The International Headache Society has established criteria for the diagnosis of migraine, to standardise headache classification for surveys and clinical trials. Nevertheless, a third of migraine headaches are bilateral, and not all throb or are accompanied by nausea and photophobia. Such neurological symptoms usually precede the headache, but may occur without an ensuing headache or may appear while a headache is in progress. However, most migraine headaches occur without such symptoms. Varieties of migraine are designated hemiplegic, basilar, ophthalmoplegic or retinal, depending on the area of the brain rendered ischaemic.
Neurological symptoms persisting for more than seven days indicate a migrainous infarction.
Genetic factors contribute about half of the susceptibility to migraine, and the gene for some kinships with familial hemiplegic migraine has been localised to chromosome 19. 4 Neurobiology of migraine Modern techniques have improved our knowledge of both aura and headache mechanisms. However, recent studies have shown that this applies to only about a third of migraine patients. Measurements of velocity of blood flow with transcranial Doppler ultrasound have shown that the middle cerebral artery dilates during migraine headache and returns to normal when the headache resolves after the injection of sumatriptan. 7 Cerebral oedema may also be a feature of some migraine attacks, as there have been two reports of skull defects bulging during the headache phase.