Acquired hair fragility in pili anulati: causal relationship with androgenetic alopecia.

Pili anulati are defined by characteristic alternating light and dark banding in the hair shaft, due to air-filled spaces between the macrofibrillar units of the hair cortex, and are regarded as a congenital hair shaft disorder without increased hair fragility. Two cases of pili anulati are presented, in which fragility of hair developed in a causal relationship with the onset of androgenetic alopecia. Accordingly, trichorrhexis-nodosa-like hair fracturing was exclusively limited to the androgenetic region. In general, secondary trichorrhexis nodosa is an unspecific finding related to excess stress of hair in relation to its fragility. With onset of hair thinning due to androgenetic alopecia, progressive reduction of hair shaft diameter may cause increased fragility in pili anulati. In this case, hair shaft fracturing occurs within the area of androgenetic alopecia and colocalizes with the air-filled cavities of pili anulati.