This article includes discussion of chronic pain and pain management. The foregoing terms may include synonyms, similar disorders, variations in usage, and abbreviations.
Neurologic pain management is now recognized as a subspecialty of neurology. The focus of this article is the evaluation of chronic pain as a symptom, including discussion of mechanism, differential diagnosis, diagnostic workup, and general management. Studies on the molecular basis of pain have provided receptors as targets for analgesic drugs. The ideal management of a chronic pain patient is by a multidisciplinary approach, including disciplines such as internal medicine, neurology, anesthesia, nursing, psychology, pharmacy, rehabilitation medicine, physical therapy, etc. Gene therapy may be used in the near future as a method of delivery of therapeutic proteins with analgesic effect to the site of action, thus, avoiding some of the undesirable effects of drugs. Viral vectors expressing proenkephalin can transduce neurons of the dorsal root ganglion, leading to release of enkephalin from nerve terminals in the dorsal horn and producing an analgesic effect.
• An understanding of basic mechanisms of pain is essential for developing management strategies.
• Acute pain can rapidly evolve into chronic pain.
• Chronic pain can be considered a neurologic disorder by itself and is also a symptom of several neurologic diseases, such as Parkinson disease.
• A large number of pharmacological as well as nonpharmacological approaches have been used for the management of chronic noncancer pain.
Historical note and terminology
Pain in antiquity. In ancient societies, pain was thought to be a result of external magical forces. Ayurvedic medicine in India included the use of herbals as analgesics, and acupuncture was used for relieving pain in ancient China. Aristotle (384 BC to 322 BC) theorized that pain was the opposite of "pleasure" and apart from the traditional "5 senses." The use of natural electricity for pain was documented in the first century. Intractable headache was treated with the discharge of the organ of electric fishes.
Understanding of the basic mechanisms of pain. In the 18th century, Erasmus Darwin started an analysis of "unpleasantness" and attributed pain to overstimulation of 1 sense of the 5 senses (Darwin 1794). The relation of pain to peripheral injury had already been recognized some years earlier. The difference between touch and pain as sensations of the skin and other organs was pointed out by Weber in the 19th century (Weber 1846). In the last decade of the 19th century, Henry Head established the referral of the pain to the skin from a deep lying structure (ie, the occurrence of arm pain following stimulation of the pericardium) (Head 1896). Head's studies of herpes zoster led to the discovery of segmental distribution of sensory nerves.
Pain in the 20th century. Several scientific studies of the anatomy and physiology of pain in the first half of the 20th century have been summarized in a book (Hardy et al 1952). Palliative neurosurgery for relief of pain started during this period. Until the 1960s, pain was considered to be an inevitable sensory response to tissue damage. Tremendous advances took place concerning the understanding and management of pain in the last few decades of the 20th century. In 1964, the Melzak-Wall gate theory emphasized the mechanisms in the central nervous system that control the perception of a noxious stimulus and, thus, integrate afferent impulses with downstream modulation from the brain (Melzak and Wall 1965). According to the neuromatrix theory of pain, the brain possesses a neural network that integrates multiple inputs, including past memories and genetic programs, to produce the output pattern that evokes pain (Melzak 1999). Discoveries in molecular biology and electrophysiology of pain continue, and the opioid receptors were identified in 1973. A cure for pain, however, has not yet been discovered.
Role of the neurologist in the management of pain. The neurologist is increasingly involved in the multidisciplinary treatment of patients with chronic pain, and neurologic pain management is now recognized as a subspecialty of neurology by the American Board of Psychiatry and Neurology. The American Academy of Neurology recognizes the undertreatment of patients with chronic nonmalignant pain and provides clear recommendations to help neurologists in the ethical and effective treatment of patients with pain. Although chronic pain can be considered a neurologic disorder and is the main symptom of migraine, pain is also a symptom of several neurologic disorders, such as stroke, peripheral neuropathies, and Parkinson disease, when it is often neuropathic pain.
Definitions. Pain is generally defined as an unpleasant sensory and emotional experience associated with actual or potential tissue damage or described in terms of such damage. Acute pain is the normal predicted physiological response to an adverse chemical, thermal, or mechanical stimulus that may be associated with surgery trauma or acute illness. Nociceptive pain is defined as pain arising from actual or potential damage to nonneural tissue and is due to the activation of nociceptors; it may be acute or chronic. Neuropathic pain is defined as chronic pain caused by nerve irritation, damage, or destruction.
Moderate or severe pain is accompanied by anxiety, and the old concept of the dual nature of pain as an emotion as well as a sensation is being recognized again. Pain can be a symptom of neurologic as well nonneurologic disorders and can be acute or chronic. The traditional separation of acute pain with recent onset and short duration (1 to 6 months) from chronic pain is not supported by studies of the molecular basis of pain. Chronic pain is usually referred to as intractable pain if it persists for 6 months or more.
The focus of this article is the evaluation of chronic pain as a symptom, including discussion of mechanism, differential diagnosis, diagnostic workup, and general management.
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