Persistent exanthema mainly on the trunk with pathologically dermal interstitial mucin as anti‐ <scp>HMGCR</scp> myopathy‐associated skin rash: Case series

Antibodies to 3-hydroxy-3-methylglutaryl-coenzyme A reductase (HMGCR) form a newly identified distinct serological marker for necrotizing autoimmune myopathy. Although anti-HMGCR myopathy is currently classified into polymyositis, skin involvement has been reported, with its characteristics only vaguely described. We retrospectively examined the clinical and histological features of non-dermatomyositis-like persistent exanthema in five anti-HMGCR myopathy patients followed up in the neurology and dermatology clinic of Huashan Hospital between December 2020 and September 2024. The exanthema presented as persistent violaceous or erythematous plaques, asymptomatic or itching, mainly distributed on the trunk, whereas dermatomyositis-specific lesions such as Gottron's sign and papules, heliotrope rash, V or shawl signs, and nailfold telangiectasia were all absent. Skin rash in 80% of patients (4/5) appeared earlier than muscle symptoms such as fatigue, proximal limb weakness, and hyperCKemia, with the median advanced time of 3 years (8 months to 7 years). Biopsies revealed scant interface dermatitis, interstitial mucin, and perivascular lymphocytic infiltrate with occasionally plasma cells and/or neutrophils. Notably, the exanthema showed no response to topical or even systemic corticosteroid but relieved after systemic immunosuppressive therapy for myopathy, in accordance with the improvement of muscle symptoms and hyperCKemia. Dermatologists should be aware of this rare entity of "pseudo-dermatomyositis", carefully evaluate muscle syndromes and carry out further investigations, including muscle biopsy and serum anti-HMGCR antibodies assays if present.