Migraine
Aetiology
Genetic
Developments in the study of genetics have made it far easier for scientists to identify and clone genes. DNA can be directly examined in order to identify genes relating to specific clinical conditions. Linkage studies can identify markers for human diseases without the knowledge of any abnormal gene product associated with the condition's aetiology. Researchers use information regarding the distance between specific DNA segments, or markers, to quantify the association between markers and particular diseases within families. Close associations allow research to follow up to identify candidate genes located in the region of that chromosome. For example, if migraine was found to be associated with a marker near to the gene coding for an aspect of the 5-HT system then the association between the DNA segments coding for this gene could be investigated in families subject to migraine. Whilst any identified gene may not be the sole cause of the condition it would have serious implications on the classification, treatment and pathophysiology of the condition.
Many twin and family studies have been carried out to investigate the involvement of genetics in the aetiology of migraine. The evidence suggests strong genetic components are present in the aetiological process of migraine. The genetic components do seem to be stronger in some types of migraine over others, for example familial studies showed greater genetic correlations in migraine without aura than migraine with aura. (Russell & Oleson, 1993). The heritability of migraine appears to only be moderate, suggesting that the aetiology is multifactorial. Gene interactions may be important but they are not the only factors to identify. Whilst research continues into the genetics of migraine it is also important to identify the other causal factors too. [10, 21, 26, 42, 44]
Environment
Environmental factors have a considerable contribution to the aetiology of migraine. Ranging from diet, including alcohol, tyramine (found in cheese and fermented foods), the flavour enhancer monosodium glutamate and phenylethylamine (found in chocolate), to changes in sleep patterns, weather and hormones the list of precipitating factors is extensive. Migraine is usually only precipitated when a number of factors are experienced within a short time frame (less than 12 hours). (Solomon et al, 1998) Elimination of multiple stimulatory factors has, in some patients, resulted in a reduction in attacks by up to 50%. Sleep has also shown to be effective in aborting migraines in many patients, suggesting that a lack of sleep may be a strong precipitating factor. [8, 51]
Pathological
It has been observed in many patients that a number of pathological factors are altered in the state of migraine, suggesting that they may have a role in the aetiology.
- Migraine appears to have a circadian rhythm, similar to conditions such as myocardial infarction and angina pectoris.
- Also there have been very specific changes in blood flow observed, including generalised systemic vasoconstriction and local cerebrovascular vasodilation in the region of the head pain.
- Platelets have been noted to release serotonin during a migraine attack.
- Neuropeptides, including CGRP (Calcitonin Gene-Related Peptide) and Sub P (Substance P), levels are elevated during a migraine attack.
These factors are more fully examined in the Pathogenesis section.
