Aura in Headaches: What Causes It?
Migraine with aura is one of the most common types of headache that are preceded by an aura, a collection of symptoms that occur just before the pain of the headache begins. A migraine aura is due to a complex set of events that happen in the brain; our understanding of exactly what causes an aura is at a relatively early stage.
What is a Migraine Aura?An aura is a phase of a migraine headache that comes on within an hour or so of the headache pain itself. It may only last a few minutes in some people. Visual disturbances are the most likely form that the aura can take. People with migraine report seeing flashes of light, wavy lines, blurred vision, temporary blindness, spots of darkness in their vision and distorted images of the world. It has been suggested that Lewis Carroll, author of the book ‘Alice Through the Looking Glass’, was a migraine sufferer who experienced aura. His story of the different adventures of Alice may have been inspired by the images he saw in one of his migraine attacks.
In some people, a migraine aura includes more than just changes in their vision. It can also make their skin and limbs feel very strange, with patches of pins and needles, numbness, muscle weakness and trembling. Some people also feel very dizzy and unable to stand during the aura phase of their headache, while others experience ringing in the ears (tinnitus) and other hearing problems.
The Difference Between Aura and Prodomal SymptomsA migraine headache, whether it is preceded by an aura or not, can also include a much longer pre-headache phase called a prodomal stage. This can come on 1-2 days before the migraine headache or the actual aura begins. The symptoms of the prodomal migraine stage are much less specific. People report feeling generally tired, unwell, and tend to yawn a lot and have a great deal of trouble concentrating. This should not be confused with an aura – this is specifically defined as being the stage immediately before the migraine itself.
What Causes a Migraine Aura?We used to think that migraine was primarily caused by problems with blood vessels within the brain but researchers now think that abnormal electrical impulses in the nervous tissue are the root cause. These affect blood vessels, but the problem begins in the nervous system, not in the blood system.
A phenomenon called cortical spreading depression has been described and this is thought to be very important in the development of a migraine. This is caused by electrical activity at a much higher level than normal that spreads through the cortex of the brain, working over the cortex like a wave. As the wave of depolarisation spreads through the brain tissue, it passes through the visual centres in the brain, which may be why an aura includes so many symptoms that affect the vision. It also spreads through the motor areas, so affects the way people can move and produces the other sensations of numbness and tingling.
As the wave of electrical activity passes through the cortex, it induces changes in the blood vessels. As the wave first reaches blood vessels, they constrict and as it passes by they expand again, which can cause pain. The wave of activity also reduces the take up of oxygen by the tissues, causing a lack of oxygen in the brain tissue, which may also be responsible for some of the experiences of aura.