A study has found that chronic pain caused by hip osteoarthritis, or OA, can actually shrink a brain region responsible for processing sensory cues and regulating consciousness and sleep.

The good news is that the brain shrinkage appears to be reversed when hip pain is relieved, in this case, through joint replacement surgery.

“The shape of your brain is changed by having painful arthritis, but in the patients I studied, who got better, the brain changed back,” says study author Stephen E. Gwilym, MD, PhD, an orthopaedic clinical research fellow at the University of Oxford in England.

Dr. Gwilym and his team studied 16 patients with hip OA. Their brains were scanned four weeks before they underwent a hip replacement and nine months after the surgery.

The researchers also scanned the brains of 16 pain-free volunteers who were matched by height, weight, age and sex to the participants with hip OA pain.

Compared to the healthy participants, people with hip OA pain had significantly less gray matter in the thalamus, a brain region that processes sensory cues, including pain.

Nine months after surgery, participants’ pain scores were virtually cut in half, and anxiety and depression scores all reduced to normal levels.

There were also no significant differences in the size of these brain regions between the two groups, suggesting that their brains had returned to a more normal state after surgery.

“We knew before that patients with chronic pain had changes in the brain. What we did not know is they are reversible after surgery,” says Michael Alexiades, MD, an orthopaedic surgeon at Hospital for Special Surgery in New York City who was not involved in the research. “I think most people think most changes to the brain are irreversible and often for the worse, and here they are showing that’s not true. The brain really can adapt one-way or the other. It’s good to show that there are things we can do to make the brain revert back to a more normal state.”

Researchers also showed that the cerebellum, the center for balance and the coordination center for walking, gets bigger in patients with chronic arthritis of the hip. But after the hip replacement it reverts back to normal size since it no longer has to work in overdrive.

“A key point is that while these patients got good pain relief, and their brain changes reversed, some patients don't get complete pain relief from hip surgery,” Dr. Gwilym says, adding that leads to the question – “do these people have brain changes which have gone 'too far'?”

Doctors say the study makes a very strong argument for getting pain under control, especially when the physical pain is associated with walking.

“This work forces the question as to the maximum time a patient should be left in pain prior to surgery if we know that these changes are ongoing,” Dr. Gwilym explains. “Some, less reversible, causes of pain are also known to be associated with similar changes and patients may hit a point where the changes are not reversible with surgery and remain in pain. This is speculation.”

Dr. Gwilym also says this study begs the question of whether or not these brain changes could affect other things too. “Certainly some of the areas which change in volume have been linked to areas that are responsible for mood and cognition. So in theory, yes, the pain may lead to changes in mood for neuroanatomical reasons,” he explains. “We saw that patients scored lower on our assessments of depression after their pain was relieved by surgery and brain changes reversed.”

Dr. Alexiades says it’s great to have this detailed way of studying improvement post-surgery.

“It’s a fascinating study from a neurological point of view and confirms what we as orthopaedists have seen for years. We can have patients walking little or horribly and after replacements go back to normal function and normal walking, and this confirms structurally their brain is agreeing with it,” Dr. Alexiades says.

The study was published in Arthritis & Rheumatism.