Scientists have long suspected a link between opioid use disorder (OUD) and chronic pain, but the brain processes that link OUD and chronic pain remain unknown.

Researchers from The Ohio State University Wexner Medical Center and College of Medicine and the University of Michigan Medical School conducted a first-of-its-kind investigation on central sensitisation in people with OUD.

The findings of the research were published in the journal PAIN Reports. Central sensitisation refers to abnormal pain processing in the brain and spinal cord. People with central sensitisation have spinal cords that are unusually good at sending pain signals to the brain, and brains that struggle to turn off those signals once they arrive.

This means people with greater central sensitisation tend to suffer more with pain than others. "Our study is the first to give patients with OUD a scale that measures central sensitisation," said Dr O. Trent Hall, lead author of the study and an addiction medicine physician in Ohio State's Department of Psychiatry and Behavioral Health.

"Our study provides the first evidence of central sensitisation underlying the chronic pain and OUD relationship and demonstrates a new tool for easily measuring central sensitisation among individuals with OUD."

Researchers recruited 141 study participants from Ohio State Wexner Medical Center's addiction treatment centre in Columbus, Ohio. As part of the study, researchers administered the American College of Rheumatology 2011 Fibromyalgia Survey Criteria via electronic survey.

Participants also responded to questions about pain interference, quality of life and items regarding pain beliefs and expectations of pain and addiction treatment.

Chronic pain may lead to OUD, and people with chronic pain and OUD have a harder time quitting opioids than people with OUD only. So, it's critical to find answers to how pain and OUD are connected in the brain.

Researchers measured quality of life across eight life domains including general health, physical functioning, mental health, social functioning, vitality, bodily pain, role limitations due to physical health and role limitations due to emotional problems. According to Hall, they found that greater central sensitisation was associated with worse quality of life among patients with OUD.

"Additionally, patients higher in central sensitisation were more likely to report pain as a major reason for why their opioid addiction first began, as well as for putting off addiction treatment, continuing and increasing their use of opioids, and fear of pain-causing OUD relapse in the future," said senior author Dr Daniel J. Clauw, director of the Chronic Pain and Fatigue Research Center at the University of Michigan.

This study suggests central sensitisation may be an important underlying factor complicating the treatment of chronic pain and OUD. This provides an example for other clinicians and researchers to measure central sensitisation in OUD, which could help them produce better treatments for people suffering from chronic pain and OUD.

As a physician with a background in both pain and addiction, Hall has cared for many patients suffering deeply from both conditions and he realises that treatment options are limited.

"It's important to me to search for new ways to help," Hall said. "But we can't create better treatments for chronic pain and OUD without first understanding how the two relate. I did this study because I believed it might offer a new window into what is happening in the brains of patients needing help with pain and addiction."