[QUOTE=Sonia59]

At my shelter we had one or two very young kittens who suddenly died, but they were not very strong and fighting to survive so it is different.
But three days ago an adult cat among the permanent residents (he was not social with humans so he could not be adopted) suddenly died. He was 4-years old. We found him half-way out of the litter box. One hour before he was perfectly OK...an autopsy has been made and we wait for the results. We were all shocked.
QUOTE]

You may find that he had a cardiac episode. Cat anatomy is similar to humans. A vagal nerve response can occur when toileting, causing the heart to slow down greatly, and when there is a heart condition present, this can be enough to cause cardiac arrest. If this is the case, the kitty went quickly with little pain.

I am a R.N. and Paramedic, and I have seen many a person on the toilet in this condition over the years.