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The visual analogue scale (VAS) is widely used for pain assessment. However, the minimum clinically relevant change remains a debated question, even with regard to the method of assessment. On the one hand, considering intra-individual change in acute pain, Todd et al suggested a crude change of 13 mm (for a 100 mm VAS) to discriminate between reporting a little less or a little more pain.1 In the study of Farrar et al the best cut off points were estimated at −20 mm (for a crude change) or −33% (for a relative change) to best discriminate between patients who require a rescue opioid dose or not.2 Similar results were also derived when considering patient self appreciation of improvement in chronic pain.3 On the other hand, Redelmeier et al suggested focusing on interindividual change to assess a minimal important difference.4 Wells et al thus observed that when a patient feels “somewhat worse” than another patient who faces him, then there is a 16% relative difference in their mean answer to a pain questionnaire.5
We performed an ancillary study to a clinical trial in order to estimate the minimum clinically relevant change in pain for patients with sciatica. The main study was a randomised double blind trial conducted to assess the effectiveness of epidural corticosteroid injections in patients with sciatica, presumably due to a herniated nucleus pulposus, for 15–180 days.6 We thus administered three epidural injections at 48 hour intervals with 2 ml prednisolone acetate (50 mg) or 2 ml isotonic saline. Self evaluation was the primary outcome, measured at day 20 on a four item scale (recovery, important improvement, poor improvement, or worse). The first two items were then pooled, thus defining a treatment success, while the last two items defined a treatment failure. As a secondary outcome we assessed the severity of pain on a VAS both at day 0 and day 20. Eighty five patients were included in the main study. However, in the present work, analyses were performed on a subsample of 75 patients for whom we had both the self evaluation at day 20 and the two pain assessments. The subsample comprised 45 men and 30 women with a mean (SD) age of 41 (11) years. Those patients had had pain for a mean (SD) duration of 49 (41) days and their mean baseline pain was 58 (16). For 39 of those 75 patients, treatment was a success. We performed analyses without taking into account the randomisation arm: there is indeed no foundation for a different relationship between self evaluation and pain assessment according to a patient’s treatment. For each patient, we calculated both the crude and relative changes in pain assessment and plotted receiver operating characteristic curves (fig 1). Table 1 shows the performance indexes associated with several cut off points. Optimal cut off points, which thus offer the best compromise between sensitivity and specificity, are estimated at −30 mm for the crude change and −50% for the relative change. These values are higher than those estimated by Todd et al1 and Farrar et al.2,3 Such discrepancies may be due to a difference in the “gold standard”. They may also be related to the length of the period of observation because we focused on a 3 week period.
In conclusion, the values we estimated may be seen as the clinically relevant long term changes in pain assessment for patients with sciatica. Such findings, however, have to be confirmed by further studies planned with this specific aim because patients included in a randomised trial may not be representative of the general population of patients with sciatica.