Danish scientists have made a key discovery in what triggers a migraine - a step that could pave the way for better treatment of the debilitating condition.

They say their findings contradict a long-held theory about why the head hurts during an attack.

It's long been thought that the throbbing pain of migraine is caused by an expansion of the arteries on the outside of the skull.

Now, the Danish research suggests there could be another reason for the pain: nerve fibres around the blood vessels become extra sensitive.

The researchers came to this conclusion after looking at MRI scans of 19 women who suffered from migraines, examining their arteries during an attack.

The women were all considered to be healthy and suffered from migraine without aura i.e. they did not suffer from visual disturbance.

The researchers explained that this means that the migraine is limited to one side of the head - enabling their other, unaffected side of the head to be scanned for differences, ScienceNordic reported.

They discovered that, contrary to popular belief, the blood vessels did not expand - leading them to think there must be another trigger.

They found that the arteries on the outside of the skull did not expand during migraine attacks - and those inside the skull were only slightly expanded on the side where the headache was felt, compared to the other side where no pain was felt.

This also raises the question of how the commonly prescribed migraine drug, sumatriptan, may work, reported the researchers in The Lancet Neurology.

Migraine patients were given the drug before having another MRI scan.

This showed that the blood vessels inside the skull were still slightly expanded even when the migraine eventually went - meaning that the drug doesn't work as previously thought.

The new theory, said the researchers, is that migraine pain is due to nerve fibres around the blood vessels becoming extra sensitive.

"Our findings are of great importance to our understanding of migraine headaches and future research on migraines," said study co-author Faisal Mohammad Amin, a PhD student at the Danish Headache Centre, Glostrup Hospital, Denmark.

"At the same time, the findings can also be used to reassure migraine sufferers who worry that their arteries are about to explode during an attack. They're not."

Commenting on the study, Helge Kasch, a headache researcher Aarhus University's Department of Clinical Medicine, told the website:

"The findings indicate that a change occurs in the nervous control of the blood vessels locally in the brain, and at the same time there is a change in the pain regulation," he says.

"Future medical treatment should also seek to ensure that the drug will function inside the central nervous system/the brain and must be able to pass the blood-brain barrier."