Migraines affect one in ten people
The visual pathway that underlies a migraine sufferer's sensitivity to light has been uncovered by Harvard scientists.
The researchers studied two groups of blind people who suffered migraine headaches.
They found light triggered a reaction in a group of brain neurons that remained active for some time.
Migraines are one-sided throbbing headaches that cause nausea and affect up to six million people in the UK.
The researchers, writing in the journal Nature Neuroscience, said they noted that even blind people who had migraines experienced sensitivity to light or photophobia.
The observation led them to the idea that the signals transmitted from the retina via the optic nerve were somehow triggering worsening of the pain.
They looked at 20 blind people who fell into two groups - the first were totally blind due to eye diseases such as retinal cancer and glaucoma.
They were unable to see images or to sense light and therefore could not maintain normal sleep-wake cycles.
Patients in the second group were legally blind due to retinal degenerative diseases, such as retinitis pigmentosa.
Although they were unable to perceive images, they could detect the presence of light and maintain normal sleep-wake cycles.
The patients in the second group described intensified pain when they were exposed to light, in particular to blue or grey wavelengths.
Rami Burstein, professor of anaesthesia and critical care medicine at the Beth Israel Deaconess Medical Centre in Boston, US, led the study.
Professor Burstein said: "While the patients in the first group did not experience any worsening of their headaches from light exposure, the patients in the second group clearly described intensified pain when they were exposed to light.
"This suggested to us that the mechanism of photophobia must involve the optic nerve, because in totally blind individuals, the optic nerve does not carry light signals to the brain.
"We also suspected that a group of recently discovered retinal cells containing melanopsin photoreceptors (which help control biological functions including sleep and wakefulness) is critically involved in this process, because these are the only functioning light receptors left among patients who are legally blind."
Lee Tomkins, director of Migraine Action, said: "We have known from previous research that this sensitivity can be triggered by the blue light waves in the light spectrum, but avoiding grey light is a new aspect, and we are now wondering if this research might help us to further understand why so many people with migraines are sensitive to low energy light sources as well."
Migraine pain is believed to develop when the meninges, the membranes that surround the brain and central nervous system, become irritated.
This stimulates pain receptors and triggers a series of events that lead to the prolonged activation of groups of sensory neurons.
The researchers also examined what happened when they injected dyes into the eyes of rats.
They traced the path of the melanopsin retinal cells through the optic nerve to the brain where they found a group of neurons that become electrically active during a migraine.
"When small electrodes were inserted into these 'migraine neurons', we discovered that light was triggering a flow of electrical signals that was converging on these very cells," said Professor Burstein.
Even when the light was removed these neurons remained activated.
"This helps explain why patients say that their headache intensifies within seconds after exposure to light, and improves 20 to 30 minutes after being in the dark."
Dr Sue Lipscombe, a GP from Brighton with a special interest in headaches, said: "It suggests that an external source is contributing to the migraine condition and it is not just a contained brain disorder that just takes its cycle but that there are outside modifying factors."