Measuring pain intensity in headache trials: which scale to use?

Pain intensity is the most clinically relevant dimension of nearly all headache attacks. Accurate, reliable measurement of pain is therefore critical to the evaluation of outcomes in clinical trials of headache treatments, but pain is inherently subjective and difficult to measure. A number of pain scales have been developed and are commonly used in clinical practice and research. Four of these are depicted in Figure 1. In headache research, an important precedent was established by the early triptan trials, which used a 4-point verbal rating scale (VRS) to measure pain intensity. International Headache Society guidelines for the conduct of controlled trials of headache treatments recommend the use of this scale or the 100mm visual analogue scale (VAS). The 4-point VRS has the virtue of simplicity, but has been criticised for statistical reasons and because the relatively small number of categories may not adequately discriminate among clinically relevant changes in pain intensity (1,2). Although the use of the 4-point VRS is an established precedent, might there be better ways to measure pain in headache trials? Somewhat surprisingly, there is a paucity of research regarding the use of pain rating scales in headache, although there are many studies evaluating their performance in other types of acute and chronic pain. Studies in headache populations are particularly important in view of previous research suggesting that the interchangeability of pain rating scales may differ based on pain aetiology (3). The study by Aicher et al. in this issue of the journal is therefore a welcome addition to the headache literature (4). The authors used information collected during a German clinical trial of an over-the-counter combination medication for acute treatment of headache. Subjects were given a 100mm VAS and asked to mark a line representing one of the categories on a 6-point VRS, chosen at random. The VRS categories were given in German as follows (English translation in parentheses): kein Schmerz (no pain); leichter Schmerz (mild pain); maessiger Schmerz (moderate pain); starker Schmerz (severe pain); ueberaus starker Schmerz (very severe pain); and staerkster vorstellbarer Schmerz (most severe pain imaginable). This was repeated with a fresh, unmarked VAS until all six VRS categories had been assessed. The same procedure was repeated at the end of the study, providing an opportunity to assess reliability. Data were analysed from 1457 subjects with a median age of 38, three-quarters of whom were women. The goals of this portion of the study, as described by the authors, were to assess both the VAS and 6-point VRS with respect to consistency of category rank order; to determine cut-off points on the VAS corresponding to the VRS categories; to evaluate how the categories of the VRS are represented on the VAS; and to assess test–retest reliability after repetition of the complete training procedure at study conclusion. Results showed that roughly three-quarters of subjects rated the six VRS categories in the same order on the VAS at the first and fourth (final) study visits. The most common inconsistencies in order (that is, categories marked on the VAS in reverse order from the VRS) were observed between mild and moderate pain (12.6% and 13.6% at visits 1 and 4), and severe and very severe pain (9.1% and 6.7% at visits 1 and 4). Receiver operating characteristic (ROC) curves were used to determine the cut-off points for VAS values that best fit the VRS categories. A non-equidistant scaling was found to be the best match, with the smallest range of VAS ratings corresponding to the extreme categories of the VRS (0–2mm for no pain and 96–100mm for most severe pain imaginable). A broader range of VAS scores corresponded to intermediate VRS categories (for example, 17–47mm for moderate and 47–77mm for severe pain). The ability of the VAS to accurately