Hamster death, is illness to blame or just old age?

[Mynsii]

This morning, my 3 year old Syrian, Gherkin, passed away in my arms. He was a rescue we adopted a year ago (his former owners, co-workers,, locked him in a conservatory with no food or water as they were bored of him), and apart from being deaf was in relatively good health for the entire time we had him.

This morning I found him lying on the bottom of his cage, freezing cold. Due to his age, I'd have put his death down to old age. But I picked him up, wrapped him in a blanket and held him close to my chest for about an hour. He warmed up and even started to walk around fine, when suddenly he toppled over to one side, gasping wildly for air, and one eye started to bulge (almost out of its socket) with a milky white fluid coming out from it. His nose started to bleed from one nostril (same side as the eye), and he began squeaking in pain and spasming. I held him close and whispered soothingly/kept him as calm and still as I could, for about 5 minutes until he passed away.

I've reared hams for over a decade and a half, and have comforted many hamsters as they died of old age. Normally it's quite peaceful and they simply fall asleep and don't wake up. I've also had to have hamsters PTS due to illnesses such as cancer/pymoetra. However I've never had a hamster pass away quite so violently from apparent old age. The screaming sound he was making as he gasped for air was horrible, and I was genuinely worried he eye was going to either pop or come completely out of the socket as it was bulging at about 2-3 times its regular size.

What do you think may have caused this? I was fully expecting his death, given his age, but the pain he seemed to suffer in his final moments has really shaken me.