Sensitivity to light, known as photophobia, is a symptom of several neurological disorders, the most common of which is migraine. During a migraine attack patients report that light makes their headache worse, forcing them to wear sunglasses while outside even in overcast weather, or in more severe cases to retire to a dark room until the migraine passes.
According to a long-standing hypothesis, migraine sufferers have a fear or aversion to light because light intensifies the pain associated with their headache. But new research led by Rami Burstein, a migraine researcher at Beth Israel Deaconess Medical Center and Harvard Medical School, Boston, US, challenges this theory.
The new work shows that light can produce a host of changes both in the autonomic nervous system—which controls functions mainly outside of conscious awareness such as breathing and heartbeat—and in emotions too. These changes, then, are key contributors to the overall experience of migraine.
“The results suggest that head pain is not the sole driver of light-induced discomfort in migraine,” says migraine researcher Andrew Russo, University of Iowa, Iowa City, US, who was not involved in the study.
The findings were published online June 26 in the journal Proceedings of the National Academy of Sciences.
A new explanation for photophobia
81 migraine patients and 17 healthy controls were recruited for the new research. Study subjects were exposed to light of various colors and intensities for 30 seconds of each and then asked to describe what they experienced while looking at the light.
Any of the 5 colors of light tested (white, blue, green, amber and red) produced more chest tightness, nausea, light-headedness, and drowsiness in the migraine patients, compared to the healthy individuals. Such symptoms are under the control of the autonomic nervous system.
Light also produced negative emotions such as irritability, anger, nervousness, neediness, and sadness somewhat more in the migraine patients though these particular results were not statistically significant. However, reports of positive emotions, such as feeling happy or relaxed, were significantly higher in the healthy control group. Thus, somehow the migraine patients were experiencing fewer positive emotions in response to light.
“Photophobia is much more complex than we originally thought. It contains not just a sensory [i.e., painful component] but also emotional and autonomic components that can be triggered by light and contributes to the whole experience of migraine,” says Rodrigo Noseda, first author of the study and also at Beth Israel Deaconess Medical Center and Harvard Medical School.
Interestingly, green light was the only color tested that was capable of producing positive emotions during an attack in the migraine patients. This may be related to previous reports of the beneficial effects of green light on pain (see RELIEF related news story).
How does it happen?
To understand how light affects the autonomic nervous system and emotions, the researchers identified nerve cells that receive signals from the retina in the eye and connect to regions of the brain and spinal cord that regulate autonomic symptoms such as dizziness, light headedness and nausea. These neurons also contained a variety of neurotransmitters such as dopamine that are known to regulate mood, which may explain how light affects emotions.
The current findings, along with previous ones from the same group, have encouraged companies to develop glasses that filter all but green light to help migraine patients better cope with daily tasks under normal light levels. And, “The ability to control the light intensity and wavelength profile in computer screens for those with sleep disturbances is gaining in popularity and could benefit migraine patients as well,” according to Noseda.
To read about the research in more detail, see the related IASP Pain Research Forum news story here.
Dara Bree is a postdoctoral fellow at Harvard University, Boston, US.