Alopecia: histologic changes.

The major types of alopecia that can be diagnosed histologically are androgenic alopecia, alopecia areata, trichotillomania, inflammatory scarring alopecias, pseudopelade, and anagen and telogen effluviums. Important to recognize are the common histologic features in chronic alopecias, which include follicular plugging, decreased numbers of hair follicles, frequent increased fibrous tracts, and superficial changes of actinic damage. Specific helpful changes to be observed are (1) the presence of actinically damaged skin that would reveal a chronic alopecic condition, (2) the presence and site of the dermal inflammation, (3) whether there is a decrease in the numbers of hair follicles and the presence of a reversal of the anagen/telogen ratio, (4) miniaturization of the hair follicle, (5) evidence of involution, and (6) the presence of scarring as demonstrated with elastic fiber stains. By assessing these histologic changes, the dermatopathologist can make a reasonably specific diagnosis of alopecia.