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A report published online Jan. 10 in Nature Neuroscience reveals how visual and pain pathways in the brain converge to produce this phenomenon.
Although the findings are unlikely to help migraine patients in the near future, "this gives us a little better insight as to the theory and mechanism behind migraine," said Dr. Michael Palm, an assistant professor of neuroscience and experimental therapeutics and internal medicine at Texas A&M Health Science Center College of Medicine, College Station, and director of the Parkinson's and Headache programs at Texas Brain and Spine Institute in Bryan.
"We are making progress in understanding this phenomenon," he said.
The Boston-based researchers report there are cells in a part of the brain called the thalamus "where information from the visual system and information from the pain system converge, and that anatomic convergence provides the first available explanation for how it could be that light makes pain worse," added Dr. Richard Lipton, director of the Montefiore Headache Center and professor of neurology and epidemiology at Albert Einstein College of Medicine in New York City.
About 85% to 90% of all migraine sufferers report having photophobia, which is when light makes the pain worse, said study senior author Rami Burstein, an associate professor of anesthesia and neuroscience at Beth Israel Deaconess Medical Center and Harvard Medical School in Boston.
"We had no clue in the world where in the world light and pain talk to each other in the brain," Burstein said. "They have completely different pathways in the brain."
"For light to make pain, those pathways would have to converge at some level," Lipton noted.
To solve the paradox, the team studied 20 blind individuals, all of whom suffered from migraines. Six participants had no light perception at all and no functioning optic nerve. These individuals also experienced no photophobia.
The remaining 14 people could sense light and dark and also experienced photophobia.
"This told us that the optic nerve is critically needed in order to produce photophobia or exacerbation of the headache by light," Burstein explained.
The researchers next discovered that a set of photoreceptors called melanopsin project onto neurons on the thalamus that also process pain signals.
"If we wanted to understand how light makes the pain worse, we needed to follow in the brain the pathways that lead from the eye into the brain using the third group of photoreceptors," Burstein said. "That is the connection so at that point we shifted to animals."
The thalamus is the brain's sensory switchboard, receiving sensory signals from different parts of the body then redirecting them to various sensory, motor and cognitive areas of the cortex.
"We identified a new pathway in the brain that originates in the eye and goes to the brain areas where neurons are found that are active during migraine attacks," Burstein said. "The light can increase the electrical activity in neurons that are active to begin with."
The findings should put to rest any thoughts that patients exaggerate their sensitivity to light, Lipton said. "This provides an anatomic and physiological basis for a common experience -- that light makes pain worse, not because you're a whiner, but because there is an anatomic pathway that links the visual system to the pathway that produces head pain," Lipton said. "That odd bit of clinical symptomatology has a firm basis in brain science."
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