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A Conceptual Model of Pain

The practicing physician's guide to the causative mechanisms of pain and how to translate these basic principles into diagnoses and treatments.
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Pain, as a concept and symptom, is discussed and described throughout professional and lay medical literature. “Pain" is the reason for initial contact with any physician for the vast majority of medical problems, e.g. abdominal pain, chest pain, limb pain, low back pain, etc. Understanding pain mechanisms, however, is often hampered by the complex neural interconnections involved in pain. Any one, or combination, of the following mechanisms can contribute to pain: neural “sensor" stimulation, neural “wire” misfiring, and “perceptron” dysfunction.

The general public often righteously complains about the lack of recognition and help for their pain and discomfort — aside from direct treatment of the underlying medical pathology. On the other hand, many physicians feel helpless, in the face of the onslaught of public indignation, to help with pain that is not supported by “standard” laboratory and/or radiological evidence, or even by “physiologically” consistent behavior; someone just can’t hurt overall!

Medical doctors depend on knowledge of the pathophysiology or at least a diagnosis to decide on treatment. Thus, to maximize success, physicians need to understand how pain is perceived. Certainly, knowing where and what the problem is, increases the likelihood of a positive outcome. Present categories of pain mentioned in medical literature are helpful, but these concepts are not organized to provide the practicing physician with handles that can help the physician more effectively treat those patients presenting with pain — particularly chronic pain.

While the Joint Commission1 now recognizes and mandates pain as the “fifth vital sign,” the present focus of Pain Medicine is “cover-up” rather than “cure.” Even with an abundance of detailed Pain Medicine literature, there appears to be a limited understanding of the basic mechanisms of pain, even within the research world. Obviously, without a reasonably detailed diagnosis reflecting the underlying pathophysiology of a given pain, treatment is no more than “hit and miss.”

In the defense of physicians, we only know what we know and cannot be expected to be omniscient in our understanding and insight of all medical problems. If we cannot “cure,” then it is good medicine to “cover-up” to reduce suffering. In either case, good medical care must be based on “diagnosis, diagnosis, diagnosis,” and a mechanistic understanding of the underlying pain pathophysiology.

Furthermore, in light of the apparent epidemic of under-treatment of pain — which can have serious legal consequences (i.e., a recent misdemeanor conviction in California) — and the sometimes necessary prescription of socially-unacceptable narcotics in treating chronic pain, the treating physician must understand the basic pathophysiology of pain to provide both optimal and medically-justifiable treatment. For all these reasons, there is an urgent need to clarify, organize and synthesize the abundance of information about pain, and apply these conclusions as simply as possible to the practical treatment of patients.

The Present State of Pain Theory and Thought

Pain is described in a myriad of ways:

  • in temporal terms: chronic pain, subacute pain and acute pain
  • in characterizations: intermittent pain, intractable pain, lancinating pain, referred pain, burning pain and dull pain
  • in acceptable diagnoses (which are all basically syndromes): phantom pain, cancer pain, vascular pain, arthritic pain, nerve pain, muscle pain, fibromyalgia, myofascial pain, sympathetically maintained pain, and complex regional pain syndrome
  • in mechanistic terms: neuropathic and nociceptive pain
  • in anatomic perceptional terms: headache, back pain, neck pain, facial pain, limb pain, abdominal pain, etc.
  • in source of origin terms: central pain as originating in the spinal cord or brain
  • in psychiatric/psychogenic terms: psychosomatic “in-the-head” pain, etc.

These descriptors variously imply the chronicity, the character, the cause or the location of this type of unpleasant sensation. To add complexity, many factors, such as culture, personality, psychosocial stressors and nutritional status, can be involved to influence the degree of pain and to confound the causal factors of the pain.

Caudill2 analyzed pain from different angles to emphasize its complexity, where pain:

  • biologically — serves as a signal that the body has been harmed,
  • psychologically — is experienced as emotional suffering,
  • behaviorally — alters the way a person moves and acts,
  • cognitively — calls for thinking about its meaning, its cause, and possible remedies,
  • spiritually — serves as a reminder of mortality,
  • culturally — tests a people’s fortitude or forces their submission.

Pain disorders are categorized in the DSM-IV-TR3 (coded for the medical condition) as follows:

  • 307.80 Pain Disorder Associated with Psychological Factors
  • 307.89 Pain Disorder Associated with both Psychological Factors anda General Pain Condition

Elsewhere, the DSM-IV-TR3 attributes neural dysfunction to pain. Again, these are only descriptive categories.

The simplest traditional categorization of pain has been “acute” and “chronic.” Acute pain is really just a result of the stimulation of a normally-functioning pain detection system and serves to alert us to avoid or minimize tissue damage. Chronic pain merely means that pain is perceived over a long period of time, which has been arbitrarily set at 6 months.

Descartes showed a basic understanding of the pain pathways in 1664.4 However, this schematic (see Figure 1) suggests that he only appreciated nociceptive pain, which implies normally functioning pain pathways. A more recent rendering of the details of pain pathways is presented in Figure 2.5

Formal classification systems (see Table 1) do exist and provide some insight into what we are trying to accomplish in this article.

The most advanced concepts are expressed by Craig,6 who states that pain is just one manifestation of the mind-body’s homeostasis system. From the patient’s point of view, the spectrum of pain control spans temporary treatments (usually pharmaceutical) in suppressing pain to permanent remission or cure of the underlying pathology/disease.

While these are all very useful concepts, they are generally academic in nature and do not provide much practical help to a physician. Concepts of pain pathophysiology, and thus classification, are abundantly available in the scientific and medical literatures. There is a need to refine and clarify all of this information and apply it as simply as possible to the treatment of pain in the physician’s office.

Last updated on: December 20, 2011
First published on: September 1, 2002