.8% P = 0.001) and in RF (mean 5.4% vs 13% P < 0.001). Late supernormality was only decreased significantly in sIBM affected TA (1.8% vs 3.6% P = 0.001) but not in RF. No consistent correlations between MVRC parameters and clinical markers of sIBM disease severity were found.

CONCLUSION

The resting sarcolemmal muscle membrane potential of sIBM muscle is depolarized relative to that of normal controls, which may be related to intramuscular amyloid deposition in sIBM.

SIGNIFICANCE: Sarcolemmal depolarization may play a role in muscle dysfunction and weakness observed in sIBM patients.