Understanding fibrosing alopecia in a pattern distribution: a retrospective study with histological challenges in the diagnosis of diffuse central alopecia.

Fibrosing alopecia in a pattern distribution (FAPD) has been described as a diffuse alopecia in the central region of the scalp with clinical and histopathological features of both androgenetic alopecia (AGA) and lichen planopilaris (LPP). Review cases that met clinical and microscopic criteria for FAPD with a minimum follow-up period of 18 months. We evaluated 28 histopathologic criteria in 35 scalp biopsies from 22 patients. The cases were separated into two groups according to the presence or absence of cicatricial fibrous tract (CFT). Low terminal to vellus hair ratio was seen in 45% of samples and 55% of biopsies presented lichenoid inflammation. Among 9 patients with CFT, four developed well-defined areas of alopecia in the follow-up. A wide range of histological features were observed and very few cases showed alterations of both AGA an LPP simultaneously. FAPD is a term that could be used either when diagnostic signs of LPP are not present or perifollicular fibrosis and higher terminal to vellus hair ratio forbid an undisputed AGA diagnosis. The presence of CFT and lichenoid inflammation may be a warning sign for actual LPP with initially diffuse distribution on the scalp and potentially scarring outcome.