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Another Benefit of Bariatric Surgery May Be Improved Pain Sensitization

January 26, 2018
Patients who had bariatric surgery experienced significantly improved pressure pain thresholds and greater overall pain relief, leading to less central pain sensitization.
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 Interview with Joshua Stefanik, MSPT, PhD and Dmitry Arbuck, MD

Bariatric surgery may provide many health benefits to patients who are obese, including alleviating type 2 diabetes, hypertension and knee pain.1,2 However, the mechanism for improvement in knee pain remains unclear. However, results of a study published in Arthritis Care & Research showed a significant decrease in pressure pain threshold (PPT) in both the patella and the wrist following bariatric surgery in comparison to patients receiving standard medical management for obesity who showed had no change in overall pain.3

Joshua Stefanik, PhD and colleagues at Northeastern and Boston Universities in Massachusetts, hypothesized that the improvement in PPT may be mediated at least in part by central sensitization as well as mechanical unloading.3

Bariatric surgery brings significant overall pain relief to patients with osteoarthritis.

Mechanisms Supporting Reduced Knee Pain Following Weight Loss

Individuals considered obese are four times more likely to have knee osteoarthritis (OA) than the general population.2 Previous evidence had demonstrated that weight management and bariatric surgery may greatly improve knee arthritis symptoms and knee pain.2,4 However, the mechanism for this pain relief was unknown.

Since people who are severely overweight often have mechanical tissue injury and persistent inflammation, which typically lead to changes in nociceptive processing.5 Peripheral and central sensitization to nociceptive input may be one of these changes, which in turn may result in more severe pain that is seen in patients with knee OA and back pain.6

“It is not clear if pain relief after weight loss is solely related to decreased mechanical loading, which would only affect knee pain, or rather if it is related to improvements in peripheral and/or central sensitization, which may have effects beyond the knee,” Dr. Stefanik told Practical Pain Management.

“Our objective was to evaluate changes in pain (at the knee and elsewhere) and pain sensitization in persons who have with knee pain and undergoing bariatric surgery compared with similarly obese persons undergoing usual medical management,” he said.

If the pain relief was related to decreased mechanical loading simply due to weight loss, only the knee pain would lessen, yet pain relief due to peripheral or central sensitization would be observed in both the knee and the wrist.3

Comparison of OA Pain Under Different Weight Management Protocols

Individuals 25-60 years old were recruited from the Nutrition and Weight Management Center at Boston Medical Center and had a body mass index (BMI) of > 35 kg/m2 with a weight-related comorbidity or a BMI of > 40 kg/m2.

Participants who were scheduled for bariatric surgery had a laparoscopic roux-en-y-gastric bypass or laparoscopic sleeve gastrectomy.3 Patients in the medical management group received lifestyle guidance addressing diet and exercise, with some also receiving medications including phentermine, lorcaserin, phentermine/topiramate, bupropion/naltrexone, or liraglutide. Both groups were told to walk 30 minutes/day and do strength training exercises twice a week.3

The pain assessments were done at baseline (up to 2 weeks before surgery for the bariatric surgery group), and at 12 months follow-up. The Western Ontario and McMaster Universities Osteoarthritis Index (WOMAC) was used to assess knee pain.

To measure mechanical pain sensitivity, the PPT was assessed with a hand-held algometer.3 The algometer was applied to the right wrist as a control site and the center of the index patella (the location of the knee usually generating the most pain). The test was repeated three times at each site and averaged, and PPT was defined as the point at which the pressure changes to slight pain.3 Greater pain sensitivity results in a lower PPT.3

Dmitry M. Arbuck, MD, president and medical director of Indiana Polyclinic in Indianapolis, who was not associated with the study said, “Although small, this is a nice and thoughtful study. The authors clearly identify study limitations and bring attention to a need for researching differences and interconnections between peripheral/nociceptive and central pain mechanisms.”

Bariatric Surgery Group Had Significantly Less Pain

Participants (n = 67) were divided into two groups: 45 in the surgery group; and, 22 in the medical treatment group, and completed both baseline and follow-up visits. In the surgery group, mean WOMAC scores decreased significantly (9.5 to 4.6; P < 0.0001)whereas in the medical group, mean WOMAC scores decreased, but the difference was not statistically significant (11.5 to 10.0; P = 0.13).3

Among patients in the surgery group, PPT at the patella increased by 38.5% and at the wrist by 30.9% (P = 0.0007 and P = 0.005, respectively), whereas there was no significant change in the pain pressure threshold among patients in the medical group.3 In addition, a downward shift in body weight was correlated with a change in WOMAC pain (0.50; P < 0.0001) and patella PPT (r= -0.33; P = 0.006), but was not correlated with wrist PPT (r= -0.4; P = 0.002).3

Dr. Stefanik and colleagues concluded that their results demonstrated that patients who had bariatric surgery showed a marked improvement in pain sensitization. Furthermore, an improved PPT at the wrist suggests that pain improvement is due to central sensitization, beyond any benefit of reduced pain associated with weight loss. However, the authors acknowledged other factors that may have contributed to pain improvement, including inflammatory mediators, and the difference in the amount of weight lost by the two groups.3

Dr. Arbuck agreed, telling Practical Pain Management, “all the factors possibly involved in pain sensation – level of inflammation associated with obesity, insulin resistance, mood and central sensitization alike involve the whole body and not just the knee, so a decrease in pain in other body areas is not surprising and may be explained by all or any of the mentioned (or yet unknown) factors. Selecting central sensitization as the only (or even as a leading) factor cannot be justified by this study.”

Study Reinforces the Importance of Weight Control in Pain Care

The authors indicate that moving forward, the next step may be to look at systemic inflammation.

“We did not have measures of systemic inflammation so future work that includes these measures may help tease out the contribution of inflammation and loading to pain and sensitization,” says Dr. Stefanik.

 

Last updated on: January 26, 2018
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