Light’s effect on migraine studied
“Scientists have worked out why light makes migraines worse, paving the way for new treatments for the crippling headaches,” reported the Daily Mail. It said that the treatments would allow sufferers to endure light without pain so they would no longer need to shut themselves in a darkened room.
This laboratory study identified neural pathways in the brain that may be involved in the worsening of migraines with exposure to light. People often find that migraines are made worse by light, and the fact that some visually impaired people (who lack image-forming sight) are also affected, led researchers to speculate that non-image-forming pathways are responsible. They studied this in rats, finding that exposure to light increased the activity along certain neural pathways.
These findings will be of interest to scientists, but it is unclear what clinical relevance they have. Whether these pathways can be targeted with treatments that reduce light sensitivity for migraine sufferers will need further research.
Where did the story come from?
The study was carried out by Rodrigo Noseda, Rami Burstein and colleagues from the Beth Israel Deaconess Medical Centre and Harvard Medical School, Boston, and the University of Utah. The research was supported by grants from the US National Institutes of Health and published in the peer-reviewed medical journal Nature Neuroscience.
The study investigated the visual pathways that may explain sensitivity to light in people who suffer from migraines. Several newspapers covered this story and described it well, although most emphasise the initial part of the study (in humans) and do not describe the laboratory study on which these conclusions are based. The suggestion by The Independent that “migraines start in the eyes’ light cells” may be misleading and is not supported by the findings of this study.
What kind of research was this?
Many people who suffer from migraine find that the migraine is made worse by light. To investigate this, the researchers looked at 20 blind people who suffered from migraine. These people had different forms of blindness, and 14 could detect light while six could not. The researchers found that those who could detect light had migraines that worsened on light exposure, while those who could not detect light were unaffected.
The researchers say there are two separate visual pathways involved in the projection of retinal images to the brain, one relating to ‘image formation’ and another relating to ‘non-image-forming’ functions. The 14 blind people who could detect light were capable of ‘non-image forming’.
This led the researchers to theorise that it may be the non-image-forming signals of the eye that activate certain neurones in the brain already known to be linked with migraines (the trigeminovascular pathway).
They went on to study this theory in rats in the laboratory, where they mapped the non-image-forming responses to light that are linked to the pathways involved in migraines.
What did the research involve?
In the laboratory part of the study, the researchers carried out experiments in rats to test their theory. They used two major techniques: single-unit recording, where an electrode is inserted into the brain to detect electrical activity generated by the neurones near its tip; and neuronal tract tracing, which can trace neural pathways from the source of a stimulus, in this case the retina, to the brain. Using these techniques they were able to map the neurones involved in non-image-forming responses to light that were linked to the pathways involved in migraines.
The techniques are complex and are well described by the researchers in this publication.
What were the basic results?
The research identified certain neurones in the brains of rats that were triggered by light. These neurones lay close to nerve cells originating from retinal ganglion cells (RGCs), which are cells in the retina of the eye, in particular to one type of RGC called intrinsically photosensitive RGCs. These are largely responsible for the light regulation associated with non-image formation.
How did the researchers interpret the results?
The researchers propose that migraines that are made worse by exposure to light are affected by the activity of neural pathways from the retina to the brain, the so-called ‘non-image-forming’ retinal pathways.
Conclusion
In this laboratory study, researchers identified neural pathways that may be involved in the exacerbation of migraines following exposure to light. Photosensitivity (sensitivity to light) is commonly associated with migraines, and the fact that some people who are visually impaired can experience this led the researchers to hypothesise that non-image-forming pathways are likely to be responsible. They were able to study this in rats, noting that exposure to light increased the activity along certain neural pathways.
These findings will be of interest to scientists who study the brain, but as yet it is unclear what clinical relevance they have.
