Hereditary myotonia in the Chow Chow

Four related Chow Chow puppies which were presented because of stiffness in their movements were shown to have myotonia similar to myotonia congenita of man. Electromyography revealed characteristic myotonic discharges. Repetitive nerve stimulation studies showed a marked fade in the compound muscle action potential (CMAP) which was most apparent at stimulation rates of 10 per sec or higher. If stimulation was continued at a rate of 10 per sec the CMAP returned to normal or sometimes greater than normal following several seconds of the reduced response. The decremental response could be exaggerated by cooling the muscle prior to repetitive nerve stimulation. A therapeutic trial to assess the efficacy of the commonly used membrane stabilizing agents, quinidine, procainamide and phenytoin, indicated that all three drugs were beneficial in the treatment of myotonia but procainamide produced the best response. Hypocholesterolaemia was documented in one case. The possibility of a multisystem membrane defect in hereditary myotonia associated with low serum cholesterol was considered.