I read with great interest the article by Suzuki and colleagues1 on myocarditis and polymyositis in myasthenia gravis (MG). Suzuki et al described 6 patients with MG with polymyositis. Onset of polymyositis was either prior to or at the same time as onset of myasthenic symptoms.1 Intramuscular lymphocytic infiltration in MG is not uncommon, and it is often interpreted as a coincidental polymyositis.2 To date, more than 30 cases of MG-associated polymyositis (MG-PM) have been desribed.2 Patients described by Suzuki et al1 had additional weakness and hyperCKemia, which supports a diagnosis of coexisting polymyositis. However, other patients with MG-PM who had been described earlier often did not have these clinical findings; therefore, a diagnosis of polymyositis may not be accurate.2 Presence of endomysial lymphocytic infiltrates is nonspecific and can be seen in either thymoma-associated MG or other inflammatory myopathies.2 A certain number of these patients with MG-PM who have subtle or no clinical findings suggestive of polymyositis may in fact represent another uncommon and underrecognized MG-associated inflammatory myopathy, immune-mediated rippling muscle disease (RMD).
Liewluck T. Immune-Mediated Rippling Muscle Disease: Another Inflammatory Myopathy in Myasthenia Gravis. Arch Neurol. 2010;67(7):896-897. doi:10.1001/archneurol.2010.124