A possible reason for why light makes the pain of migraines worse is described online in Nature Neuroscience this week. The work reports a previously unknown connection between light-sensitive cells in the eye and certain nerve cells in the brain that are crucial to migraine pain perception.
Rami Burstein and colleagues found that many blind migraine sufferers still avoid light. However, blind patients who had lost the entire eye, or the optic nerve that connects the eye to the brain, did not avoid light. It therefore appeared that light exacerbation of migraine pain does involve light detected in the eye, presumably via a special class of retinal neurons that are intrinsically light sensitive and contribute to the regulation of daily rhythms, but that are known not to contribute to vision.
To test this idea, the team studied rats, where they could directly search for connections from the retina to pain areas in the brain. They found that retinal axons, including some from the intrinsically light sensitive neurons, were indeed sending connections to a group of nerve cells in the area of the brain, known as the thalamus, that are already known to receive and transmit pain signals related to migraine. This connection may explain how light exacerbates migraine pain.