Listed here are some problems that may affect your dogs eye(s). Veterinary consultation in these matters are a MUST & your vet should always be consulted if you are at all worried.
Entropion - Inward turning of eyelids
Ectropion - Outward turning of eyelids
Cherry Eye - Prolapsed Gland of the 3rd eyelid
Below are descriptions of these conditions that affect the over all health of a St Bernards eye.
Entropion is the turning in of the eyelid, allowing the eyelashes to rub on the cornea and can cause complications. The direct cause of this is some what complicated, but is primarily due to the massive size of the Saints head which commonly has larger eye sockets. While the actual eye ball can often be of normal size, if the socket is too large, the eye balls seems to be small but instead is just set back into the socket. Thus, it is common to find the lower eyelid "hitting" the lower part of the eye ball instead of gently rolling up and over to the middle of the eye to meet the upper eyelid with each blink or squint. Not only can this cause irritation, but another concern for this condition can occur when the dog reaches an older age. Because the eye ball has a "fat" layer behind it, as the dog progressively ages, the "fat" layer thins and can cause the eye to sink back even deeper. When considering potential puppies, try to look at their parents eyes. Although the standard calls for a smaller eyed Saint, it is important that the eye not be set back into a large deep socket or problems may arise as the dog ages.
Ectropion is an outward turning of the lower eyelid, which is condition that is mainly inherited due to the genetic make up of the more massive type St Bernard heads. Typically a dog that has a massive head, will also have extra skin that can accommodate their growing skulls. Since the head type is genetic, these dogs will often have more elasticity also which allows their skin to stretch over the head. The combinations of these issues can lead to ectropion, which leaves the eye exposed to irritation and/or infection. Surgery can be performed to correct it. Again, when considering potential puppies, try to evaluate their parents eyes yourself.