The vascular theory of migraine suggested that changes in the blood vessel size and blood flow were responsible for the development of migraine attacks. This theory went out of fashion and for the past 20 years most headache experts thought that the process of migraine begins in the brain and not blood vessels. A new study by researchers at the University of Pennsylvania seems to again implicate blood vessels as the culprit.
Brain is supplied by four blood vessels that come up from the neck into the brain – two carotid and two vertebral arteries. At the base of the brain they connect with each other making a circle of Willis. Thomas Willis was a 17th century English physician who first described this circle. This circle ensures good blood flow to the brain even if one or even two of the four blood vessels become occluded. Only a third of the population actually has a complete circle connecting all four arteries, while in the rest the circle is incomplete.
This is not a new finding – a group of French physicians reported this discovery in 2009. However, the current study showed that having incomplete circle affected cerebral blood flow and this may be contributing to the process of triggering migraines.
This abnormality appears to be particularly common in those who have migraine with aura. The study looked at 170 people from three groups – a control group with no headaches, a group that had migraine with aura, and a group that had migraine without aura. An incomplete circle of Willis was more common in people with migraine with aura (73 percent) and migraine without aura (67 percent), compared to a headache-free control group (51 percent).
One of the authors commented that “People with migraine actually have differences in the structure of their blood vessels — this is something you are born with” and, “These differences seem to be associated with changes in blood flow in the brain, and it’s possible that these changes may trigger migraine, which may explain why some people, for instance, notice that dehydration triggers their headaches.” A very interesting observation was that “Abnormalities in both the circle of Willis and blood flow were most prominent in the back of the brain, where the visual cortex is located. This may help explain why the most common migraine auras consist of visual symptoms such as seeing distortions, spots, or wavy lines”. It is also possible that the increased risk of strokes in patients with migraine with aura is due to this anatomical defect.
It is most likely that having an incomplete circle of Willis is only one of many predisposing factors. Unfortunately, we cannot do much about this congenital abnormality, but we do have many ways to prevent migraine headaches even without fixing this problem directly.
