The term distichiasis refers to abnormal hairs which arise from the eyelid margin. Normally the eyelid margin is identified by small openings of ducts leading from glands deep within the eyelid. These meibomian glands are responsible for producing lipid for the surface of the tear film and are modified sweat glands. Normally they do not contain hair producing cells, but occasionally something goes wrong in embryonic development and hairs grow from these glands. The hairs are not always but usually irritating to the surface of the eye because they stimulate pain receptors in the surface of the cornea. A common sign of distichiasis is a watery discharging eye.
In some dogs such as the Staffordshire Bull Terrier the hairs can be very stiff and the discomfort they produce can cause continuing spasm of the eyelids and eventually entropion secondary to the chronic spasm. In other breeds, such as the American Cocker Spaniel the hairs might be quite fine and will not necessarily cause significant irritation but might still stimulate excessive tearing.