Migraine is a chronic disorder characterized by recurrent moderate to severe headaches often in association with a number of autonomic nervous system symptoms. The word derives from the Greek ἡμικρανία (hemikrania), "pain on one side of the head",from ἡμι- (hemi-), "half", and κρανίον (kranion), "skull". Typically the headache is unilateral (affecting one half of the head) and pulsating in nature, lasting from 2 to 72 hours. Associated symptoms may include nausea, vomiting, photophobia (increased sensitivity to light), phonophobia (increased sensitivity to sound) and the pain is generally aggravated by physical activity.Up to one-third of people with migraine headaches perceive an aura: a transient visual, sensory, language, or motor disturbance which signals that the headache will soon occur.Occasionally an aura can occur with little or no headache following it. Migraines are believed to be due to a mixture of environmental and genetic factors. About two-thirds of cases run in families.Fluctuating hormone levels may also play a role: migraine affects slightly more boys than girls before puberty, but about two to three times more women than men.Propensity for migraines usually decreases during pregnancy.The exact mechanisms of migraines are not known. It is, however, believed to be a neurovascular disorder.The primary theory is related to increased excitability of the cerebral cortex and abnormal control of pain neurons in the trigeminal nucleus of the brainstem.