A case of anti-nuclear matrix protein 2 antibody-positive dermatomyositis sine dermatitis with a challenging diagnosis due to the absence of typical skin manifestations.

Dermatomyositis (DM) is classically associated with distinctive skin manifestations, such as Gottron's sign or heliotrope rash. Cases of DM without these skin manifestations - termed DM sine dermatitis (DMSD) - have recently been reported, with an increasing association with anti-nuclear matrix protein 2 (NXP-2) antibody. We report a 26-year-old Japanese man who presented with muscle weakness and myalgia but without the characteristic skin manifestations of DM. Laboratory findings showed only mildly elevated levels of myogenic enzymes, and initial screening for myositis-specific antibodies (MSAs) was negative. However, the muscle biopsy demonstrated necrotic and regenerative muscle fibres with perifascicular expression of myxovirus resistance A. The findings were consistent with DM. Further MSA testing revealed positivity for the anti-NXP-2 antibody, confirming the diagnosis of DMSD. This case presented a diagnostic challenge because of the absence of typical skin rashes and only mild elevation of myogenic enzymes. Clinicians should be aware of DMSD as a potential diagnosis, particularly in patients with muscle symptoms but lacking the characteristic DM skin manifestations. In a myositis diagnosis, proactive muscle biopsy and MSA testing, including the anti-NXP-2 antibody, are crucial for ensuring early diagnosis and treatment.