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.