4.1.1 Visual Analogue Scale
The visual analogue scale (VAS) was first described by Hayes and colleagues100 as an instrument to quantify pain intensity. It features a linear scale ranging from ”no pain” to ”worst pain ever”, and the individual marks the intensity of their pain on a 100 mm line. This scale has been commonly used in various populations (e.g., children101, patients with chronic non-cancer pain102, individuals with juvenile idiopathic arthritis103). In patients with OUD, several studies have utilized the VAS to assess pain104-106. Notably, similar VAS tools have been used to measure other opioid-related clinical phenomena, such as opioid craving and withdrawal107, 108.
Several studies have examined the use of the VAS to assess pain in patients with OUD undergoing opioid switching or taper. For example, Muriel and colleagues conducted an observational study in 138 patients with OUD and co-occurring chronic pain undergoing a 6-month opioid taper104. They examined whether CYP2D6 (an enzyme involved in opioid metabolism) phenotypes (poor vs. extensive vs. ultrarapid metabolizers) affected the severity of opioid withdrawal symptoms and pain using VAS and the Opioid Withdrawal Scale (OWS). In the context of significant opioid tapering, CYP2D6 ultrarapid metabolizers demonstrated more severe opioid withdrawal symptoms and higher VAS pain scores compared to extensive and poor metabolizers. This suggests that the VAS may be used to quantify the severity of chronic pain experienced during opioid tapering104.
Veldman and colleagues conducted an observational study examining the effects of switching 43 persons with OUD and co-occurring chronic pain from full μ-opioid receptor agonists to buprenorphine-naloxone106. Using the VAS, pain levels were measured at baseline, while on full agonists, and again two months after switching to buprenorphine-naloxone. Change scores indicated that patients showed a significant reduction in pain scores on the VAS following the transition; further, they also demonstrated increased pressure and electrical pain thresholds and tolerance, suggesting reduced hyperalgesia106.
Taken together, these studies demonstrate that the VAS appears to be an acceptable and valid method for evaluating pain severity in populations with co-occurring OUD and chronic pain. Notably, the unidimensional nature of the VAS pain assessment portends significant limitations in elucidating the multifaceted experience of chronic pain in persons with OUD.
In an additional adjacent study, Nielsen and colleagues also employed the VAS to measure perceived pain severity in a randomized trial comparing ketamine to placebo for acute postoperative pain in 147 patients with chronic pain patients with a history of daily opioid use105. Evaluating VAS scores, the investigators found that ketamine reduced the need for opioids during the 24 hours after surgery compared to placebo. A follow-up pain assessment conducted six months post-surgery continued to show that patients treated with ketamine had greater improvements in pain relief as measured by the VAS compared to those who received the placebo105. Of note, it is unclear if these patients actually met diagnostic criteria for OUD, so caution is suggested in generalizing these findings to this population.