The primary cause of entropion is a genetic or conformational anatomic abnormality in the eyelids. It most commonly affects purebred dogs and is considered inherited in Chow Chows, English Bulldogs, Irish Setters, Labrador and Golden Retrievers, Rottweilers, Shar-Peis, Saint Bernards, Great Danes and Chesapeake Bay Retrievers, among other breeds.
Entropion most commonly involves the lateral lower eyelids, but the upper eyelid can be affected in breeds with a heavy brow, such as bloodhounds, Shar-Peis and Chow Chows. The medial canthus (corner of the eye where the upper and lower eyelids meet) is most commonly affected in brachycephalic breeds including Shih Tzus, Pekingese, English Bulldogs, Pugs and Cavalier King Charles Spaniels. Primary entropion is also common in large breed male cats.