In a normal canine eyelid structure, hairs called eyelashes grow forth from the rim of the eyelid, pointing outward, and are used much like bronchial cilia to trap dust and foreign bodies from entering the sensitive membrane of the eye. There are, however, instances where this goes wrong, and it usually manifests in one of three ways. Ectopic cilia, when the eyelash grows through from the outside to the inside of the eyelid; trichiasis, which eyelashes start growing normally but turn inward; and the most severe: distichiasis, when hairs begin growing from the inside of the eyelid pointed towards the eye. In special circumstances, distichiasis is compounded with a secondary symptom called narrow palpebral fissure in which the opening afforded by a dog's eyelid is significantly smaller than normal. When this happens, it makes treatment especially problematic, and thus it's this compound problem that this article concerns itself with.

Any breed can find themselves affected by narrow palpebral fissure distichiasis, but the most common victims seem to be the Spaniels, the toy breeds, the terriers, the Boxer, Welsh Corgi, Collie, Dachshund, Doberman Pinscher, German Shepherd, Irish Setter, Great Dane, Pug, Samoyed, and Saint Bernard.

When the hairs growing inward from the eyelid are fine and soft, many times there will be no secondary problems. However, if the hairs are coarse and stiff, they'll tend to puncture the eye causing incredibly severe irritation. Dogs will tend to squint and paw at the affected eye, giving you the first warning signs that you might have a problem on your hands. Because distichiasis can cause corneal ulceration if the problem is severe enough, it's important to take any of these signs seriously.

The pressure of a narrow palpebral fissure can make diagnosing and spotting the distichiasis rather difficult. In general, a veterinarian is only able to locate all of the offending hair follicles in a case like this after multiple corneal ulcers have been treated and their point of origin noted. Dye tests and magnification can also be useful tools for diagnosis in a typical case, but again, the presence of the narrow palpebral fissure can occlude these findings.

Treatment for narrow palpebral fissure distichiasis is in two stages. Firstly, any corneal ulcers are treated with antibiotics to ward off secondary infections. Then, depending on their location, the errant hairs will either be removed with electrolysis or, if the problem is particularly severe, the follicle itself will be frozen and destroyed with cryosurgery. Generally speaking, the latter type of surgery has shown better results.

Dogs who have historically shown themselves as prone to having distichiasis are not usually good candidates for breeding, as the disease is an inherited one.