DEFINITIONS & CLASSIFICATION OF PAIN
Like other conscious sensations, normal pain per-ception depends on specialized neurons that func-tion as receptors, detecting the stimulus, and then transducing and conducting it to the central nervous system. Sensation is often described as either proto-pathic (noxious) or epicritic (nonnoxious). Epicritic sensations (light touch, pressure, proprioception, and temperature discrimination) are characterized by low-threshold receptors and are generally con-ducted by large myelinated nerve fibers. In con-trast, protopathic sensations (pain) are detected by high-threshold receptors and conducted by smaller, lightly myelinated (Aδ) and unmyelinated (C) nerve fibers.
Pain is not just a sensory modality but an experi-ence. The International Association for the Study of Pain defines pain as “an unpleasant sensory and emotional experience associated with actual or potential tissue damage, or described in terms of such damage.” This definition recognizes the inter-play between the objective, physiological sensory aspects of pain and its subjective, emotional, and psychological components. The response to pain can be highly variable among different individuals as well as in the same person at different times.
The term nociception is derived from noci (Latin for harm or injury) and is used to describe neural responses to traumatic or noxious stimuli. All nociception produces pain, but not all pain results from nociception. Many patients experience pain in the absence of noxious stimuli. It is there-fore clinically useful to divide pain into one of two
categories: (1) acute pain, which is primarily due to nociception, and (2) chronic pain, which may be due to nociception, but in which psychological and behavioral factors often play a major role. Table 47–1 lists terms frequently used in describing pain.Pain may also be classified according to patho-physiology (eg, nociceptive or neuropathicpain), etiology (eg, arthritis or cancer pain), or the affected area (eg, headache or low back pain). Such classifications are useful in the selection of treatmentmodalities and drug therapy. Nociceptive pain is caused by activation or sensitization ofperipheral nociceptors, specialized receptors that transduce noxious stimuli. Neuropathic pain is the result of injury or acquired abnormalities of periph-eral or central neural structures.
There are differences in pain perception related to gender and age. Research has confirmed differ-ences in pain experiences and coping strategies between genders, and there is ongoing investigation into exactly how this processing differs. Brain activa-tion differs between genders, with men particularly influenced by the type and intensity of a noxious stimulus. Brain imaging patterns differ as well. Some of these differences decrease with age and may dis-appear after age 40.
Acute pain is caused by noxious stimula-tion due to injury, a disease process, or theabnormal function of muscle or viscera. It is usu-ally nociceptive. Nociceptive pain serves to detect, localize, and limit tissue damage. Four physiologi-cal processes are involved: transduction, transmis-sion, modulation, and perception. This type of pain is typically associated with a neuroendocrine stress response that is proportional to the pain’s intensity. Its most common forms include post-traumatic, postoperative, and obstetric pain as well as pain associated with acute medical illnesses, such as myo-cardial infarction, pancreatitis, and renal calculi. Most forms of acute pain are self-limited or resolve with treatment in a few days or weeks. When pain fails to resolve because of either abnormal healing or inadequate treatment, it becomes chronic (below). Two types of acute (nociceptive) pain—somatic and visceral—are differentiated based on origin and features.
Deep somatic pain arises from muscles, ten-dons, joints, or bones. In contrast to superficial somatic pain, it usually has a dull, aching quality and is less well localized. An additional feature is that both the intensity and duration of the stimulus affect the degree of localization. For example, pain following brief minor trauma to the elbow joint is localized to the elbow, but severe or sustained trauma often causes pain in the whole arm.
convergence of visceral and somatic afferent input into the central nervous system. Thus, pain associat-ed with disease processes involving the peritoneum or pleura over the central diaphragm is frequently referred to the neck and shoulder, whereas pain from disease processes affecting the parietal surfaces of the peripheral diaphragm is referred to the chest or upper abdominal wall.
Chronic pain is pain that persists beyond the usual course of an acute disease or aftera reasonable time for healing to occur; this heal-ing period typically can vary from 1 to 6 months. Chronic pain may be nociceptive, neuropathic, or mixed. A distinguishing feature is that psychologi-cal mechanisms or environmental factors frequently play a major role. Patients with chronic pain often have attenuated or absent neuroendocrine stress responses and have prominent sleep and affective (mood) disturbances. Neuropathic pain is classically paroxysmal and lancinating, has a burning quality, and is associated with hyperpathia. When it is also associated with loss of sensory input (eg, amputa-tion) into the central nervous system, it is termed deafferentation pain. When the sympathetic systemplays a major role, it is often termed sympatheticallymaintained pain.
The most common forms of chronic pain include those associated with musculoskeletal disor-ders, chronic visceral disorders, lesions of peripheral nerves, nerve roots, or dorsal root ganglia (including diabetic neuropathy, causalgia, phantom limb pain, and postherpetic neuralgia), lesions of the central nervous system (stroke, spinal cord injury, and mul-tiple sclerosis), and cancer pain. The pain of most musculoskeletal disorders (eg, rheumatoid arthritis and osteoarthritis) is primarily nociceptive, whereas pain associated with peripheral or central neural disorders is primarily neuropathic. The pain asso-ciated with some disorders, eg, cancer and chronic back pain (particularly after surgery), is often mixed. Some clinicians use the term chronic benign pain when pain does not result from cancer. This termi-nology should be discouraged, however, because pain is never benign from the patient’s point of view, regardless of its cause.