Pain: More Than Just a Number

I n this issue of Regional Anesthesia and Pain Medicine, Gilron and Jensen 1 have provided an excellent primer related to the multiple and challenging issues confronted in selecting an appropriate primary outcome measure for use in clinical trials assessing the efficacy of treatments for acute and chronic pain. Beyond making a simple recommendation, the authors describe key methodological concerns such as reliability and validity, review the merits and drawbacks of certain measurement approaches, discuss clinically meaningful differences, and highlight the notion that instruments should be selected to address the specific aims of a given trial. In this context, they conclude that ‘‘research on primary outcomes in analgesic trials supports the conclusion that single-dimension measures (eg, visual analog scales, numerical rating scales, and verbal rating scales) of both pain intensity and pain relief are valid and reliable measures of treatment effect. ’’ However, they also make it clear that single-dimension measures are not the final destination. Their collective body of work, role as collaborators on the IMMPACT (Initiative on Methods, Measurement, and Pain Assessment in Clinical Trials), and content of their review emphasize the notion that ‘‘pain is a complex, multidimensional, sensory, and emotional experience that is individually perceived and described in many different ways. ’’ Cartesian philosophy, dictating that pain signals traveled from the periphery along dedicated pathways to the brain, where the impulse was experienced as pain, dominated the field until less than 50 years ago. In 1965, the gate-control theory of pain put forward by Melzack and Wall forever changed how pain was conceptualized. Their theory proposed that pain did not travel uninterrupted along a dedicated line from point A to point B, but rather modulation of peripheral sensory input could take place in the dorsal horn of the spinal cord. Such spinal level influences could play a significant role in determining both the presence and severity of pain. Since then, research using functional magnetic resonance imaging suggests that, in addition to this spinal control of pain, supraspinal processes can influence the experience of pain as well. Thus, it seems that pain can be controlled at three or more levels in the central nervous system (ie, peripheral, spinal, supraspinal), and in the spirit of the gate-control theory, three aspects of pain are sensed by cortical regions: (1) ‘‘sensory,’’ registering the location of the pain and level of discomfort; (2) ‘‘affective,’’ reflective of the emotional valence associated with the pain; and (3) ‘‘cognitive/evaluative,’’ processing of thoughts regarding the meaning of the pain. It is now clear that the pain experience (especially in those with chronic pain) is the result of an intricate information processing network that is far more sophisticated than a simple unidirectional signal relayed by specific pain fibers to the brain. Advances in neuroscience have also made it clear that there are different types of pain including ‘‘nociceptive’’ pain that is generally considered adaptive and protective and ‘‘pathological’’ pain caused by abnormal function of the central nervous system. Nociceptive pain results when noxious stimuli (eg, tissue injury or inflammation) are appropriately detected, in contrast to pathological pain that can be the result of either damage to the central nervous system (eg, neuropathic pain) or augmentation of sensory signals in the central nervous system, as is the case in fibromyalgia. Gilron and Jensen note that instruments that account for diverse pain qualities and help distinguish between neuropathic and nociceptive pain are already being used in pain treatment trials. Properly characterizing pain will be pivotal in future studies as data suggest that different types of pain respond to different classes of analgesics. Pain assessment strategies that can help EDITORIAL