January 25, 2010

How Light Boosts Migraine Pain

Photo of a woman holding her hand to her head

Most migraine sufferers know that light can intensify headache pain. A new study of blind patients with migraine may help explain why. The finding ultimately may lead to new approaches for calming severe light-induced headaches.

More than 1 in 10 people nationwide experience recurring headaches known as migraines. They're often described as a pulsing or throbbing in one side of the head. Other symptoms include nausea, vomiting and extreme sensitivity to sound. Exposure to light often triggers or intensifies the pain, but the underlying mechanism has been unclear.

To gain a better understanding of light's role, Dr. Rami Burstein of Harvard Medical School and his colleagues evaluated 20 migraine sufferers who were also blind. Six participants were unable to detect any light, either because their optic nerves had been damaged or their eyes removed due to disease. The remaining 14 were unable to perceive images, but their eyes could detect some light, even if they were not aware of it. Their sleep-wake cycles were normal, whereas the other 6 had disrupted sleep patterns. The study was supported by NIH's National Institute of Neurological Disorders and Stroke (NINDS) and by Research to Prevent Blindness.

As reported in the January 10, 2010, online edition of Nature Neuroscience, the researchers found that light exposure intensified migraine pain in the 14 people with some light detection but not in the remaining 6 who were totally blind. The researchers concluded that the optic nerve, which carries light signals to the brain, must be key to light-induced migraine. But because the 14 had faulty rods and cones—the main light-detecting and image-producing cells in the eye—the scientists suspected that some other type of light-detecting cell must contribute to light-sensitive pain.

The scientists turned to rats to gain a better sense of the brain pathways that might be involved. They focused on rare light-sensing cells in the eye called intrinsically photosensitive retinal ganglion cells (ipRGCs). These cells, discovered only a decade ago, are crucial for maintaining sleep-wake cycles and for pupil response to light, but play no role in image formation.

The researchers traced the path of ipRGC signals through rat optic nerves, where they later converged on brain cells that transmit pain. Exposure to light rapidly activated the ipRGCs and the pain-transmitting cells, which previously had been linked to migraine pain. When the light was removed, the brain cells remained activated for several minutes.

"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," says Burstein.

The findings point to a non-imaging-forming pathway for light-induced migraines, although the scientists note that additional mechanisms may be involved. "Clinically, this research sets the stage for identifying ways to block the pathway so that migraine patients can endure light without pain," Burstein says.

—by Vicki Contie

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