The nasolacrimal duct drains tears from the spectacle into the oral cavity. It can get blocked by substrate, hair from prey or other small items the snake ingests.
Some animals can be predisposed to infections of this duct. Either way, by infection or by blockage, the tears are unable to drain from the eye into the mouth and the eye begins to bulge.
I've had it occur from a blockage via ingestion in a black-headed python. The snake's eye ended up filling up with clear fluid and bulging. Prior to that, he showed a disinterest in food and was rubbing his face on objects in his enclosure. It was also fairly obvious that he was having issues seeing out of that eye.
The Vet diagnosed it immediately and removed the blockage - fur and a bit of aspen inside of his mouth. This was followed by a spurt of clear fluid and immediately his eye was back to normal.
As a precaution against potential secondary infections, he was put on Gentamyacin antibacterial eye drops for 14 days. He has had no reoccurrence.
May I ask what issues you are having?
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