Fine structural alterations of muscle fibers in diseases accompanied by myotonia.

The authors have reported the results of examination by electron microscopy of two muscle biopsy specimens from cases of myotonia congenita and three cases of myotonia dystrophica. They have stated that "peripheral annular formation" was a frequently observed alteration in the myotonia congenita cases. In the myotonia dystrophica cases there were additionally disorganized myofibrils in the subsarcolemmal region and inclusion body vacuoles morphologically connected with the sarcolemma. The term "peripheral annular formation" refers to the situation in which the peripheral myofibrils of the muscle fiber fracture and the fragments retract and form a helical sheath around the central myofibrils of the same muscle fiber.