more info
ACVS - Atlantoaxial Instability
I think the fact that you did not describe any seizure activity it affected his diaphragm and stopped breathing and why they could not revive him when you got there. Did you see any seizures? My friends dog had a massive seizure and they could not stop the seizure activity at the specialty hospital emergency with any amount of drugs so she had to put her down. It sounds like davinci did not have that from what you describe but sounds like he stopped breathing from what i read.