Migraine is a chronic disorder characterised by recurrent episodes of headache and associated symptoms. It occurs in all age groups though incidence declines with progressing age and the disease is rarely found before puberty. In the years prior to puberty, migraine is more common among boys than girls. By the onset of puberty, migraine is more prevalent in girls. The prevalence of migraine peaks in both sexes during the most productive years of adulthood (25-55 years) and then has a predilection for females in a ratio of 2:1. In the age span 30-39 years, prevalence reaches 24% in women and 7% in men. Up to 12% of the populations suffers from a form of migraine, migraine without aura being the most common accounting for up to 75% of cases.
Migraine is a syndromic disorder of the brain that is in most instances inherited. As with most common diseases, the genetic basis of migraine is likely to be complex and in some individuals may be based on the additive effect of more than one genetic source. Individuals prone to migraine have a genetic threshold that renders them susceptible to an acute migraine attack depending upon the balance between excitation and inhibition at various levels of the nervous system. Subtle abnormalities, involving membrane channels, receptor families, and enzyme systems have been linked to migraine in certain groups and individuals.
The importance of inheritance in migraine has long been recognised. One early general population based study found that the risk of migraine in relatives of migraine patients was three times greater than that of relatives of non-migraine control subjects. However segregation analysis does not identify any single Mendelian pattern of inheritance in the common forms of migraine. The common forms of migraine are likely to be complex genetic disorders, meaning that multiple genes at different genomic sites act in tandem with environmental factors to confer both the susceptibility to and the characteristics of the disease in affected individuals.
Migraine has been associated with cardiac right-to-left shunts, mostly with patent foramen ovale (PFO) or atrial septal defect (ASD), though the evidence supporting this correlation is disputed. A high quality observational study among 1101 stroke free subjects who were evaluated for cardiac shunting did not show an association between migraine and PFO or ASD.