A myopathy associated with myotonia in the dog

The pathology of two cases of canine myopathy associated with myotonia are presented. The changes were interpreted as dystrophic. The most obvious features were a rounding on cross section and variation in fibre size with numerous internal nuclei, many of which formed chains. Degeneration and regeneration were seen and there was a slight increase in perimysial and endomysial connective tissue. Only one ringed fibre was seen, but no sarcoplasmic masses. Enzyme histochemistry failed to demonstrate any selective Type I fibre atrophy. The peripheral and central nervous systems were normal in both cases.