Pattern alopecia (androgenetic balding)

The essential histologic finding in pattern alopecia is progressive miniaturization of the follicular unit. This occurs predominantly in the central scalp, and in an asynchronous fashion. The biopsy will demonstrate variability in hair diameter. Focal spongiotic infundibulofolliculitis is common (mild seborrheic folliculitis). In long-standing cases, solar elastosis as well as elastotic degeneration of the fibrous tract remnants may be seen. Advanced pattern alopecia demonstrates a marked increase in vellus hairs (hair shaft diameter < inner root sheath diameter). Large sebaceous glands may be present, especially in males.

Telogen effluvium

Telogen effluvium has many telogen hairs. Telogen effluvium caused by premature interruption of anagen growth (such as by a febrile illness or crash diet) lacks the miniaturization of pattern alopecia. The shortened anagen cycle of pattern alopecia results in an altered anagen/telogen ratio, but not to the extent seen after febrile illness or crash dieting.

Trichotillomania (trichotillosis)

Traction alopecia

Alopecia areata

The lymphocytes appear to target melanocytes within the hair bulb. White hairs are spared. The inflammation results in damage to the hair matrix, tapered hair shafts with fracture, and miniaturization of the follicular unit.

Syphilitic alopecia

Alopecia mucinosa

Folliculotropic mycosis fungoides (cutaneous T-cell lymphoma)