A rare form of painful nondystrophic myotonia.

Objective: In this paper we report a painful nondystrophic myotonia which has not been previously described. Pain is a rare symptom in myotonia. We report a myotonic disorder in a 34-year-old woman and her 14-year-old daughter. Painful cramps occur during and after exercise in the mother, and both patients can demonstrate unusual contractions in the tongue. In the present study we try to evaluate the mechanisms behind the unique finding of trains of high amplitude of positive waves, not seen in the earlier known myotonic conditions.

Methods: Clinical investigations and electromyography with single and dual channel recordings and muscle morphometry were performed.

Results: The electromyographic recordings reveal positive waves, fibrillation potentials and myotonic discharges. In addition, extraordinary findings were made of trains of high frequency positive potentials with very high amplitudes and with conduction block along the muscle fibres.

Conclusions: In this new form of myotonia with likely dominant heredity, the specific finding of trains of high amplitude positive waves indicates ephaptic transmission within bundles of neighbouring muscle fibres.