A case of inclusion body myositis masquerading as statin-induced myopathy

Background: Inclusion body myositis (IBM) is a common acquired myopathy in individuals older than 50 years of age. This idiopathic inflammatory myopathy carries an insidious progression with frequent delays in diagnosis and a high incidence of misdiagnosis resulting in significant morbidity and disability for affected individuals. Clinical features include asymmetric weakness predominantly affecting the quadriceps and/or finger flexors, and a slow, progressive course leading to loss of ambulation and dysphagia. Creatinine kinase (CK) levels are usually less than 10 times normal and erythrocyte sedimentation rate (ESR) and C-reactive protein (CRP) fall within normal limits. Myositis-specific autoantibodies are typically absent in patients with IBM; however, highly specific positive cytoplasmic 5'-nucleotidase 1A (cN1A) autoantibodies can help distinguish IBM from other forms of myositis, such as polymyositis. There is an association of IBM with autoimmune diseases, such as Sjögren's syndrome, sarcoidosis, and some lymphoproliferative disorders. Unlike many other autoimmune diseases, the condition typically affects males more frequently with a male-to-female ratio of approximately 3:1. Mean age of symptom onset ranges from 61 to 68, with over 20% of patients developing symptoms in their forties.

Case presentation: A 71-year-old male patient with a long-standing history of hypertension and dyslipidemia was evaluated for complaints of progressively worsening proximal muscle weakness, fatigue, and dyspnea for more than 2 years in duration. He had been prescribed 40 mg of atorvastatin and was presumed to have statin-induced myopathy after taking the drug for 1 year. Physical examination revealed an otherwise healthy male with proximal muscle weakness with stable vital signs. CK was within normal limits and CRP was 6.6. He was noted to have a negative ANA screen extensive myositis and HMGCR antibody, but was found positive for cytoplasmic 5'-nucleotidase 1A (cN1A) autoantibodies, thus confirming suspicion of IBM. MRI of the thigh demonstrated generalized muscular atrophy with no active inflammation. Given the prolonged nature of the disease in the patient, no active inflammation was noted. He is responding well to tapering doses of steroids and other immunomodulatory options being discussed with him.

Conclusion: This case illustrates the importance of appropriate recognition of IBM, and the challenges associated with diagnosis and management. This acquired myopathy may be mistaken for other conditions such as statin-induced myopathy and should be considered in the evaluation of progressive muscle weakness in individuals over 50 years of age. Appropriate intervention is essential to prevent progressive disability with rapid loss of strength and function in affected individuals.