Migrains and physical activity

  On numerous occasions within the last 4 years I have had episodes of heat exhaustion typically characterized by nausea, light-headedness, disorientaion, tingling in the fingers, spotty & tunnel vision, and subsequent severe migrain headaches lasting for 3-5 hrs.  This condition is brought on by moderate to heavy exercise, typically compounded by, but not limited to, hot and humid conditions.  This occurs despite absurdly high levels of water intake. This is only brought on by heavy physical activity.  I do have a history of dehydration and heat related problems despite water intake.
  My military career, as well as my interest in exercise and sports, is in jeopardy and would like to get a grasp on what is going on. Thanks for your time.
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Generally the advice when there are obvious precipitating factors for
migraine headaches is to avoid the precipitating factor, in your case
at least in the context of your military career the option to do this
would appear to be limited.
The options are very restricted in this situation, either you avoid the
preciptiating factors or try medications which prevent the onset of
migraines, either using them constantly or before you start to exercise.
One of the more commonly used medications is propanolol however it use is
not compatible with strenuous exercise.
Another preventative medication is Depakote, its use in this type
of situation is unusual, but might be worth a try, otherwise pou will have
to modify to some extent, the exertion which is leading to the problem.