1. INTRODUCTION
The convergence between pain and opioid use disorder (OUD) continues to be a complex problem and significant public health concern. Pain is a pervasive issue, with over half of the U.S. adult population reporting some form of pain within the last three months1. More specifically, chronic pain, defined as persisting pain lasting over three months, affects over 100 million adults in the U.S., and chronic low back pain ranks as one of the 10 leading causes of reduction in disability-adjusted life years2, 3. Considering health care, forensic, production loss, and life loss costs, the economic burden of chronic pain has exceeded $600 billion annually4.
In parallel, the consequences of the OUD continue to escalate amidst the opioid crisis, as we witness consecutive yearly records for fatal opioid overdoses and opioid-related hospitalizations in the U.S., now above 60,000 opioid-involved overdose deaths per year5. Pain is pivotal to this crisis, particularly as the initial wave of the opioid epidemic was the result of inadequately prescribed opioids for chronic pain6. An added layer of complexity is the pervasive stigma associated with OUD, often resulting in labeling patients with pain as ”opioid-seeking.” This hampers appropriate medical care in the context of acute and chronic pain7. Such is the need for specialized care that considers the consequences of co-occurring pain and OUD, for which clinicians and other stakeholders have been attempting to develop innovative strategies, including specialized clinics that are capable of jointly managing pain and OUD2, 3, 8, 9. A persistent challenge, however, is distinguishing between pain-related and OUD-related phenomena, due to their clinical and neurobiological similarities10.
Several approaches have been used to assess both acute and chronic pain among persons with OUD. For example, Delorme and colleagues recently conducted a meta-analysis to evaluate the prevalence of chronic pain among persons with OUD receiving buprenorphine or methadone11. The analysis included 23 studies, using a variety of pain assessment tools, with the Brief Pain Inventory (BPI) most frequently used (n=12). Four studies relied on a simple binary question, asking about the presence or absence of pain. One study utilized a pain numerical rating scale12, 13, while another combined the BPI questionnaire with a numerical rating scale14. A unique approach was taken in one study, where a custom-made questionnaire was used to gauge pain levels15. The other two determined the presence of chronic pain based on the prescription or dispensation of analgesic medication longitudinally. This wide variation in assessment tools illustrates the lack of consensus in methods used to understand and quantify the experience of chronic pain among patients with OUD, highlighting the need for empirically-supported consensus in this area.
In this narrative review, we discuss the fundamental approaches to pain measurement in persons with OUD. First, we describe biological, psychological, and social aspects of the pain experience among people with OUD. Our discussion ranges from molecular opioid-related phenomena to healthcare disparities. Second, we review methods to assess pain including: (1) traditional self-reported methods, such as visual analog scales and structured questionnaires; (2) behavioral observations (e.g., antalgic and pain-avoidant behaviors) and physiological indicators (e.g., heart rate, blood pressure); (3) and laboratory-based approaches such as functional brain imaging, electroencephalography, and quantitative sensory testing. We discuss the influence of relevant clinical phenomena in the assessment of pain among persons with OUD, including tolerance, heightened pain sensitivity (i.e., hyperalgesia), and pain exacerbated by opioid withdrawal. Finally, we will outline strategies for improving pain assessment in OUD and implications for future research.