analgesia

analgesia In contrast to anaesthesia, which signifies loss of feeling — including such sensations as heat and cold, consciousness being optional — the Oxford English Dictionary defines analgesia as insensibility to pain; painlessness. In medical terms anaesthesia would imply the total relief of pain which is necessary for a surgical operation, while analgesia would provide a varying amount of relief for a painful condition. This difference is laboured here because of inconsistency in the use of the words by anaesthetists, for some of whom anaesthesia implies loss of consciousness — hence the insistence on the usage local (and regional, and spinal) analgesia rather than local anaesthesia. However, it is common to speak in the same breath about spinal anaesthesia and epidural analgesia, while the patch of skin insensitivity that results from cutaneous nerve damage is invariably referred to as anaesthesia. Since painlessness is the normal condition, analgesia here will refer to the relief of pain, acute or chronic, and the subject will be considered from its anatomical, physiological, pharmacological, and psychological aspects.

The anatomical approach implies the relief of pain by surgery or by injection. Nerve compression, which may occur at several sites, most commonly the sciatica that results from disc protrusion in the spinal canal, or the carpal tunnel syndrome at the wrist, can be cured by operation; and, rarely, severe intractable pain, such as trigeminal neuralgia, which affects the face, may, as a last resort, be treated by division or destruction of the appropriate nerve. Analgesia may also be produced by the injection of a local anaesthetic, generally into the epidural space around the spinal cord, for the relief of pain in childbirth, or when analgesia is required for some days after an operation, or after a major injury which involves fractures of the ribs.

Two recent advances in the understanding of the physiological mechanisms of pain and its suppression have suggested new approaches to pain relief, and have explained the efficacy of some very old methods. The gate control theory of pain, first proposed in 1965 by Patrick Wall (physiologist at University College, London) and Ronald Melzack, has given rise to the use of transcutaneous electrical nerve stimulation (TENS) by means of a small battery-operated apparatus, to produce analgesia. This involves the electrical stimulation of nerves at or adjacent to the painful region, which enter the spinal cord at about the same level. It has been used with some effect for the treatment of chronic pain, and to produce relief during childbirth. The gate control theory also offers a physiological explanation for the efficacy of such psychology-laden folk remedies as rubbing the offending part; the application of cold or of counterirritants such as camphor; cupping; and moxibustion (the burning of small piles of moxa, the common mugwort, on the skin, to produce a blister).

The discovery in 1973 of opiate receptors in the central nervous system led to the search for, and discovery of, endogenous analgesic substances, the endorphins a year later, the assumption of the scientists having been that the receptors must be there for a physiological purpose. The release of these hormones at times of stress explains the phenomenon that pain may not be felt until some considerable time after the injury — the legendary footballer who continues to play with a broken leg, for example. This observation of delayed pain in the wounded, described by the Harvard anesthesiologist Henry Beecher (1907–76) during the 1943 North Africa campaign, was already well-known to earlier army surgeons. Richard Wiseman (1622–76), caring for the injured during the English Civil War, advised that wounds should be cleaned and dressed as soon as possible, before pain began to be felt. The discovery of endorphins has given rise to hopes that analgesics with more specific sites of action than opiates, and without their undesirable side-effects such as constipation, respiratory depression, and addiction, might be synthesized. Both the gate control theory and endorphin release have been invoked in attempts to give physiological respectability to acupuncture for the relief of pain.

The greatest part of pain relief, however, is dependent on pharmacological agents. The relief of acute, intermittent pain, such as during childbirth or dentistry, can be achieved rapidly and effectively by the intermittent inhalation of an analgesic gas or vapour: nitrous oxide, or until recently, when it was judged too expensive to manufacture in pure form, trichlorethylene (trilene). Otherwise pain relief involves the administration, either by mouth or by injection, of analgesic drugs. These come in gradations of effectiveness, and with different mechanisms of action, which make them more appropriate either for acute or chronic requirements. The basis of the most potent analgesics is still the opiates. Opium, the dried juice of the poppy seed capsule, is one of the oldest drugs known. It was mentioned by Homer, and by Aristotle, but until 1805, when the German apothecary Friedrich Wilhelm Sertürner (1783–1841) prepared pure crystals of the active principle — to which the French scientist Gay-Lussac gave the name morphine — it was available only as a crude, unstandardized preparation. Sertürner's researches opened the door to the isolation of many alkaloids, and eventually to the synthesis of morphine derivatives such as diamorphine (heroin), and codeine.

During the 1980s two methods of administration developed which made the patient less dependent on the attention of others; the battery operated syringe pump and electronic, fail-safe, patient-controlled systems. The first may be used when there is the need to provide continuous analgesia in advanced cancer, and the second to relieve pain postoperatively.

Of the milder analgesics, salicylic acid was isolated from willow bark, and its acetyl derivative, better known as aspirin, was prepared in 1897, and was hailed as a wonder drug for its analgesic and antipyretic (fever-controlling) properties. During the first half of the twentieth century it was used for the control of both acute pain and chronic inflammatory conditions such as rheumatic fever. For acute use it has largely been replaced by paracetamol, which came on the market in 1953 and has generally proved a safer analgesic, certainly in children.

Drugs used for the control of long-standing pain include the non-steroidal anti-inflammatory drugs (NSAIDS). The first of these, ibuprofen, resulted from the screening of more than one thousand compounds in the laboratories of Boots of Nottingham, and was patented in 1964. Since then a number of non-steroidals effective in chronic conditions, such as rheumatoid arthritis, have been synthesized, and have been found effective also in relieving postoperative pain. They, and also acetylsalicylic acid, act by preventing the synthesis of prostaglandins, which are to a large extent the cause of the pain, swelling, redness, and fever characteristic of inflammation.

States of ‘altered consciousness’, hypnosis, autohypnosis, euphoria, and psychosomatic conditions such as hysteria, may be accompanied by insensitivity to pain. Hypnosis has been used successfully for obstetric pain relief, and has even been able to produce the profound analgesia necessary for surgical operations, but it is a very time-consuming process. However, even such a simple measure as relieving anxiety can be effective in reducing the severity of pain. Who has not experienced the relief which comes from just making an appointment with the doctor or dentist? In recent years an attempt has been made to relieve psychologically certain — mercifully rare — states of chronic pain that are not susceptible to relief by any of the conventional methods. Often no anatomical or pathological cause can be found, and the condition becomes all-absorbing and is characterized by a state of alienation from ordinary everyday life. Attempts to produce relief have invoked research into medical anthropology — into, for example, the trance-like state which may be entered into to relieve the pain of certain initiation rites. Another approach has been to attempt to dissociate the physical pain from the sufferer's response to it, involving an attempt to rebuild a life around the pain. While much abstruse philosophy has been written about the theory behind this movement, essentially it involves listening to the sufferer, taking his symptoms seriously, and finding some means of taking his mind off them.

David Zuck

Bibliography

Melzack, R. and and Wall, P. (1992). The challenge of pain. Penguin Books, Harmondsworth.
Rey, R. (1998). The history of pain. Harvard University Press, Cambridge, MA.
Sneader, W. (1985). Drug discovery — the evolution of modern medicines. Wiley, Chichester.

See also opiates and opoid drugs; pain.