Researchers in the US believe they have discovered why light can make the pain of migraine worse, even in some blind people: they found photoreceptors in a part of the brain called the thalamus appear to process both light and pain, revealing for the first time that it is possible for neural pathways for pain and non-image-forming light sensitivity to converge.
The study was the work of senior author Dr Rami Burstein, an associate professor of anesthesia and neuroscience at Beth Israel Deaconess Medical Center and Harvard Medical School in Boston, Massachusetts, and colleagues, and appeared online in advance on 10 January in Nature Neuroscience.
Burstein told the media that up to 90 per cent of migraine sufferers experience photophobia, where light makes the migrane hurt more.
“We had no clue in the world where in the world light and pain talk to each other in the brain,” he said. “They have completely different pathways in the brain.”
But for light to make pain worse, the pathways have to converge somewhere, thought the researchers.
So first they studied 20 blind migraine sufferers, 6 of whom had no light perception at all, no functioning optic nerve, and experienced no photophobia.
The remaining 14 blind migraine sufferers could sense light and dark and also had photophobia. Burstein and colleagues wrote:
“We found that exacerbation of migraine headache by light is prevalent among blind individuals who maintain non-image-forming photoregulation in the face of massive rod/cone degeneration.”
Burstein said this suggested that the optic nerve is critical to photophobia, ie for the light to exarcebate the headache.
In the next stage of the study, the researchers studied neural pathways in rats.
With “single-unit recording and neural tract tracing” they found “dura-sensitive neurons in the posterior thalamus whose activity was distinctly modulated by light”.
The thalamus is the brain’s sensory switching center: it receives sensory signals from different parts of the body and redirects them to various sensory, motor and cognitive areas of the brain’s cortex.
The researchers wrote that the cell bodies and dendrites (filamentous fingers through which cells link up and pulse electrical signals to each other) of these “dura/light-sensitive neurons were apposed by axons originating from retinal ganglion cells (RGCs), predominantly from intrinsically photosensitive RGCs, the principal conduit of non-image-forming photoregulation”.
Burstein and colleagues proposed that these findings suggest:
“Photoregulation of migraine headache is exerted by a non-image-forming retinal pathway that modulates the activity of dura-sensitive thalamocortical neurons.”
Essentially, as they explained to the media, they had discovered a group of photoreceptors (known as melanopsin, a light sensitive pigment) projects onto neurons on the thalamus that also process pain signals.
Burstein said:
“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.”
“The light can increase the electrical activity in neurons that are active to begin with,” he explained.
One expert said these findings should put to rest any suggestion that patients exaggerate their sensitivity to light; they are not whining or imagining their symptoms.
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, commented that:
“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.” .
“”That odd bit of clinical symptomatology has a firm basis in brain science,” added Lipton, who was not an author of the study.
However, while the findings help us understand more about migraine and photophobia, they are unlikely to help migraine patients in the near future, 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.
“This gives us a little better insight as to the theory and mechanism behind migraine,” said Palm, who was also not an author of the study.
“We are making progress in understanding this phenomenon,” he added.
“A neural mechanism for exacerbation of headache by light.”
Rodrigo Noseda, Vanessa Kainz, Moshe Jakubowski, Joshua J Gooley, Clifford B Saper, Kathleen Digre & Rami Burstein.
DOI:10.1038/nn.2475
Source: Beth Israel Deaconess Medical Center HealthDay News.
Written by: Catharine Paddock, PhD