Central Sensitisation and Cutaneous Allodynia in Migraine
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Cutaneous allodynia, pain resulting from application of a non-noxious stimulus to normal skin, is a recently described symptom of migraine, with a potential role in directing optimal treatment for migraine attacks. Manifestations of cutaneous allodynia include discomfort when combing the hair, shaving, and wearing glasses, contact lenses, earrings or tight clothing.
The exact mechanism by which a migraine attack is triggered is not known, but it has been theorised that, in some patients, once the attack has begun, central neurons can propagate information about the pain process without the need for further external stimuli. This process is termed central sensitisation. The trigeminal nerves, which innervate intracranial and extracranial tissues, account for head pain and other symptoms in migraine. The first-order neurons in the trigeminal ganglion receive input from the durai blood vessels, which is transmitted to second-order neurons in the trigeminal brain stem nuclear complex and is finally sent to the third-order neurons in the thalamus. Studies in humans and animals have shown that migraine pain progresses along this neural pathway, with throbbing head pain occurring early in the attack (sensitisation of first-order neurons), followed by central sensitisation and cutaneous allodynia within the referred pain area (second-order) and finally extracephalic allodynia (third-order). The data also indicate that once central sensitisation is established in the second-and third-order neurons, migraine treatment designed to prevent the initiation of central sensitisation can lessen the pain to some extent but cannot reverse it. Thus, treatment affecting the initiation of central sensitisation should be administered immediately after the onset of migraine pain to prevent intracranial hypersensitivity and the establishment of allodynia.
The serotonin 5-HT1B/1D agonist anti-migraine agents (the ‘triptans’) block meningeal nociceptor transmission at presynaptic sites in the dorsal horn. Studies have shown that triptan therapy can abort pain prior to the development of central sensitisation, but not after allodynia has been established. Therefore, in the subset of patients who report symptoms of cutaneous allodynia with migraine attacks, early initiation of triptan therapy is currently the best intervention to achieve rapid, complete and sustained pain relief.
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