All medical conditions require a risk assessment, as every venture under the water does for all of us. We all have different degrees of risk acceptance, too. For example, I have seen a woman with stage IV breast cancer post about whether it's safe to dive on chemo . . . she had a terminal diagnosis, and her risk assessment is going to be totally different from those of us who are well.
Someone who had seizures at one point, but has been seizure-free for 10 years off meds, probably has a higher risk of having a seizure than someone who has never had one, but such a person can drive . . . and having a seizure when driving has to be quite close to the same lethal risk as having one when diving. On the other hand, no one blinks at a grossly overweight, older smoker who doesn't normally exercise at all, who probably has as high a risk of a lethal event when diving as that asymptomatic seizure patient.
A few conditions carry a predictable likelihood of problems severe enough that most of us would not advise someone to take the risk. A history of spontaneous pneumothorax is one, since the recurrence rate is high, and the lethality of a pneumo underwater is going to be very high. Significant asthma with air trapping is another. Seizures requiring daily medication are another. Brittle diabetes is another, although most Type II and many well-controlled Type I's are diving safely. Uncontrolled hypertension, poorly controlled arrhythmias, congestive heart failure, narcolepsy . . . the list is long, and the conditions where I personally would say diving isn't worth the risk are the ones where a bad outcome is a) predictable and b) not within the diver's control. But in certain circumstances, like the one I referred to above, a diver might make the personal decision that the risk was acceptable to them. Then the question becomes, is it acceptable to their buddy or anyone ELSE who is or might be involved in the dive. And that's a much harder question.
