Ask anyone who suffers from migraine headaches what they do when they are having an attack, and you are likely to hear "go into a dark room." And although it is long been known that light makes migraines worse, the reason why has been unclear. The condition is known as photophobia.
It was the observation that even blind individuals who suffer from migraines were experiencing photophobia that led the study's senior author Rami Burstein and colleagues to hypothesise that signals transmitted from the retina via the optic nerve were somehow triggering the intensification of pain.
The investigators studied two groups of blind individuals who suffer migraine headaches. Patients in the first group were totally blind; 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; although they were unable to perceive images, they could detect the presence of light and maintain normal sleep-wake cycles.
"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," explains Burstein. "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."
In the laboratory, the scientists performed a series of experiments in an animal model of migraine. After injecting dyes into the eye, 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 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," says Burstein. "This increased their activity within seconds."
MEDICA.de; Source: Beth Israel Deaconess Medical Center