The underlying causes of migraines are unknown. However, they are believed to be related to a mix of environmental and genetic factors. They run in families in about two-thirds of cases and rarely occur due to a single gene defect. While migraines were once believed to be more common in those of high intelligence, this does not appear to be true. A number of psychological conditions are associated, including depression, anxiety, and bipolar disorder, as are many biological events or triggers. Genetics Studies of twins indicate a 34% to 51% genetic influence of likelihood to develop migraine. This genetic relationship is stronger for migraine with aura than for migraines without aura. A number of specific variants of genes increase the risk by a small to moderate amount. Single gene disorders that result in migraines are rare. One of these is known as familial hemiplegic migraine, a type of migraine with aura, which is inherited in an autosomal dominant fashion. Four genes have been shown to be involved in familial hemiplegic migraine. Three of these genes are involved in ion transport. The fourth is an axonal protein associated with the exocytosis complex. Another genetic disorder associated with migraine is CADASIL syndrome or cerebral autosomal dominant arteriopathy with subcortical infarcts and leukoencephalopathy. One meta-analysis found a protective effect from angiotensin converting enzyme polymorphisms on migraine. The TRPM8 gene, which codes for a cation channel, has been linked to migraines. Triggers Migraine may be induced by triggers, with some reporting it as an influence in a minority of cases and others the majority. Many things such as fatigue, certain foods, alcohol, and weather have been labeled as triggers; however, the strength and significance of these relationships are uncertain. Most people with migraines report experiencing triggers. Symptoms may start up to 24 hours after a trigger. Physiological aspects Common triggers quoted are stress, hunger, and fatigue (these equally contribute to tension headaches). Psychological stress has been reported as a factor by 50 to 80% of people. Migraine has also been associated with post-traumatic stress disorder and abuse. Migraine episodes are more likely to occur around menstruation. Other hormonal influences, such as menarche, oral contraceptive use, pregnancy, perimenopause, and menopause, also play a role. These hormonal influences seem to play a greater role in migraine without aura. Migraine episodes typically do not occur during the second and third trimesters of pregnancy, or following menopause. Dietary aspects Between 12 and 60% of people report foods as triggers. There are many reports that tyramine – which is naturally present in chocolate, alcoholic beverages, most cheeses, processed meats, and other foods – can trigger migraine symptoms in some individuals. Likewise, monosodium glutamate (MSG) is frequently reported as a trigger for migraine symptoms. Environmental aspects A 2009 review on potential triggers in the indoor and outdoor environment concluded that while there were insufficient studies to confirm environmental factors as causing migraine, "migraineurs worldwide consistently report similar environmental triggers". The article suggests that people living with migraine take some preventive measures related to indoor air quality and lighting.
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