The introduction to an article on chronic pain contains this statement on why pain was never a vital sign:
Pain is always subjective in that each individual learns the application of the word through their own experiences.
More from the article:
Cognitive and emotional factors have a critically important influence on pain perception and these relationships lie in the connectivity of brain regions controlling pain perception, attention or expectation, and emotional states. Imaging studies have confirmed that activity of afferent and descending pain pathways are altered by attentional state, positive emotions, and negative emotions among many other factors unrelated to the pain stimulus itself. The physiology of central pain amplification at the level of the brain takes into account these important connections. There are now numerous studies that demonstrate that patients with chronic pain have alterations in brain regions involved in cognitive and emotional modulation of pain (5). This complex interplay may explain why patients with long-term chronic pain develop anxiety and depression, but also why those with cognitive distortion and psychological distress are at increased risk for chronic pain and central amplification of pain.
These principles have been known in one form or another for decades. It is astonishing that health care professionals for a time acquiesced to a false dogma of pain as an objective and measurable parameter. This dogma and its consequences over the last 15 years represents one of the most shameful recent trends in medicine. The author puts it this way:
One of the most distressing recent developments in medicine is the explicit focus on eliminating pain — manifested as pain as the fifth vital sign and relief of pain being used to measure the quality of a health care facility.
The article points out that although pain is referable to some form of tissue damage there are, especially in the case of chronic pain, complex brain body relationships that come into play. Here are some examples given in the article:
Those patients with one pain condition are more likely to develop another, more centralized form of pain. For example, patients with inflammatory or degenerative joint disease, for example, are almost four times as likely to also have fibromyalgia, the prototypical musculoskeletal central pain amplification syndrome (7). Centrally maintained pain, in contrast to nociceptive or neuropathic pain, is usually multifocal, difficult to precisely localize, moves from site to site, and may have variable pain descriptions.
Chronic pain syndromes, such as chronic headaches, temporomandibular disorder, fibromyalgia, irritable bowel syndrome (IBS), interstitial cystitis/irritable bladder, pelvic pain, and others, cluster together in an individual (8) (Figure 2).
There is much more. The article is available as free full text.