Hair Casts: What are they and why do they form?

What are hair casts?

Hair casts are thin, elongated, cylindrical concretions that encircle the hair shaft. Hair casts range in size from 2-7 mm and can be easily moved and dislodged. The term was coined back in 1957 by Kligman.

Hair casts (sometimes called “pseudonits”) can be easily differentiated from true “knits" because hair casts slide along hairs when grabbed with the fingers. True nits are glued to the hairs and don’t move. Hair casts are usually asymptomatic and particularly common in young women.

Hair casts are said to be "primary" in nature when not associated with an underlying scalp disorder and "secondary" when associated with an underlying disorder. Common secondary causes include psoriasis, seborrheic dermatitis, pemphigus and traction alopecia and scarring alopecia. Many other causes are possible too including hair sprays and deodorants.

Hair casts are thought to represent material from both the internal root sheath and the external root sheath of the hair follicle.

The photo here shows casts in a patient with the scarring alopecia lichen planopilaris.