The histologic findings of scalp involvement in dermatomyositis are not well characterized on account of a lack of large series.

To systematize the histologic features of scalp involvement in dermatomyositis on horizontal and vertical sections.

A descriptive, prospective, cross-sectional study recruited 31 patients with pathologically and serologically confirmed dermatomyositis in Mexico City, Mexico, from June 2014 to June 2015. A total of 36 scalp biopsy specimens from 20 patients with scalp involvement in dermatomyositis were processed as 20 vertical and 16 horizontal sections.

Dilated capillaries and diffuse mucin deposition were detected in all biopsy specimens, followed by interface dermatitis. Partial or segmental thickening of the basement membrane, hyperkeratosis, atrophic epidermis, and acrosyringeal hypergranulosis with hyperkeratosis were other very common findings. Preserved follicular architecture, with intact or slightly atrophic sebaceous glands, was present in most horizontal sections. There was decreased follicular density, with a terminal-to-vellus ratio of 4:1 and telogen count of 10.3%. Eosinophils were present in 15% and 25% of horizontal and vertical sections, respectively.

No special stains performed.

Scalp involvement in dermatomyositis shows nonscarring pattern on horizontal sections that is consistent with chronic telogen effluvium. Telangiectasia and mucin are universal histologic features; eosinophils and acrosyringeal hypergranulosis with hyperkeratosis are new findings.