Pain: Assessment and Measurement

Measurement of Pain in the Laboratory

Experimental heat, mechanical, electrical, chemical, and other forms of stimulation are extremely useful in studies of pain in both man and animals. Pain is inferred in animals from a range of physiological measurements and from reactive and operant behaviors. Those uncomfortable with human verbal reports of pain have promoted the same type of measurements for pain in humans using physiological measurements (such as spinal reflexes, cortical evoked potentials, or functional neuroimaging) and behavioral indices ranging from observed measures of grimacing and bracing to measures of social and work activities. However, pain is only truly defined in humans, based on the unique ability to describe and evaluate private experience.

Laboratory methods provide a degree of control over the subjective measurement process that is not possible with clinical pain measurement. These methods demonstrate that human subjects can reliably rate pain, distinguish between dimensions of sensory intensity and unpleasantness, and discriminate different pain qualities. They also provide valuable information about rating scale behavior that can be applied to clinical pain assessment. Many traditional psychophysical procedures, which focus on relations between stimulus-evoked sensations, have been applied to the evaluation of pain sensation. These methods can be divided into those that assess the pain threshold and those that evaluate the suprathreshold range from pain threshold to pain tolerance.

The term pain threshold is often used to describe general pain sensitivity, such as “he has a high pain threshold.” The specific meaning of pain threshold refers to the distinction between sensory qualities of nonpainful and painful stimulation. As the magnitude of a potentially painful stimulus is increased, the pain threshold marks the transition from the absence of pain sensation to the presence of pain sensation, and is quantified as the amount of stimulus intensity needed to evoke a painful sensation. In the laboratory, the pain threshold is determined by established psychophysical procedures that minimize biases that influence the result. These include the classical method of limits and the method of constant stimuli. The method of limits uses alternating ascending and descending series of continuous or discrete stimuli. These alternating series control for directional errors such as anticipation, of indicating the presence of pain before the sensation becomes painful, and errors of habituation, of continuing to use the non-painful responses after the stimulus has become painful. To further control biases, both the starting [Page 714]point and amount of change in successive stimuli can be randomly varied. For assessment of the pain threshold, this method is usually modified to use only ascending series to prevent unacceptably painful stimulation at the beginning of a descending series. The method of constant stimuli presents discrete stimuli of varying intensities that span the range from certain judgments of nonpain to certain judgments of pain. Thus, these stimulus values span the “gray area” of uncertain responding. The result of this method is usually plotted as a probability function extending for 0 to 100% probability, and a stimulus value corresponding to a specific value (such as 50 or 75%) is defined as the pain threshold.

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