An occurrence of immune-mediated necrotizing myopathy following treatment of polymyositis in a Filipino female: A case report

ABSTRACT

Background: Necrotizing myopathy has been previously described but was not included in the Peter and Bohan criteria until 2004, when immune-mediated necrotizing myopathy (IMNM) was distinguished from polymyositis (PM) based on immunologic and histopathologic differences. IMNM is currently a well-recognized autoimmune myopathy and represents up to 20% of these cases. Case A 60-year- old female with biopsy-proven PM achieved sustained clinical remission with Rituximab. Her co-morbid conditions include hypertension, diabetes mellitus, and dyslipidemia. The patient noted a recurrence of gradual progressive, proximal muscle weakness and easy fatigability after receiving her first mRNA Covid-19 vaccine. Four months after onset of symptoms, CK Total was 9600 U/L. Rituximab was administered and muscle weakness and total CK levels (1247 U/L) improved within 10 days. She was prescribed rosuvastatin and fenofibrate for dyslipidemia within 7 days of completing the rituximab course. Two weeks later, proximal muscle weakness recurred. She became wheelchair-bound and experienced dysphonia. MMT score was 2/5 in proximal muscles and total CK total increased to 19,935 U/L. The patient received Methylprednisolone 500 mg IV once a day for 3 days. She had a good response with resolution of dysphonia and improvement of MMT to 4/5 on shoulder abduction and hip flexion on the 6th hospital day. She was discharged on oral methylprednisolone at 1 mg/kg/day. Muscle biopsy was consistent with an immune-mediated necrotizing myopathy, revealing necrotic fibers, intracellular macrophages, fatty infiltrates, irregular staining patterns on NADH stain with no evidence of endomysial inflammation, perifascicular atrophy, ragged red fibers, or rimmed vacuoles. Antibodiy against 3-hydoxy- 3- methylglutarylcoenzymeA reductase (HMGCR) result is pending but the other myositis-specific antibodies are negative.(including anti-SRP). Conclusion(s) IMNM is an autoimmune myopathy associated with anti-HMGCR and anti-SRP antibodies that clinically present similarly to polymyositis. The temporal occurrence of worsening muscle weakness with initiation of statin therapy make statin toxic myopathy or immune mediated necrotizing myopathy as diagnostic considerations. This case emphasizes the need to re-evaluate the etiology of new onset muscle weakness in patients with idiopathic inflammatory myopathy and highlights the role of myositis-specific antibodies and muscle biopsy in confirming the diagnosis.