RMD is a rare muscle hyper-excitability disorder, characterized by stiffness, cramps, mounding and rippling of the muscle. RMD can be hereditary (hRMD) or immune-mediated (iRMD). The mosaic pattern of sarcolemma caveolin-3 expression in iRMD distinguishes it from hRMD.

To describe the features of a series of patients with iRMD.

Retrospective review of clinical and investigatory findings of RMD patients presenting to our institution in the past decade.

We identified 6 patients with iRMD and one with hRMD. IRMD patients presented with: muscle rippling and cramps (4) and cramps followed by rippling (2). Coexisting autoimmune diseases were present in 3, manifesting within 6 months of iRMD symptom-onset in 2. Two patients developed urticaria around iRMD symptom-onset.  CK was elevated in 5 patients (range 336 – 2600 U/L). AChR Antibodies were positive in 3 patients, associated with clinical myasthenia and an abnormal SFEMG in 1 patient; 4 patients had anti-striated muscle antibodies without thymoma. EMG showed myopathic changes in all patients accompanied by fibrillation potentials or increased insertional activity in 4, most commonly in paraspinal and rippling muscles. Rippling was electrophysiologically active in 3 patients and silent in 1. Muscle biopsy revealed an active myopathy in 4 patients with perimysial inflammatory infiltrates in 3. Caveolin-3 expression was patchy in all iRMD muscle biopsies, and accompanied by reduced dysferlin expression in 2/3. One patient had a history of breast cancer. Four patients received immunotherapy, including IVIG (3), PLEX (1), and steroids (2). Post-treatment data was available in 3 patients: one patient had symptom resolution, 1 marked improvement and 1 no benefit.

iRMD was more frequent than hRMD. All iRMD patients had myopathic EMG changes; rippling was electrically active in 3/4. An inflammatory myopathy was present in half. Differentiation from hRMD is essential for proper pharmacological treatment as 2 of 3 improved on immunotherapy.