A migraine headache is caused by changes in your nervous system. Migraines may progress through four stages: prodrome, aura, attack and postdrome. Not everyone experiences all the stages.

Forty to 60% of people with migraines experience a prodrome phase — subtle changes one or two days preceding the attack. This may include constipation, depression, diarrhea, drowsiness, food cravings, or hyperactivity and irritability. Alternatively, you might not notice any symptoms.

About 20% of people with migraines experience a more distinct migraine warning sign in a second phase, called the migraine aura.

Auras are usually visual but can also be sensory, motor or verbal disturbances. Visual auras are most common.

A visual aura is like an electrical or chemical wave that moves across the visual cortex of your brain. The visual cortex is the part of your brain that processes visual signals. As the wave spreads, you might have visual hallucinations.

The best known visual aura is called a fortification spectrum because its pattern resembles the walls of a medieval fort. It may start as a small hole of light, sometimes bright geometrical lines and shapes in your visual field.

This visual aura may expand into a sickle- or C-shaped object, with zigzag lines on the leading edge. As it moves, it may appear to grow. Auras are not the same for all people, so you also might experience bright spots or flashes. Auras are sometimes accompanied by a partial loss of vision referred to as a scotoma. Auras commonly last 10 to 30 minutes.

A sensory aura is also common. It can occur at the same time as the visual aura, directly afterwards or simply on its own. A sensory aura begins as a tingling in one limb or a feeling of numbness that travels up your arm over 10 to 20 minutes. The sensation can spread to one side of your face and tongue.

Another aura causes transient speech or language problems referred to as dysphasic aura. In the rarest of auras, the limbs and possibly the face on one side of your body might become weak; this is referred to as hemiplegic migraine.

A migraine aura usually precedes the migraine attack but can also occur during the attack. A migraine aura can also occur without an associated headache. The migraine attack itself can last as little as four hours or it can last several days. The attack is followed by a postdrome phase, where you might feel drained or washed out.