The care of people with pain has been wrought with ineffective and unnecessary treatment, including the misuse of opioids, largely because we do not have an accurate conceptualization of pain. The absence of animal and human models of central nervous system (CNS) pain processing ensures that our understanding of pain will remain incomplete for the foreseeable future, but enough evidence exists to help family physicians develop an understanding of pain that goes beyond what we learned in medical school and that can help us more effectively treat patients with pain.
In this review, we will briefly discuss the established concepts of nociceptive and neuropathic pain. And then, with those concepts in mind, we will explore a third type of pain that for lack of a better term, we will call “pain for psychological reasons.” We hypothesize that this pain may be the consequence of changes in nervous system function that arise from developmental trauma, other traumatic experiences in a patient’s life, or mental health disorders. It is this third type of pain that may offer us insights into conditions such as fibromyalgia.
While we do not yet have validated diagnostic criteria for this third type of pain, we believe that there is enough information to present initial criteria so that one may distinguish it from nociceptive and neuropathic pain.
Nociceptive and neuropathic pain: The current paradigm
Nociceptive pain. The sensory pain experience, or nociceptive pain, is produced by noxious stimuli that either damage, or are capable of damaging, tissues (eg, burns, cuts, fractures, inflammation, and increased pressure in a hollow viscus). Noxious stimuli are detected at the molecular level by specific pain sensory receptors embedded in our tissues called nociceptors.
The process by which noxious stimuli lead to the experience of sensory pain consists of 4 steps—transduction, transmission, modulation, and perception—which are described in “From periphery to brain: The process of nociceptive pain.”1-4
Neuropathic pain. While nociceptive pain can be easily traced from a peripheral nociceptive fiber to the brain and typically resolves when the nociceptive stimulus stops, neuropathic pain (NPP) results from changes to the function of the nervous system and is typically caused by injury to the nerves. Such changes, referred to as neuronal sensitization, may not quickly resolve, as is the case with postherpetic neuralgia. In fact, the changes can become permanent. NPP fundamentally differs from nociceptive pain because it results from changes in the central processing of pain that can lead a person to perceive pain sensations even in the absence of tissue pathology.
Common causes of NPP that persists even after tissue damage has healed include trauma (eg, amputation of a limb), ischemia (eg, pressure palsy), disease (eg, the metabolic injury of diabetes or the injury caused by a shingles infection), and drug treatment (eg, chemotherapy). The underlying mechanisms of NPP and the neuronal plasticity (the ability of the nervous system to rewire itself) that initiate and then maintain NPP are important areas of active research that may eventually lead to the development of more effective treatments.
Timing is critical. Neuroplastic changes in the nervous system following nerve injury are time-dependent. Synaptic plasticity can occur within seconds to minutes, while cellular plasticity occurs within hours to days. Synaptic and cellular plasticity happen relatively fast and may be reversible.