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Algopathy—Acknowledging the Pathological Process of Pain Chronification

A case must be made that the best way forward in pain management requires a concept that presents a clear distinction between and an accounting of the chronicity, central sensitization, plasticity, and wind-up of pain.
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Despite its ubiquity, sustained, long-term pain still defies definition. The traditional medical/scientific approach to pain has relied on a rigid dichotomy between mind and body, resulting in pain that is considered either physical (eg, related to tissue injury) or psychological (eg, pain with no obvious physical causation).1 This duality in pain, however, was challenged in the 1970s and 1980s by a move to recognize a mind-body connection and a more holistic approach to patient care.2

Defining pain requires acknowledgement of the chronicity, central sensitization, plasticity, and wind-up of pain

Pain may arise from damage to non-neural tissue (nociceptive pain), the bones and muscles (somatic pain), the internal organs (visceral pain), or nerve damage or aberrant neural processing (neuropathic pain), and may be associated with inflammation, which in turn causes pain.3 In effect, many patients present with multiple mechanisms of pain that also contribute to their perceived pain.

The basis for our modern understanding of pain is that it is a complex biopsychosocial phenomenon that can vary widely among individuals.4 Many interlinking factors contribute to the inherent subjectivity of pain: age, gender, genetic factors, demographics, ethnicity, culture, familial beliefs, and past experiences. Other factors, such as stress and catastrophizing, play a role in pain perception.4 Pain, therefore, is both a physical symptom and an emotional response to that symptom.

This growing appreciation for the complexity of pain has implications in defining pain, including how clinicians and their patients talk about pain. Pain is essentially an experience;5 by helping patients articulate their specific experience, clinicians may to better respond to and manage that pain.

Taxonomies of Pain

Scientific taxonomies of pain may help shape our clinical thinking but are not always intuitively grasped by patients. Nevertheless, pain taxonomies are useful in that even with imperfect definitions of various pain types, they have allowed practitioners to classify different pain experiences in ways that help in the diagnosis and treatment of them. The International Association for the Study of Pain has published a lexicon of pain terminology and taxonomy to aid in and formalize the communication of the pain experience.6

To reinforce the complexity of pain terminology, consideration of the differences in nociceptive versus neuropathic pain pathways is a good exercise.

Nociceptive pain, for instance, refers to the pain that results from a noxious stimulus at the periphery, which is then transmitted via the neural network to the spinal cord and from there upward to the brain for interpretation and response. The brain then processes this signal and may take action in an adaptive mechanism aimed at preventing further injury to the body. A good example of this might be burning one’s hands on a hot pot—the nociceptive signal results in both pain perception and the quick withdrawal from the hot surface. Nociceptive pain may be further subdivided into somatic pain (nociceptive pain that occurs in somatic tissue, or muscles and bones) and visceral pain (nociceptive pain in internal organs). Nociceptive pain is perceived via sensory receptors in the peripheral somatosensory nervous system.6

Neuropathic pain, on the other hand, is caused by a lesion, disease, or damage to the somatosensory nervous system, although a precise definition eludes taxonomists. Neuropathic pain is regarded as pain that results from aberrant neural processing, not from a noxious stimulus. Neuropathic pain can be peripheral, such as peripheral diabetic neuropathy, or centralized (eg, fibromyalgia, phantom limb pain). Neuropathic pain typically presents without structural damage or injury to the body and can be difficult to diagnose.

A simplistic yet helpful way to think of nociceptive versus neuropathic pain is that nociceptive pain primarily relates to what could be called “hardware,” while neuropathic pain often (but not always) relates to the “software” of the body. Neuropathic pain may originate from a “hardware” problem (eg, causalgia, multiple sclerosis, Parkinson’s disease) or as a “software” problem, arising from conditions such as depression, anxiety, or post-traumatic stress disorder. Examples of these various pain types appear in Table 1.

It is important for clinicians to distinguish between nociceptive and neuropathic pain sensations, as these types of pain respond to different treatments, in order to manage the intensity of the pain.

Temporal Aspects of Pain

Pain is routinely described as acute, subacute, or chronic. While it is typical and easy to differentiate these categories by the length of time that the pain persists, acute pain is fundamentally different from chronic pain in more ways than just the duration of the experience.

While there is no consensus on the length of time pain needs to be present to be defined as acute, common definitions exist for different conditions. For example, low back pain lasting less than 4 weeks in duration is acute, whereas subacute low back pain persists for 4 to 12 weeks, and chronic low back pain after 12 weeks. Other definitions for acute pain range from 72 hours to 1 or 2 weeks and so on.7-11

The term “persistent pain” is sometimes used clinically and appears in the literature, although it is not always clear if and how it differs from the older and more established “chronic pain. “Acute pain is typically nociceptive pain that follows a trajectory of decreasing pain intensity as tissue heals. It is thought that acute pain is adaptive, such that it encourages the individual to rest, withdraw, and protect the injury.

In contrast, chronic pain is often maladaptive, as it persists long after the tissue has healed. Chronic pain involves centralization or a migration from the peripheral to central nervous systems (central sensitization). Acute pain may transition into chronic (centralized) pain in a process known as chronification or wind-up,12,13 although the exact mechanisms behind this, and why it occurs in some patients but not in others, remain unclear.

In keeping with this model for defining pain, subacute would represent the transitional state from acute to chronic pain. During this stage, the emotional component of the pain experience increases even as the physical component decreases. In fact, it may be helpful to think of acute pain as perception colored by emotions, while chronic pain is emotion colored by perception.

Chronic pain is problematic for patients to talk about, difficult for clinicians to assess, and challenging to treat. Chronic pain is often diffuse or migratory and may be intermittent, all qualities that make it easy for the patient to doubt the validity of the clinical symptoms. Chronic pain may also be characterized by unpredictable flares.14

Last updated on: May 16, 2017
First published on: May 15, 2017
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