Histopathologic features of alopecia areata incognito: A review of 46 cases

Background: Alopecia areata (AA) incognito represents a variant of AA characterized by acute diffuse hair thinning. Dermoscopy shows yellow dots and short regrowing hairs. The differential diagnosis with telogen effluvium (TE) and androgenetic alopecia may be difficult. Methods: In order to establish histopathological criteria for the diagnosis of AA incognito, we evaluated retrospectively 92 specimens (46 horizontal and 46 vertical) of 46 patients diagnosed with AA incognito within 1 year. All specimens were assessed for 20 features, including hair counts and follicular ratios. The numbers were compared with 46 control specimens, consisting of 21 cases of TE and 25 cases of androgenetic alopecia. Results: The following main criteria are proposed: (a) preserved number of follicular units and decreased number of terminal follicles; (b) increased number of telogen structures (mean count of 37%) with presence of at least one telogen germinal unit or/and one small telogen follicle (c) decreased terminal:vellus ratio (mean ratio of 3.3: 1) and (d) dilated infundibular openings. Conclusion: Two histopathologic clues for AA incognito include the presence of dilated infundibular openings and small basaloid aggregates of cells with round, irregular or polygonal shape, lack of hair shaft and no apoptosis in the outer root sheath, corresponding to small telogen follicles.