Anti-nuclear matrix protein 2 antibody-positive dermatomyositis associated with smouldering myeloma mimicking anti-melanoma differentiation-associated gene 5 antibody-positive dermatomyositis: A case of digital gangrene, palmar papules, and scrotal rash.

Dermatomyositis (DM) is an idiopathic inflammatory myopathy with characteristic cutaneous manifestations and systemic complications, including malignancy and interstitial lung disease (ILD). Myositis-specific autoantibodies (MSAs) define distinct disease subtypes, but significant clinical heterogeneity can still occur. Here, we report a unique case of smouldering myeloma-associated anti-nuclear matrix protein 2 (anti-NXP2) antibody-positive DM mimicking clinical features of anti-melanoma differentiation-associated gene 5 (anti-MDA5) antibody-positive DM, including digital ischemia and palmar papules (i.e. inverse Gottron's papules), and severe synovitis without clinically evident myopathy. Additionally, the patient exhibited a rare scrotal rash. This case expands the known spectrum of cutaneous and systemic manifestations in anti-NXP2 antibody-positive DM and emphasises the heterogeneous nature of DM, where significant clinical mimicry can occur despite the presence of a well-defined MSA. It highlights the need for further research into the immune mechanisms and biomarker profiles driving these mimicking DM phenotypes, which could improve early diagnosis, risk stratification, and targeted therapeutic strategies.