A comparative study of histopathologic features in alopecia areata and pattern hair loss.

Background: When peribulbar infiltrates are absent, other histopathologic findings are necessary to distinguish alopecia areata (AA) from pattern hair loss (PHL). The purpose of this study is to determine which histopathologic features are most useful for differentiation.

Methods: A retrospective slide review was conducted of AA and PHL scalp biopsy specimens from 2014 to 2019 at a tertiary referral center.

Results: Ninety-six cases were retrieved, of which 38 were AA. Peribulbar infiltrates were identified in 24 AA (63.2%) cases. A catagen/telogen shift was observed more frequently in AA than PHL (25 cases, 65.5% vs. 10 cases, 17.2%; p ≤ 0.0001). Lymphocytes (4 cases, 10.5% vs. 1 case, 1.7%; p = 0.058) and melanin (25 cases, 65.8% vs. 5 cases, 8.6%; p ≤ 0.0001) in fibrous tracts were more common in AA. Apoptotic bodies within vellus hairs were more frequently identified in AA (32 cases, 84.2% vs. 37 cases, 63.8%; p = 0.030). Small dystrophic follicles were also more common in AA (16 cases, 42.1% vs. 1 case, 1.7%; p < 0.0001).

Conclusions: Common features of AA other than peribulbar infiltrates include a catagen/telogen shift, melanin in fibrous tracts, and small dystrophic follicles. Practitioners should consider these features when distinguishing AA from PHL in specimens without peribulbar infiltrates. The retrospective design limits our ability to exclude multifactorial alopecia, such as telogen effluvium.