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While I wrote the post, I did think about nausea and the fact that you wrote earlier about throwing up in a FFM. But I have no experience with that, I just know how to do it through an ordinary reg.
A convulsion or seizure is simple: lights out, like the flip of a switch. You won't have any memory of it. So yes, you're completely out of control.
Can you feel it coming? Small chance.
As I understand it, sometimes you get a warning (see VENTID or ConVENTID for the signs). Sometimes you don't. When you do, it may well be like the warning I got when I saw the bicyclist coming at me in my lane in his tuck position as I was riding in a curving underpass--it gave me a chance to scream before impact. On the other hand, that may be all the warning you need. In one famous case, a diver getting such a warning signalled for an air share from a buddy and thus was able to get a safe gas in his mouth with a hand holding it there when he seized.
Schwob what they said ^^^^^^^^
...Have protein drinks for breakfast!