What evidence is there for this?The severity of the convulsion is dependent on the ppo2 and critically the time breathing the mix. The sooner the depth is reduced the quicker the diver will recover, as you can’t change the mix.
I suspect that it's one of the many myths surrounding oxygen toxicity - another being that they come on almost instantaneously. In my case there was at least five minutes between me becoming aware of the onset of symptoms and unconsciousness. I believe that was due to me actually being aware of what was happening and staying on top of the situation until I was safe. I'm not sure how much longer I could have done so.
The sensation of toxing was actually very pleasant and I can fully understand why anyone who was not realising what was happening would just go with it until they succumb. To anybody watching, they would appear to have just started fitting with no warning.
I have no idea what the survival rate is for divers toxing underwater but I imagine that it's not high. So there is limited first-hand evidence of what happened leading-up to the incident.
Anecdotal evidence from people witnessing such events should probably be taken with a large pinch of salt. My buddy had no idea what was happening when I suffered mine and would certainly not have been much use if he'd tried to deal with it underwater. That's no reflection on his ability as a diver or his rescue skills.