Appendix
A
EXCERPTS
FROM “PATHOPHYSIOLOGY OF PAIN”
Pain is “the perception of a noxious stimulus that begins in the dorsal horn and involves the entire spinal cord and brain.” The International Association for the Study of Pain (IASP) defines pain as “an unpleasant sensory and emotional experience which we primarily associate with tissue damage or describe in terms of such damage, or both.”
Pain can be described in terms of sensory, emotional, and cognitive components. These three components of pain are reflected in the mechanisms of the transmission and modulation of painful stimuli. Such mechanisms are mediated through the nociceptor neurons, the spinal cord processes, and the cerebral or brain processes.
Many different taxonomies exist for the classification of pain. In the clinical setting, a physician must infer the pathophysiology of a pain syndrome from his or her patient’s clinical evaluation. The most common of these inferred pathophysiologies may be described as nociceptive, neuropathic, and mixed.
For more information, please see Pain Assessment.
Pain is termed “nociceptive” when the clinical evaluation suggests that it is sustained primarily by the nociceptive system. Nociceptive pain is pain that is proportionate to the degree of actual tissue damage. A more severe injury results in a pain that is perceived to be greater than that caused by a less severe injury.
Such pain serves a protective function. Sensing a noxious stimulus, a person behaves in certain ways to reduce the injury and promote healing (eg, pulling his finger away from a hot object). This “good” pain serves a positive function.
Some examples of nociceptive pain are acute burns, bone fracture, and other somatic and visceral pains.
Neuropathic pain occurs through central nervous system (CNS) changes, such as the processes of “wind-up” phenomenon and central sensitization that can occur in patients with a prolonged exposure to noxious stimuli or nerve injury, or through peripheral nervous system (PNS) changes, such as neuroma formation. It is disproportionate to the degree of tissue damage; it can occur without nociception.
Also called neurogenic pain, neuropathic pain occurs when pathophysiologic changes become independent of the inciting event, thus serving no protective function.
Neuropathic pain does not serve a positive function for the overall health of the person. Some examples of neuropathic pain are painful diabetic and other peripheral neuropathies, deafferentation and sympathetically-maintained pains, and nerve inflammation, compression, or laceration.
For more information, please see Pain Assessment and Neuropathic Pain.
In a given patient, components of continued nociceptive pain may coexist with a component of neuropathic pain. Patients with persistent back and leg pain following lumbar spine surgery (failed low-back-surgery syndrome) represent a common example. Some patients with complex regional pain syndrome (reflex sympathetic dystrophy or causalgia) may develop painful complications that are nociceptive (eg, joint ankylosis, myofascial pain) and that coexist with the underlying neuropathic pain.
Idiopathic pain may be defined as pain that persists without any identifiable organic lesions or as pain that is disproportionate to the degree of tissue damage.