Jan. 12, 2010 -- Researchers in Boston say they’ve gained new insight into why light makes migraine headaches so painful.
The culprit, they say, is a new pathway that underlies sensitivity to light during migraine episodes in blind people and people with normal eyesight. Scientists at Beth Deaconess Medical Center report their findings in the online journal Nature Neuroscience.
Migraines, which afflict more than 30 million people in the United States, are painful headaches for which sufferers often seek relief by going into dark rooms. Migraines are believed to develop when the system of membranes surrounding the brain and central nervous system, called the meninges, becomes irritated.
That stimulates pain receptors and triggers a series of events that lead to prolonged activation of groups of sensory neurons, the scientists report.
“This explains the throbbing headache and accompanying scalp and neck-muscle tenderness experienced by many migraine patients,” says Rami Burstein, PhD, professor of anesthesia and critical care at Beth Israel and Harvard Medical School.
People with migraines also are also typically aggravated by light with worsening of symptoms compared to being in the dark. This causes many migraine sufferers to wear sunglasses, often at night, Burstein says.
It was the observation that even blind people who suffer migraines were experiencing sensitivity to light, referred to as photophobia, that led Burstein and Rodrigo Noseda, PhD, to hypothesize that signals transmitted from the retina to the optic nerve were triggering the intense headaches, the authors say.
The scientists studied two groups of blind people with migraines. Patients in one group were totally blind because of eye diseases, unable to see images or sense light.
People in a second group were legally blind, but were able to detect the presence of light.
“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, in particular blue or gray wavelengths,” Burstein says in a news release. “This suggested to us that the mechanisms of photophobia must involve the optic nerve, because in totally blind individuals, the optic nerve does not carry light signals to the brain.”
He says they suspected that a group of recently discovered retinal cells that help control biological functions, including sleep and wakefulness, were critically involved in this process, because “these are the only functioning light receptors left among patients who are legally blind,” Burstein says.
In laboratory experiments involving animals, they traced the path of specific cells through the optic nerve to the brain. A group of neurons then became electrically active.
“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,” Burstein says. “This increased their activity within seconds.”
When the light was taken away, the neurons remained activated, which explains “why patients say that their headache intensifies within seconds after exposure to light, and improves 20 to 30 minutes after being in the dark,” Burstein says.
The discovery of the new pathway will provide scientists a new path to follow in trying to solve photophobia, he says.
The study, the authors write, reveals “a mechanism for the exacerbation of migraine headache by light.”
“Clinically, this research sets the stage for identifying ways to block the pathway so that migraine patients can endure light without pain,” Burstein says.