Why Pain Is Not Always Tissue Damage

Redefining Pain

The International Association for the Study of Pain defines pain as "an unpleasant sensory and emotional experience associated with, or resembling that associated with, actual or potential tissue damage." The critical word in this definition — added in a significant revision in 2020 — is "resembling". Pain can occur in the complete absence of tissue damage and can be of identical subjective quality and intensity whether or not tissue damage is present. This is not a philosophical distinction but a neurobiological one with profound practical implications for clinical practice.

The older model of pain — sometimes called the Cartesian or biomedical model — conceptualised pain as a simple alarm signal: damage occurs in the periphery, signals travel up the spinal cord, and pain is experienced in proportion to the degree of damage. Clinical experience and decades of pain research have comprehensively disproved this model. Pain intensity correlates poorly with tissue damage severity, identical injuries produce widely varying pain responses between individuals, and pain regularly persists long after tissues have healed. The contemporary understanding is that pain is a protective output generated by the brain based on its evaluation of available information about potential danger — not a faithful report of peripheral damage.

Phantom Pain and the Brain's Construction of Pain

The most compelling evidence that pain does not require tissue damage comes from phantom limb pain — the experience of pain in a limb that no longer exists. Amputees frequently experience their missing limb as painful, burning, cramping, or crushed — yet there is, by definition, no tissue in that location to be damaged. The pain is generated entirely within the brain's representation of the body (the cortical body schema), without any peripheral input from the painful region. This demonstrates unambiguously that the brain is capable of producing a complete, convincing, and disabling pain experience from central mechanisms alone.

Phantom pain is not a curiosity — it is a window into the general mechanism of pain production. The same brain processes that generate phantom pain operate in all pain experiences. In acute pain, they are heavily informed by peripheral nociceptive input; in chronic pain, particularly centrally sensitised states, the contribution of peripheral input diminishes and the brain's own sensitised processing becomes the primary driver.

The Distinction Between Nociception and Pain

Nociception — the detection and transmission of potentially damaging stimuli by peripheral nociceptors — is not the same as pain. Pain requires the brain's interpretation of nociceptive signals as threatening. The brain can suppress nociceptive signals before they reach conscious awareness (as in stress-induced analgesia, such as a soldier who sustains a serious wound in battle and reports no pain until reaching safety), and it can generate pain in the absence of significant nociceptive input (as in central sensitisation states). The modulating influences — cognitive, emotional, contextual, and attentional — are not merely psychological add-ons to a fundamentally physical process; they are integral to the process itself, operating at the level of the brainstem and spinal dorsal horn through descending modulatory pathways.

Clinical Implications

The implication for musculoskeletal practice is significant. Effective treatment of pain requires attending to all the contributors to the brain's threat evaluation — the peripheral nociceptive input (addressed through manual therapy, exercise, and load management), the central sensitisation state (addressed through graded exposure and pain neuroscience education), and the cognitive and emotional contributors (addressed through patient education, stress management, and psychological support where needed). A treatment approach that focuses exclusively on tissue — assuming that eliminating pain requires finding and fixing a specific structural abnormality — will be inadequate for a substantial proportion of patients with chronic pain.