Conference Coverage

Green light therapy: A stop sign for pain?



– Exposure to green light therapy may significantly reduce pain in patients with chronic pain conditions, including migraine and fibromyalgia, an expert reported at the scientific meeting of the American Pain Society.

“There’s a subset of patients in every clinic whose pain doesn’t respond to medical therapies,” said Mohab M. Ibrahim, MD, PhD. “I always wonder what could be done with these patients.”

Dr. Ibrahim, who directs the chronic pain clinic at the University of Arizona, Tucson, walked attendees through an experimental process that began with an observation and has led to human clinical trials of green light therapy.

“Despite being a pharmacologist, I’m really interested in nonpharmacologic methods to manage pain,” he said.

Dr. Ibrahim said the idea for green light therapy came to him when he was speaking with his brother, who experiences migraines. His brother said his headaches were alleviated with time outside, in his back yard, or in one of the many parks in the city where he lives.

Knowing that spending time in nature had salutary effects in general, Dr. Ibrahim, an anesthesiologist and pain management specialist, wondered whether exposure to the sort of light found in nature, with blue skies and the green of a tree canopy, could help control pain.

To begin with, Dr. Ibrahim said, “the question was, do different colors have different behavior aspects on animals?”

Dr. Ibrahim and his collaborators exposed rats to light of various wavelengths across the color spectrum, as well as white and infrared light. They found that the rats who were exposed to blue and green light had a significantly longer latency period before withdrawing their hands and feet from a painfully hot stimulus, showing an antinociceptive effect with these wavelengths similar to that seen with analgesic medication.

“At that point, I decided to pursue green light and to forego blue, because blue can change the circadian rhythm,” said Dr. Ibrahim, adding, “Most pain patients have sleep disturbances to begin with, so to compound that issue is probably not a good idea.”

Dr. Ibrahim and his colleagues wanted to determine whether the analgesic effect had to do with rats seeing the green light or just being exposed to the light. Accordingly, the researchers fitted some rats with tiny, specially manufactured, completely opaque contact lenses. As a control, the researchers applied completely clear contact lenses to another group of rats. “I can’t tell you how many times we got bit, but by the end we got pretty good at it,” said Dr. Ibrahim.

Only the rats with clear lenses had prolonged latency in paw withdrawal to a noxious stimulus with green light exposure; for the rats with the blackout contact lenses, the effect was gone, “suggesting that the visual system is essential in mediating this effect,” noted Dr. Ibrahim.

This series of experiments also showed durable effects of green light exposure. In addition, the analgesic effect of green light did not wane over time, and higher “doses” were not required to achieve the same effect (as is the case with opioids, for example) (Pain. 2017 Feb;158[2]:347-60).

Clues to the mechanism of action came when Dr. Ibrahim and his colleagues administered naloxone to green light-exposed rats. “Naloxone reversed the effects of the green light, suggesting that the endogenous opioid system plays a role in this,” he said, adding that enkephalins were increased two- to threefold in the green light-exposed rats’ spinal cords, and astrocyte activation was reduced as well.

Similar experiments using a rat model of neuropathic pain showed a reversal of pain symptoms with green light exposure, offering promise that green light therapy could be effective in alleviating chronic as well as acute pain.

Moving to humans, Dr. Ibrahim enrolled a small group of individuals from his pain clinic who had refractory migraine into a study that exposed them either to white light or to green light. “These are patients who have failed everything…They have come to me, but I have nothing else to offer them,” he said.

The study had a crossover design. Participants in the small study had baseline pain scores of about 8/10, with no significant drop in pain with white light exposure. However, when the white light patients were crossed over to green light exposure, pain scores dropped to about 3/10. “That’s a greater than 50% reduction in the intensity of their migraine.”

Similar effects were seen in patients with fibromyalgia: “It was exactly the same story…When patients with white light exposure were crossed over [to green light], they had significant reductions in pain,” said Dr. Ibrahim.

“Their opioid use also decreased,” said Dr. Ibrahim. Medication use dropped in green light-exposed patients with migraine and fibromyalgia from an aggregate of about 280 morphine milligram equivalents (MME) to about 150 MME by the end of the study. The small size of the pilot study meant that those differences were not statistically significant.

“A multimodal approach to manage chronic pain patients is probably the best approach that we have so far,” said Dr. Ibrahim. An ongoing clinical trial randomizes patients with chronic pain to white light or green light therapy for two hours daily for 10 weeks, tracking pain scores, medication, and quality of life measures.

Future directions, said Dr. Ibrahim, include a study of the efficacy of green light therapy for patients with interstitial cystitis; another study will investigate green light for postoperative pain control. Sleep may also be improved by green light exposure, and Dr. Ibrahim and his colleagues plan to study this as well.

Dr. Ibrahim reported that his research was supported by the National Institutes of Health. He reported that he had no relevant conflicts of interest.

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