A 73-year-old female presents with muscle rippling in her legs and arms for the past two years, precipitated by tapping, or bumping into things. Needle EMG showed bursts of low-amplitude, sharply contoured muscle activity only with percussion.  EMG was electrically silent otherwise, and did not show increased insertional activity, ?brillation potentials, myotonic discharges, or neuromyotonic discharges. Workup revealed AChR binding of 5.90 nmol/L and modulating of 79 nmol/L.  Chest CT negative for thymoma. One year later she developed double vision. Genetic testing revealed a pathogenic CAPN3 mutation c.550del (p.Thr184fs) and negative CAV3 mutation. Repeat EMG did not show evidence of myopathy. CK was elevated at 690 U/L and TSH was elevated to 32.9 mIU/L. After levothyroxine adjustment, repeat TSH was normal and repeat CK was 306 U/L. She declined steroids, which has been associated with improved response in patients with RMD-MG.  

Our patient had RMD-MG with high-frequency discharges being brief and reportedly distinct from neuromyotonia. These discharges were noted to occur with percussion of the muscle and not spontaneously. There are cases of electrically silent RMD-MG which have had exceptionally high AChR antibody levels; but our patient had a value of 5.90 nmol/L. Although her RMD was not known to be due to CAV3, the relevance of the pathogenic mutation in CAPN3 is presently unclear.