Researchers have identified the brain regions that allow some people to feel less pain in one area of the body when they are subjected to pain in another area, according to a study in NeuroImage.

The reaction, known as conditioned pain modulation (CPM) or “pain inhibits pain,” is used to assess patients’ physiological response to pain. People’s bodies to varying degrees inherently inhibit the sensation of pain, research has shown. Identifying the brain circuitry involved in CPM may lead to more effective treatments for postoperative pain and chronic-pain conditions, such as fibromyalgia and irritable bowel syndrome, which are associated with decreased CPM, the researchers said.

Pain in two body locations triggered greater activity in various regions of the brainstem, the part of the brain that connects to the spinal cord, in people with decreased CPM compared with those who had functional CPM, according to the study, performed at the University of Sydney in Australia.

The researchers measured brain activity in 54 participants in their early 20s during two magnetic-resonance imaging scans. During the first MRI, a thermal device secured to the right side of the mouth was heated eight times at 15-second intervals to a moderately painful temperature based on individual pain thresholds. The device activated a number of regions of the brainstem, which correlated with lip pain ratings by the participants.

The second MRI followed the same format, but after the fourth temperature increase, the participants received an unexpected painful injection of a harmless solution to the lower leg.

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In 23 of the participants, ratings of lip pain intensity during the second MRI were reduced by 29% compared with the first MRI, an indication of CPM analgesia, the researchers said. In contrast, pain increased by 3.7% in the other participants. A greater reduction in brain activity was associated with greater CPM pain relief. No gender differences were found.

Caveat: The study was small and involved young men and women without chronic pain conditions.

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