Unreliability of the Visual Analog Scale in Experimental Pain Assessment: A Sensitivity and Evoked Potentials Study

Background: Pain is a universal but subjective experience, making it difficult to obtain objective information about the experiential dimensions of pain. Although the visual analog scale (VAS) is ubiquitously used in pain assessment, its reliability has been questioned. The properties of this rating scale, especially its anchor points likely to be reinterpreted by subjects, may bias the results. Objective: To determine whether the VAS commonly used to assess experimental pain is a reliable tool for obtaining objective information about the experiential dimensions of pain and to assess whether subjects may erroneously interpret anchor points of the classical pain-VAS, ranging from "no pain" to "unbearable/worst pain. " Study Design: A randomized, controlled prospective trial. Setting: Laboratory of cognitive neurosciences in France. Methods: Forty healthy volunteers were enrolled. We analyzed subjects' ratings of the same high-intensity (painful) and low-intensity (non-painful) thermal laser stimulations on 2 computerized VAS during 2 successive sessions: the classical pain-VAS (" no pain" - "unbearable pain") and a pleasantness-VAS ("very unpleasant" - "very pleasant"). Concomitantly, somatosensory evoked potentials (SEPs) were recorded. We investigated the correspondence between these psychophysical measures and specific somatosensory evoked potential (SEP) components elicited by thermal stimulation as a function of its intensity. Results: Low-intensity thermal laser stimulations rated as painful on the pain-VAS were labeled pleasant on the pleasantness-VAS. The cerebral responses following these low-intensity thermal stimulations reflected activation of C-fibers, known to convey non-painful warm sensations, and not activation of Ad-fibers, which transmit painful heat stimulations. SEP results therefore agreed with subjects' ratings on the pleasantness-VAS rather than on the pain-VAS. Limitations: Study limitations include the lack of SEP and psychophysical measures of thermal stimulation intensities eliciting a neutral sensation / corresponding to subjects' pain threshold. Conclusions: Taken together, our psychophysical and SEP results suggest that healthy individuals reinterpret the " no pain" anchor on the classical pain-VAS commonly used in the experimental assessment of pain, by rating the intensity of the stimulation rather than pain perception.