There seems to be general agreement that a key element is blood flow changes in the brain. People who get migraine headaches appear to have blood vessels that overreact to various triggers.
Scientists have devised one theory of migraine which explains these blood flow changes and also certain biochemical changes that may be involved in the headache process. According to this theory, the nervous system responds to a trigger such as stress by causing a spasm of the nerve-rich arteries at the base of the brain. The spasm constricts several arteries supplying blood to the brain, including the scalp artery and the carotid or neck arteries.
As these arteries constrict, the flow of blood to the brain is reduced. At the same time, blood-clotting particles called platelets clump together-a process which is believed to release the neurotransmitter serotonin. Serotonin acts as a powerful constrictor of arteries, further reducing the blood supply to the brain.
Reduced blood flow decreases the brain's supply of oxygen. Neurological symptoms signaling a headache, such as distorted vision or speech, may then result, similar to symptoms of stroke.
Reacting to the reduced oxygen supply, certain arteries within the brain open wider to meet the brain's energy needs. This widening or dilation spreads, finally affecting the neck and scalp arteries. The dilation of these arteries triggers the release of pain-producing substances called prostaglandins from various tissues and blood cells. Chemicals which cause inflammation and swelling, and substances which increase sensitivity to pain, are also released. The circulation of these chemicals and the dilation of the scalp arteries stimulate the pain-sensitive nociceptors. The result, according to this theory: a throbbing pain in the head.
More recent neuroimaging techniques seem to show that migraine is primarily a disorder of the brain (neurological), not of the blood vessels (vascular). A spreading depolarization (electrical change) may begin 24 hours before the attack, with onset of the headache occurring at about the time of maximum brain coverage. The effects of migraine may persist for some days after the main headache has ended. Many sufferers report a sore feeling in the area where the migraine was, and some report impaired thinking for a few days after the headache has passed.
In 2005, research was published indicating that in some people with a patent foramen ovale (PFO), a hole between the upper chambers of the heart, migraine might result and that the occurrence of migraines might end if the hole were blocked. Several clinical trials are currently under way in an effort to determine if a causal link between PFO and migraine can be found. Early speculation as to this relationship has centered on the idea that the lungs detoxify blood as it passes through. The PFO allows blood to go directly from the right side of the heart to the left without passing through the lungs.
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