Eyelid abnormalities are common in dogs and cats, with dogs being more frequently affected. Other than eyelid tumors, most eyelid abnormalities are inherited or breed-related.
In dogs, breed-related eyelid abnormalities include entropion (rolling in of the eyelid), ectropion (rolling out of the eyelid), euryblepharon (excessive eyelid), lagophthalmos (incomplete eyelid closure), trichiasis (normal hair contacting the eye), ectopic cilia (eyelash protruding from underneath the eyelid), and distichia (abnormal eyelashes contacting the eye).
In cats, the most common non-cancerous eyelid abnormality is eyelid agenesis, or an absence of a small or significant part of the upper eyelid. This is not an inherited condtion, but is seen predominantly in domestic shorthair cats. The second most common eyelid abnormality in cats in entropion.