Migraines are usually broken into two classifications: migraines with aura — which used to be called classic migraine — and migraines without aura — which were once called common migraines.
In migraines, auras are considered a warning sign. They are a pre-migraine symptom. Aura occurs before the pain during a phase that is sometimes called the prodrome. Aura does not occur in all people with migraines. More than a third of migraine sufferers, 36 percent, say their migraine is accompanied by aura. Those who do experience aura, don’t necessary experience aura with each migraine. Also, there are some people who experience migraine aura without head pain.
When migraine sufferers undergo MRI (magnetic resonance imaging) during aura, in certain patients, the changes in the brain (officially called a cortical spreading depression) can be seen slowly moving across part of the brain. It is thought that auras are caused by a wave of nerve signals or impulses that move across the brain, causing a disruption in normal activity in those parts of the brain. These changes are thought to cause the changes in vision or feelings of numbness.