Symptomatic myopathies in sarcoidosis: disease spectrum and Myxovirus resistance protein A (MxA) expression.

OBJECTIVES: Symptomatic myopathy in sarcoidosis patients is not always due to sarcoid myopathy (SM). We investigated the clinical and pathological spectrum including myxovirus resistance protein A (MxA) expression among sarcoidosis patients.

METHODS: We reviewed the Mayo Clinic database (May 1980 - December 2020) to identify sarcoidosis patients with myopathic symptoms and pathological evidence of myopathy.

RESULTS: Among 5,885 sarcoidosis patients, 21 had symptomatic myopathy. Eight carried a diagnosis of sarcoidosis 5.5 years (median) prior to myopathy onset. Eleven patients had SM. The remaining had non-sarcoid myopathies (5 IBM, 1 immune-mediated necrotizing myopathy, 1 nonspecific myositis, 2 nonspecific myopathy and 1 steroid myopathy). Estimated frequency of IBM is 85 per 100 000 sarcoidosis patients. The following features were associated with non-sarcoid myopathies (p< 0.05): 1) predominant finger flexor and quadriceps weakness, 2) modified Rankin scale (mRS) >2 at time of diagnosis, 3) creatine kinase > 500 U/l, and 4) absence of intramuscular granulomas. Sarcoplasmic MxA expression was observed in scattered myofibers in 3 patients, 2 of whom tested for dermatomyositis-specific autoantibodies and were negative. Immunosuppressive therapy led to improvement in mRS ≥ 1 in 5/10 SM, none of the 5 IBM, and 3/3 remaining patients with non-sarcoid myopathies.

DISCUSSION: Symptomatic myopathy occurred in 0.36% of sarcoidosis. IBM was the second most common cause of myopathies after SM. Frequency of IBM in sarcoidosis is higher than general population. Recognition of features suggestive of alternative etiologies can guide proper treatment. Our findings of abnormal MxA expression warrants a larger study.