Doctors believe the underlying cause of migraines is related to changes in the blood flow in certain areas of the brain, with subsequent alterations in the blood vessels in that region. Chemical reactions may lead to swelling and irritation of the blood vessels (thought to contribute to the characteristic “throbbing” quality of a migraine headache).The specific cause of tension-type headache is unknown. Doctors believed at one time that chronic muscle tension was the underlying factor, but there now appears to be a more complicated combination of factors that are involved.