At the Mayo Clinic, skin biopsy specimens of 55 patients with dermatomyositis (DM) were reviewed. Twenty-four patients had specimens taken from areas of clinical DM, whereas 31 patients had specimens obtained over muscle biopsy sites, without concern for the presence of DM. From this study, three conclusions were drawn: (1) Poikiloderma on a well-selected skin biopsy may be diagnostic of DM, if correlated with a compatible clinical history. (2) The histopathology of DM can resemble lupus erythematosus. (3) Mucin in an otherwise nonspecific skin biopsy, although not diagnostic, is suggestive of DM.
Janis JF, Winkelmann RK. Histopathology of the Skin in DermatomyositisA Histopathologic Study of 55 Cases. Arch Dermatol. 1968;97(6):640-650. doi:10.1001/archderm.1968.01610120030004