Objectives Sporadic inclusion body myositis (sIBM) is a relentlessly progressive inflammatory myopathy of unclear aetiology. We used a novel technique to assess muscle velocity recovery cycles (MVRC) that provides information on the polarisation state and T-tubule function of myocytes.1 This study aimed to identify differences in MVRC parameters in sIBM patients that could provide pathophysiological insights or act as disease markers.
Methods Twenty patients meeting ENMC 2011 criteria as clinicopathologically or clinically defined sIBM were recruited. MVRC recordings were obtained from tibialis anterior (n=19) and rectus femoris muscles (n=14). Data was captured using QTRACS threshold tracking software employing the M3RC3 recording protocol with subsequent data analysis through the MAnal8 program. Excitability parameters were compared with control muscle data obtained from older (>50yo) healthy individuals without neuromuscular disease.
Results MVRC on tibialis anterior in sIBM patients demonstrated a prolonged muscle relative refractory period (MRRP: 5.91 ms vs 4.41 ms, p<0.001) alongside reduced early supernormality (5.8% vs 8.6%, p=0.002) when compared with controls. Late supernormality was reduced to a lesser extent, (1.6% vs 3.4% p=0.002) with extra conditioning stimuli producing a reduced increment of late supernormality (2.50% vs 5.66%, p<0.001). A similar pattern of prolonged MRRP and reduced supernormality was seen in recordings from sIBM patient rectus femoris versus controls. No significant confounding differences between sIBM and control patients in age, skin temperature or serum potassium were present.
Conclusions MVRC changes consistent with depolarisation of the resting muscle membrane potential of tibialis anterior and rectus femoris was found in sIBM patients when compared to normal controls in this cross-sectional study. This may reflect mitochondrial (2) or Na+/K+ ATPase pump dysfunction,3 induced by the widespread muscle disruption and destruction seen in sIBM. Longitudinal studies may determine the reliability of MVRC in assessing disease progression and treatment responses.
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