On the Need to Update the Definition of Pain

Pain, in particular chronic pain, is a private and subjective experience. While many studies have elucidated some of the intricacies of the chronic pain experience, the definition of pain has not adapted to current knowledge.^1,2

In fact, the precise definition of pain varies depending on literature source. Indeed, individual levels of pain tolerance, and varying descriptors used to express pain, illustrate the challenge of a unifying definition that address the spectrum of pain experience.

The challenges, when attempting to define pain were highlighted by Joanna Bourke, PhD, professor of History, at Birkbeck, University of London: while acknowledging that pain is difficult to both describe and define, she argues that the sensation of pain refers to the experience, not what is experienced.

So that the injury or  noxious stimulus, in by themselves may not cause pain, but that the sensation of pain might rather be caused by the way in which the injury or stimulus is individually evaluated.³

Professor Bourke then posed the question: “What actually is pain?” According to her, this question may best be answered by noting that pain ”can refer to a heart attack and a heartache. It is a pinprick and an ocular migraine, we can feel pain when the limb that ‘feels it’ is not present (ie, phantom limb sensations); many people do not feel pain even when dreadfully injured (as in combat or extreme sports). They can suffer, yet be lesion-free, as in chronic pain states.”

Professor Bourke explained that pain can evoke complex emotions, adding that “so-called ‘noxious stimuli’ can excite a vast array of emotions, including distress (eg, face-to-face with a torturer), fear or panic (eg, crashing through the car windscreen), anticipation or surprise (eg, the moments after a knife or heart attack), relief (eg, self-cutting), or inspire joy (eg, childbirth)”.³

In the era of evidence-based medicine and multimodal and assessment-centered pain management, a clear definition of pain is essential to make the most of available pain treatment options. Despite an improved understanding of pain pathophysiology, clinical decisions continue to be made based on the International Association for the Study of Pain (IASP) 1994 definition of pain, which remains the current internationally recognized definition of pain.^2,5  

In fact the core elements of the 1994 definition are rooted in the seminal work dating from 1964.⁵ The IASP 1994 definition of pain states that “Pain is an unpleasant sensory and emotional experience associated with actual or potential tissue damage, or described in terms of such damage.”

According to this definition, pain is associated with actual or potential tissue damage. The definition focuses on patient self-report, and clearly acknowledges that pain in the absence of tissue injury or a pathophysiological cause, is simply psychological, and can therefore not be considered ‘true pain’.

As an example, pricking of a finger does not lead to a pain sensation, as “it is an experience that resembles pain, but is not unpleasant.⁵ The focus on self-report in the IASP definition does not accommodate individuals who are unable to experience or describe their pain because of language barrier or impaired cognitive capacities, oversight which led to a revision of the definition in 2002.

The current definition of pain focuses on a unidimensional physical cause that must include tissue and/or physiological injury, thus excluding cognitive and social factors known to often be associated with chronic pain. Some have argued that the current definition of pain is outdated and partly redundant.⁵ For example, contrary to the current definition, pain can be experienced in the absence of tissue, as  shown in imaging studies.⁶

Researchers have proposed a revised definition of pain, so as to “better capture the essence of what we presently understand to be pain and how it would better equip those who try to control pain.”⁵ In this definition, it is stated that “Pain is a distressing experience associated with actual or potential tissue damage with sensory, emotional, cognitive, and social components.”

This revised definition acknowledges the importance of the subjective experience and accommodates social interactions, sensations and emotions. Authors recommend incorporating a broad range of verbal and non-verbal behaviors as part of the pain assessment, in order to adequately capture the complexity of the pain experience.

An official revised definition of pain will require a consensus agreement and may take several years before being universally accepted.