A Validation of Pain Assessment Using the Visual Analog Scale and Ratio Production Methods Brian P. Dyre Intellective Consulting and Services, LLC Nicholas Roome and Tristen Beaudoin University of Idaho Prepared By: Intellective Consulting Limited, 667 Indian Hills, Moscow, Idaho 83843 PWGSC Contract Number: W7719-165316/001/TOR Technical Authority: Justin Hollands, 416-635-2073 Disclaimer: The scientific or technical validity of this Contract Report is entirely the responsibility of the Contractor and the contents do not necessarily have the approval or endorsement of the Department of National Defence of Canada. Contract Report DRDC-RDDC-2017-C055 DRDC Date of Publication: March 2017 © Her Majesty the Queen in Right of Canada, as represented by the Minister of National Defence, 2017 © Sa Majesté la Reine (en droit du Canada), telle que représentée par le ministre de la Défense nationale, 2017 PAIN METRICS 1 A Validation of Pain Assessment Using the Visual Analog Scale and Ratio Production Methods Brian P. Dyre Intellective Consulting and Services, LLC Nicholas Roome and Tristen Beaudoin University of Idaho Technical Report Prepared for: Defence Research and Development Canada (DRDC) Contract No. W7719-165316/001/TOR Aircrew Strain Metrics DRDC Toronto Research Centre 1133 Sheppard Ave. W., Toronto, ON M3K 2C9 PAIN METRICS 2 Abstract This research aimed to support development of a valid and reliable psychophysical pain measure with ratio scaling properties. Such a measure is important for assessing the effectiveness of design and procedural interventions introduced to reduce neck strain/pain in Griffon helicopter aircrew. This study included two experiments. Experiment 1 determined whether productions of pain ratios in a dual-stimulus procedure possess ratio scaling properties by testing whether the mathematical property of commutativity is maintained in ratio production of pain. We found commutativity to be maintained, which implies the underlying representation of pain in a pattern of produced ratios of pain has ratio properties. Experiment 2 aimed to determine whether visual analog scales (VAS) can reliably represent these ratio pain productions. In this experiment, we presented participants an experimentally-induced pain stimulus and a numeric ratio. Participants then adjusted a second pain stimulus to represent the given ratio of pain relative to the first stimulus. We then retested participants with the pain stimuli they produced at different ratios and had them rate the pain intensity using VAS. The results demonstrate that VAS scores are linearly related to the produced ratios of two independent pain stimuli. These results suggest that pain can be represented on a ratio scale and that VAS scores can validly represent this underlying ratio scale. Because percent change in VAS pain responses are a reasonable representation of percent changes in actual pain levels experienced, we conclude that the VAS is a convenient and accurate method for assessing interventions aimed at pain reductions in aircrew. PAIN METRICS 3 A Validation of Pain Assessment Using the Visual Analog Scale and Ratio Production Methods This research examined a novel dual-stimulus ratio-production approach to measuring subjective pain to develop a valid and reliable psychophysical pain measure with ratio scaling properties. Such a measure is needed to assess the effectiveness of design and procedural interventions introduced to reduce neck strain/pain in Griffon helicopter aircrew, many of whom are being benched or grounded due to neck pain. Justification of Approach Physiological indicators of pain are not useful for evaluating design or procedural interventions aimed at reducing pain. Musculoskeletal injuries can result in chronic pain that endures long after injured tissues have finished healing and the peripheral nociceptive activity has returned to normal. Chronic pain typically involves a neuropathic component: pain sensations are the result of damage to peripheral nerves or to the central nervous system (CNS) itself. Neuropathic pain is considered to be a symptom of neuropathic disease, a disease whose most reliable indicator is the report of a subjective pain experience by a patient in the absence of a nociceptive stimulus or obvious damage to peripheral nociceptors or C and A-delta nerve fibers (Cervero, 2009). It is a sad irony that while the subjective effect of chronic pain is profound and obvious to the sufferer, the objective effects on central nervous system physiology are exceedingly difficult to measure. Indeed, subjective reports of pain are the primary diagnostic tool for assessing neuropathic pain (Baron, 2009). For any disease, our abilities to diagnose and evaluate treatment are limited by the reliability and validity of our measures of disease symptoms. Pain presents a particularly challenging disease symptom to measure due to its inherent subjectivity, multi-dimensional nature, and unclear scientific classification (a percept vs. a state; Cervero, 2009). Pain is a PAIN METRICS 4 multidimensional construct comprised of quality (e.g., burning, stabbing, throbbing) and intensity. The focus of this report is the measurement of pain intensity rather than quality. Measurement of the suprathreshold subjective intensity of pain can be achieved using psychophysical scaling techniques such as magnitude estimation and cross-modality matching. For magnitude estimation, many clinicians and researchers employ discrete categories where a sufferer indicates pain intensity on a fixed 10 or 20 pt. bounded scale, labeled numerically, and sometimes including propositions (Brennum et al., 1992; Melzack, 1975). Common cross- modality matching tasks ask sufferers to adjust grip strength or adjust the spatial placement of a point on a line to indicate a scale value (a visual analogue scale or VAS). These methods all share a common feature: the two end points of the scales provide bounds that—in principle— serve to enhance the reliability of numerical estimates or produced matches. However, it is probable that fixed bounds at the ends of the response scales (referred to by psychophysicists as moduli with constant subjective values) sacrifice validity of measurement for reliability; that fixed anchors increase the consistency of responses, but at the cost of potentially misrepresenting the underlying subjective state by imposing a particular number system on respondents (Fernandez et al. 1991). For example, a participant may experience a strong stimulus early in a set of trials and assign it a very high value (or grip as hard as they can), not knowing that an even stronger stimulus is possible. When the stronger stimulus is perceived, a ceiling effect artificially compresses the pain measure associated with that stimulus. Conversely, when sufferers or participants are presented a limited range of pain intensities, they might expand their responses to better fit the range of the scale (Gracely & Eliav, 2009). Consistent with these observations, Hollands and Dyre (2000) showed that scale boundaries significantly bias judgments of proportions, and that changes in the locations of boundaries will affect the magnitude and direction of those biases. PAIN METRICS 5 A more valid ratio scale of stimulus intensity may be obtained using a free modulus procedure, where participants are given no anchors and are encouraged to assign whatever numeral they feel appropriately represents a particular stimulus (Stevens, 1975). Such scales avoid the bias created by fixed scale bounds. However, Stevens’ (1975) assumption that such numerals represent an internal ratio scale of stimulus intensity has been shown repeatedly to be false: participants use and interpret numerals in a manner that violates the properties of ratio scales (Ellermeier & Faulhammer, 2000; Zimmer, 2005). In effect, numerals are simply one more type of stimulus that generates its own unique internal intensity scale (Narens, 1996; Luce, 2002). Thus, even for free modulus magnitude estimation tasks, the numerals that are reported do not necessarily represent the perceived stimulus intensity. Traditional subjective pain measurement techniques such as single stimulus magnitude estimation are analytically incapable of separating the effect of subjective response scales from the internal experience representation of subjective pain. As a result, such techniques can only produce pain scales with ordinal (rather than ratio) properties, and cannot be used to provide estimates of changes in pain intensity that are precise enough to test measures introduced to address neck strain in aircrew. Clearly a more valid method of assessing a pain sufferer’s true subjective experience of pain independent of their concepts of numerals or other response scales is needed. Objectives This study aimed to provide data to help develop a valid and reliable psychophysical pain measure with ratio scaling properties to assess the effectiveness of design and procedural interventions introduced to reduce neck strain/pain in Griffon helicopter aircrew. One commonly-used measure of pain is the visual analog scale (VAS; Price et. Al. 1983). The VAS presents a horizontal line bounded at the left end with the label “no pain” and at the right end with the label “maximum pain intensity imaginable.” Participants place a mark on the line that is PAIN METRICS 6 presumed to represent the percentage of “maximum pain intensity imaginable.” Price et al. (1983) tested VAS scores for 2 thermal pain stimuli, a standard and a second stimulus intensity