What is entropion?
Entropion is an abnormality of the eyelids in which the eyelid “rolls” inward. This inward rolling often causes the hair on the surface of the eyelid to rub against the cornea (outer part of the eyeball) resulting in pain, corneal ulcers or corneal erosions. This corneal damage can also result in corneal scarring, that can interfere with vision.
Most dogs will squint, hold the eye shut and tear excessively (epiphora). Interestingly, many flat-faced dogs with medial entropion (involving the corner of the eyes near the nose) exhibit no obvious signs of discomfort. In most cases, both eyes are affected
The above is an excerpt from: http://www.vcahospitals.com/main/pet-health-information/article/animal-health/eyelid-entropion-in-dogs/1009
