Why the study?
How is muscle membrane function affected in patients with myotonia congenita compared to normal controls when assessed by velocity recovery cycles?
How is muscle membrane function affected in patients with myotonia congenita compared to normal controls when assessed by velocity recovery cycles?
Muscle velocity recovery cycles reveal that chloride conductance in myotonia congenita becomes important primarily during muscle fiber depolarization, with differential responses between dominant and recessive subtypes.
Velocity recovery cycles may distinguish myotonia congenita subtypes by membrane excitability; hypothesis-generating and should not yet change practice.
INTRODUCTION: Myotonia congenita (MC) is caused by congenital defects in the muscle chloride channel CLC-1. This study used muscle velocity recovery cycles (MVRCs) to investigate how membrane function is affected. METHODS: MVRCs and responses to repetitive stimulation were compared between 18 patients with genetically confirmed MC (13 recessive, 7 dominant) and 30 age-matched, normal controls. RESULTS: MC patients exhibited increased early supernormality, but this was prevented by treatment with sodium channel blockers. After multiple conditioning stimuli, late supernormality was enhanced in all MC patients, indicating delayed repolarization. These abnormalities were similar between the MC subtypes, but recessive patients showed a greater drop in amplitude during repetitive stimulation. CONCLUSIONS: MVRCs indicate that chloride conductance only becomes important when muscle fibers are depolarized. The differential responses to repetitive stimulation suggest that, in dominant MC, the affected chloride channels are activated by strong depolarization, consistent with a positive shift of the CLC-1 activation curve.
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Tan et al. (2013) studied this question.
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