Complex Neuromuscular Presentation: Acquired Rippling Muscle Disease in a Patient with Ocular Myasthenia Gravis and Pathogenic CAPN3 Mutation
Andrea Zhang1, Olimpia Carbunar2
1University of Miami, 2Neurology, University Of Miami
Objective:
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Background:
Rippling muscle disease (RMD) is a rare condition characterized by muscle tensing, hypertrophy, and self-propagating rippling induced by stretching or percussion. It occurs in both familial and sporadic forms, and the familial form is typically associated with mutations in caveolin-3 (CAV3) gene. Limited cases of acquired RMD in patients with acetylcholine receptor (AChR) antibody-positive myasthenia gravis (MG) have been reported. We present a unique case of acquired RMD in an adult with ocular MG and pathogenic CAPN3 without any limb girdle muscle weakness. 
Design/Methods:
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Results:

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, fibrillation 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.  

Conclusions:

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.

10.1212/WNL.0000000000208730
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