Superimposed Segmental Juvenile Dermatomyositis: A Case Report.

Superimposed segmental juvenile dermatomyositis (SSJDM) is a rare and underrecognized variant of juvenile dermatomyositis (JDM) characterized by an initial, long-standing, well-defined lesion following Blaschko lines, later accompanied by classic cutaneous and systemic features. SSJDM likely results from early developmental genetic mutations causing loss of heterozygosity and type 2 cutaneous mosaicism. We describe an eight-year-old girl with morpheiform SSJDM and review the previously reported cases, analyzing clinical features, treatment, and outcomes. We also propose a clinical classification for the segmental presentations, which include morpheiform, linear calcinosis, lichenoid plaques, and erythematous-scaly plaques. This report aims to improve recognition of atypical segmental lesions to enable earlier diagnosis and treatment of JDM.