Journal of Manipulative and Physiological Therapeutics
Original ArticlesResponsiveness of visual analogue and McGill pain scale measures*
Introduction
Responsiveness or sensitivity to change is the ability of a measure to detect clinically important change in a patient's health status over time. A prerequisite for determining responsiveness is an external criterion, independent of the measure under investigation, that is accepted as evidence of a clinically important change. The consequences of an outcome measure that lacks responsiveness are the same as a diagnostic test that has poor sensitivity or specificity. The outcome measure may suggest that the patient has improved when they have not (a false alarm) or fail to detect true improvement (a miss). For researchers, measures that lack responsiveness are of concern because they reduce precision and therefore increase sample size requirements.
The McGill Pain Questionnaire (MPQ) includes a list of 78 pain descriptors that can be used to provide a quantitative measure of pain. In the original description of the instrument, subjects were asked to choose words from the list to describe their current pain1; however, the instrument has been used to quantify average pain and most intense pain. A pain rating index (PRI) can be calculated based on the sum total of the scale or rank values for chosen descriptors. Separate scores for sensory, affective, evaluative, and miscellaneous subcategories can also be provided.
Although Melzack has stated that the MPQ “…is sufficiently sensitive to detect differences among different methods to relieve pain,”1 there have been few formal investigations of the responsiveness of the MPQ. One critical question is whether the MPQ is as responsive as simpler pain measures, such as the visual analogue scales (VAS) or numerical rating scales (NRS). This issue is relevant to researchers and clinicians because VAS and NRS measures are quicker and simpler to administer and easier to translate into other languages. Another important question is whether Melzack's recommendation that the MPQ be used to rate current pain maximizes the responsiveness of the measure, or whether it would be better to adapt it to rate average pain over a defined period.
Two previous studies have evaluated the influence of the time reference on the responsiveness of pain measures.2, 3 Both noted that the responsiveness of a pain measure was greater when subjects judged their pain over a preceding period (eg, 1 week) rather than at the point in time when they completed the measure. In the study by Jensen et al,3 patients judged their pain over the past 2 weeks, whereas in the study by Bolton and Wilkinson,2 the time frame was the past week. Studies that have compared the responsiveness of the MPQ with alternate pain measures have consistently demonstrated that the MPQ is less responsive than simpler pain measures such as the VAS or NRS.4, 5, 6, 7 However, because the time reference was not always explicitly stated in these studies, it is unclear whether the reported lower responsiveness of the MPQ is a feature of the instrument itself or is because of the current pain time reference originally advocated by Melzack.1
The aim of the current study was to evaluate this hypothesis by using outcome data available from a clinical trial of 2 rehabilitation protocols after spinal surgery. Patients completed both the MPQ and VAS to rate pain over the last 24 hours, making it possible to provide a head-to-head comparison of the responsiveness of the 2 measures without the confounding effects of time reference. Additionally, because patients also completed the VAS to measure current pain, we were able to replicate earlier studies that found that the responsiveness of a pain measure was greater when used to judge pain over a preceding period rather than at the exact moment the measure was completed. Finally, the responsiveness of the subscale of the MPQ and the MPQ measure based on the number of words chosen was calculated.
Section snippets
Patients
The study sample consisted of patients who had undergone lumbar diskectomy, fusion, or laminectomy at a private hospital in Sydney, Australia. These patients were participating in a randomized controlled trial evaluating 2 postoperative rehabilitation regimens. Patients were excluded from participation if they were unable to read English, had malignant disease, or had undergone microdiskectomy. Eighty-one patients joined the trial; 5 patients were lost to follow-up, leaving 76 patients. A
Results
The responsiveness indices for the 3 pain measures are given in Table 2. The AUC and t statistic analyses, in which subjects are divided into those that improved and those that did not, are repeated for a range of cut-offs on the 7-point GPE scale. Column 1 uses the most strict cut-off forEmpty Cell AUC t Empty Cell Empty Cell 1 2 3 1 2 3 Rho VAS-24 0.729 0.856 0.836 3.322 6.050 2.968 0.555 VAS-now 0.565 0.727 0.644 0.624 3.343 1.164 0.258 MPQ 0.597 0.700 0.688 1.488 2.878 1.579 0.277
Discussion
The principal finding of this study was that the MPQ was less responsive to clinical change than the VAS-24. This finding has been reported previously4, 5, 6, 7; however, because the time reference for the MPQ-PRI was often not clearly stated, the source of the reduced responsiveness was unclear in those studies. In this study, we controlled for the time reference by having subjects complete both an MPQ and VAS to rate pain over the last 24 hours. Under this condition the MPQ was less
Conclusion
Our study showed that the VAS is more responsive to clinical change than the MPQ. We would advise readers to use the VAS in preference to the MPQ when measuring pain in clinical trials and clinical practice.
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Submit reprint requests to: Dr. Christopher Maher, School of Physiotherapy, Faculty of Health Sciences, University of Sydney, Lidcombe, NSW 2141, Australia.