Myotonia

Abstract

The muscle membrane in myotonia congenita is characterized by a normal resting potential with a greatly increased resting resistance usually attributed to a decrease in membrane chloride permeability (P_cl). In this report, the hypothesis that decreased P_clalone can account for the repetitive action potentials of myotonia is tested with a mathematical model of the muscle membrane and is shown to be valid. Reduction of P_clto 20% of control values will produce myotonic activity in response to a single stimulus. Membrane resistance and potential approximate those found experimentally. The model predicts that increasing external K⁺will aggravate myotonia due to a reduction of P_cl, while decreasing K_outwill prevent repetitive spiking. Further, myotonia can be prevented by reducing peak membrane sodium permeability or by shifting the voltage dependency of the membrane rate constants for sodium in a depolarizing direction. These results are shown to correlate well with clinical observation and provide explanations for the action of drugs effective in the treatment of myotonia.