The ambiguity with which the term pain is used is responsible for some of our difficulty in understanding it. One aspect, the easier to comprehend, is the transmission of impulses along certain pathways in response to potentially tissue-damaging stimuli, that is, nociception. Far more abstruse is its quality as a mental state intimately linked to emotion, the quality of anguish or suffering that defies definition and quantification, or “a passion of the soul,” in the words of Aristotle. This duality (nociception and suffering) is of practical importance, for certain drugs or surgical procedures, such as cingulotomy, may reduce the patient’s reaction to painful stimuli, leaving awareness of sensation largely intact. Further complexities are that the symptom of pain may persist despite interruption of neural pathways that abolish all sensation (i.e., denervation dysesthesia or anesthesia dolorosa), or that pain may continue to be perceived from the absent part of an amputated limb (“phantom pain”). Finally, pain can be evoked by almost any sensory modality, such as touch, pressure, heat, or cold, if it is intense enough.
It is apparent that few physicians are capable of handling difficult and unusual pain problems and it is sometimes to the neurologist that other physicians may turn for help with these matters. Although much has been learned about the anatomy of pain pathways, their physiologic mechanisms, and which structures to ablate in order to produce analgesia, the effective management of pain by medical and surgical means remains a considerable clinical challenge. The practice of pain medicine challenges every thoughtful physician, for it demands a high degree of skill in medicine, neurology, and psychiatry. More problematic are patients who seek treatment for pain that appears to have little or no structural basis; further inquiry may disclose that fear of serious disease, worry, or depression has aggravated some relatively minor ache or that the complaint of pain has become the means of seeking attention, drugs, or monetary compensation. There is also the “difficult” pain patient, in whom no amount of investigation brings to light either medical or psychiatric illness. Further complicating almost all aspects of pain medicine is concern about narcotic dependence, tolerance, and addiction, which may be informed by societal and political forces. Finally, the physician must be prepared to manage patients who require relief from intractable pain caused by established and incurable disease.
For more than a century, views on the nature of pain sensation have been dominated by two major theories. One, the specificity theory, was from the beginning associated with the name of von Frey. He asserted that the skin consisted of a mosaic of discrete sensory spots and that each spot, when stimulated, gave rise to one sensation—either pain, pressure, warmth, or cold; in his view, each of these sensations had a distinctive end organ in the skin and each stimulus-specific ...