No one really knows what causes migraine headaches, but several theories have been put forward involving genetics, blood vessels, the nervous system, a brain chemical called serotonin – also known as 5-hydroxytryptamine (5HT) – and a heart defect.
As it has been observed that migraine often runs in families, some researchers believe genetics does play a part in migraine causes. They believe that many of the migraine sufferers are born with a predisposition to migraines.
There is a long held view migraines occur when the blood vessels constrict and then expand. The resulting fluctuations in blood flow in the brain leads to the start of migraine headaches.
The Nervous System
This view reinforces the blood vessel theory in that a migraine attack begins with oversensitive brain cells triggering the nerves to release brain chemicals. These brain chemicals cause the blood vessels in the brain to swell, leading to throbbing pain and other migraine symptoms.
The hypothalamus is the part of the brain that is associated with much of the nervous system and it controls appetite and emotions, among several other things. This may explain prodromal symptoms such as food cravings and mood swings.
It has been suggested that levels of serotonin or 5-hydroxytryptamine (5HT), are low at the beginning of an attack. Serotonin is important for normal brain function and affects the size of the blood vessels. It is thought that drugs called triptans – also known as 5HT agonists – constrict the blood vessels in the brain by balancing the levels of serotonin.
An injection of serotonin has been shown to end an attack. However, this is not recommended as a treatment for migraine headache as this has a number of adverse side effects.
Recent research has suggested a possible link between migraine with aura and a hole in the heart, which is known medically as a patent foramen ovale (PFO). PFO is a small hole in the wall that divides the two upper chambers of the heart (the atria). All babies have this hole whilst in the womb, so that blood is circulated more efficiently, The hole usually closes after birth, but in some people, it stays open.
Tests show that 60% of migraine sufferers have larger than average PFOs, compared to 10% of non-sufferers. It is thought that the problem can lead to impurities not being filtered out of the blood properly. In those without PFO, all of the blood returning to the heart being pumped around the body is cleaned and filtered by the lungs.
However, in those with the condition, unfiltered blood may get through the hole. It is believed that when this unfiltered blood reaches the brain, it can trigger a migraine in some people. It is possible to close a PFO using corrective surgery but, following a trial involving more than 100 people with severe migraines, concerns have been raised over the risks involved, especially given the small reduction in migraine frequency that was reported.
All of the above-mentioned factors may be interrelated and linked to the causes of migraine headaches. For example, a migraineur could inherit an oversensitivity to stimuli, such as bright light, loud noise or physiological disturbances, including blood sugar changes, altered sleep patterns or dehydration. Any of these could trigger the release of serotonin, causing the blood vessels first to narrow and then expand, could cause a migraine. In some cases where a migraineur inherits a PFO, similar changes in the brain as a result of the PFO may then cause a migraine.