The two big risks associated with concussions are seizures and lethal brain swelling with a second incident. Concussions are graded according to the severity of the initial injury (length of loss of consciousness, severity of original neurologic abnormalities) and the recommendations to avoid activities with a risk of repeat head injury (eg. football or other active sports) is determined by the original grade. It can range anything from one to six months AFTER the resolution of ALL symptoms, including headaches.
Scuba doesn't present a major risk of repeat head injury (unless you do your descents head first
) so that's not too relevant.
What is relevant is the risk of post-injury seizures. Again, that risk is higher if the original injury was more severe. Seizures underwater are a lethal risk, so that possibility needs to be examined seriously.
Since I don't know any of the details of your original injury, I can't tell you much more that's useful. I would highly recommend consulting with a neurologist who can review the records of your original injury and give you accurate information about the risks in your particular situation.
You should also be aware that there are measurable neuropsychiatric abnormalities after severe concussion -- difficulty concentrating, problems with short-term memory, etc. -- that could affect how safely you can dive. You should certainly be more careful than usual, inform your buddy of your prior injury, and possibly keep the dives more conservative than normal until you have a chance to see how you are doing.
Hope this is helpful.
