Histopathology of alopecia areata, acute and chronic: Why is it important to the clinician?

Alopecia areata (AA) is often easy to diagnose but a scalp biopsy for horizontal sectioning is routine in this research clinic. The characteristic histological feature of AA is the peribulbar and intrabulbar mononuclear cell infiltrate, which occurs in the acute stage of the disease but may be absent in biopsies taken at a later stage. AA evolves through acute, subacute, chronic, and recovery phases. Increased numbers of terminal catagen and telogen hairs are found in the acute and perhaps subacute stages with increased numbers of miniaturized, vellus-like hairs in the subacute and chronic stages. Thus, it is important for clinicians and pathologists to recognize the different phases of AA, so that in the absence of the classic findings of a peribulbar lymphocytic infiltrate, a diagnosis of AA can still confidently be made.