Knowledge of these factors, together with the physical examination, including maneuvers designed to reproduce and relieve the pain and ancillary diagnostic procedures, enable the physician to identify the source of most pains and the diseases of which they are a part. Nevertheless, the severity of pain is often difficult to assess reliably. Extreme degrees of pain are betrayed by the patient’s demeanor but lesser degrees can be roughly estimated by the extent to which the pain has interfered with the patient’s sleep, work, and other activities, or by the patient’s need for bed rest. Some physicians find it helpful, particularly in gauging the effects of analgesic agents, to use a “pain scale,” that is, to have the patient rate the intensity of his pain on a scale of zero (no pain) to 10 (worst pain) or to mark it on a line (the Visual Analog Pain Scale). It has been our experience that this effort to quantify pain is often unhelpful to the neurological analysis as patients rarely rate pain as trivial once they have decided to consult a physician about the problem. For most patients, pain that necessitates medical consultation is, by definition, considered to be serious. This general approach is put to use every day in the practice of general medicine.
In addition to the pains caused by the more common and readily recognized diseases of each organ system, there remain a significant number of chronic pains that fall into one of four categories: (1) pain from an obscure medical disease, the nature of which has not yet been disclosed by diagnostic procedures; (2) pain associated with disease of the central or peripheral nervous system (i.e., neurogenic, or neuropathic pain); (3) pain associated with psychiatric disease; and (4) pain of unknown cause.
Here the source of the pain is usually in a bodily organ and is caused by a lesion that irritates and destroys nerve endings. Consequently, the term nociceptive pain is often used even though it is ambiguous. It usually means an involvement of structures bearing the origin of pain fibers. Cancer is the most frequent example. Osseous metastases, tumors of the kidney, pancreas, or liver, peritoneal tumor implants, invasion of retroperitoneal tissues or the hilum of the lung, and infiltration of nerves of the brachial or lumbosacral plexuses can be extremely painful, and the origin of the pain may remain obscure for a long time. Sometimes it is necessary to repeat all diagnostic procedures after an interval of a few months, even though at first they were negative. From experience one learns to be cautious about reaching a diagnosis from insufficient data. Treatment in the meantime is directed to the relief of pain, at the same time instilling in the patient a need to cooperate with a program of expectant observation.
These terms are sometimes used interchangeably to designate pain that arises from direct stimulation of nervous tissue itself, central or peripheral, exclusive of pain as a consequence of stimulation of C fibers by lesions of other bodily structures (i.e., the nociceptive pain described above). The term “neuropathic” has been attached more often to pain arising in the peripheral nervous system. This category comprises a variety of disorders involving single and multiple nerves, notably trigeminal neuralgia and those caused by herpes zoster, diabetes, and trauma; neuromas and neurofibromas, polyneuropathies of diverse type; root irritation from a prolapsed disc; and spinal arachnoiditis. Neurogenic pain of central origin includes the thalamic pain syndrome of Dejerine-Roussy and spinal cord injuries; and parietal lobe infarction such as the cases described by Schmahmann and Leifer. As a rule, lesions of the cerebral cortex and white matter are associated not with pain but with hypalgesia. Schott (1996) has reviewed the clinical features that characterize central pain. Particular diseases giving rise to neuropathic pain are considered in their appropriate chapters but the following remarks are applicable to the entire group.