Clinical syndromes associated with IMNM defined by MSA antibodies
• Anti-signal recognition particle (SRP): patients present with a very acute and severe onset of myositis with significant muscle pain due to muscle fiber necrosis. Has increased incidence of cardiac involvement (myocarditis) and dysphagia. May have arthritis but no ILD or rash. Poor overall response to corticosteroid therapy with 5YS of 30%. The autoantibody is directed against a 54-kDa protein in an RNA complex involved with intracytoplasmic protein translocation. ANA is negative.
• Anti-3-hydroxyl-3methylglutaryl-coenzyme reductase (HMGCR; 200/100 kDa): patients present with severe muscle disease with high CPK (90% >5000 U/L). No ILD or rash. Up to 80% have a history of statin exposure frequently for >1 year. Lovastatin and simvastatin more likely than other statins. If not on a statin, rule out an associated cancer or ingestion of red yeast rice and certain mushrooms (oyster, shitake) which contain statin-like compounds. Can be resistant to therapy. The autoantibody is directed against the HMGCR enzyme. Over 70% of patients have the DRβ1∗11:01 class II HLA allele.