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New Concepts in Acute Pain Therapy: Preemptive Analgesia ALLAN GOTTSCHALK, M.D., PH.D., Johns Hopkins Medical Institute, Baltimore, Maryland DAVID S

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New Concepts in Acute Pain Therapy: Preemptive Analgesia ALLAN GOTTSCHALK, M.D., PH.D., Johns Hopkins Medical Institute, Baltimore, Maryland DAVID S New Concepts in Acute Pain Therapy: Preemptive Analgesia ALLAN GOTTSCHALK, M.D., PH.D., Johns Hopkins Medical Institute, Baltimore, Maryland DAVID S. SMITH, M.D., PH.D., University of Pennsylvania School of Medicine, Philadelphia, Pennsylvania Pain, which is often inadequately treated, accompanies the more than 23 million surgi- cal procedures performed each year and may persist long after tissue heals. Preemptive O A patient informa- analgesia, an evolving clinical concept, involves the introduction of an analgesic regi- tion handout on pain control, written by the men before the onset of noxious stimuli, with the goal of preventing sensitization of authors of this article, the nervous system to subsequent stimuli that could amplify pain. Surgery offers the is provided on page most promising setting for preemptive analgesia because the timing of noxious stimuli 1985. is known. When adequate drug doses are administered to appropriately selected patients before surgery, intravenous opiates, local anesthetic infiltration, nerve block, subarachnoid block and epidural block offer benefits that can be observed as long as one year after surgery. The most effective preemptive analgesic regimens are those that are capable of limiting sensitization of the nervous system throughout the entire peri- operative period. (Am Fam Physician 2001;63:1979-84,1985-6.) ain is thought to be inadequately laparotomy,8,9 herniorrhaphy10 and mastec- See editorial 11 on page 1924. treated in one half of all surgical tomy. Moreover, even low levels of residual procedures.1 In addition to imme- pain are associated with decreased physical diate unpleasantness, painful expe- and social function as well as a diminished riences can imprint themselves perception of overall health.9 Questions that Pindelibly on the nervous system (Figure 1), confront a physician whose patient is about to amplifying the response to subsequent noxious undergo surgery include: “What is the process stimuli (hyperalgesia) and causing typically by which painful stimuli sensitize the nervous painless sensations to be experienced as pain system?” and “How can this sensitization be (allodynia). A chronic condition sometimes prevented to reduce short-term and long- develops that produces continuous pain long term pain and its consequences?” after surgery. Prior painful experiences are a known predictor of increased pain and anal- Pain Response gesic use in subsequent surgery.2,3 Both the peripheral and the central ner- The process by which the nervous system vous systems (CNS) are involved in the per- becomes sensitized is active early in life, as ception of pain, with the spinal and demonstrated by the enhanced pain-related supraspinal components of the CNS playing behavior that occurs in circumcised boys dur- key roles12 (Figure 2).13 The transduction of ing subsequent vaccination compared with noxious stimuli begins with peripheral noci- boys who were not circumcised4 and boys ceptors. Signals from these nociceptors travel who were circumcised after application of a primarily along small myelinated A and local anesthetic cream.5 Long-term painful unmyelinated C fibers with soma lying in the sequelae of surgery in adults occur following dorsal root ganglion. Their axons synapse in amputation of an extremity,6 thoracotomy,7 the dorsal horn of the spinal cord, where the neurons of laminae I, II and V are most involved in the perception of pain. The signals then travel along the spinothal- Pain accompanies almost all surgical procedures but is amic tract of the spinal cord to the thalamus treated inadequately in more than one half of patients. and the cortex. Large fiber inputs from other sensory modalities and descending pathways MAY 15, 2001 / VOLUME 63, NUMBER 10 www.aafp.org/afp AMERICAN FAMILY PHYSICIAN 1979 can modulate activity in the dorsal horn, the initial injury. Experimentally, this second where these descending pathways may pro- phase is associated with a growing region of vide a physiologic explanation for the hypersensitivity around the point where the increased pain experienced by patients who noxious stimulus was initially applied. have high levels of depression and anxiety.3,9 The process through which the neurons of Painful stimuli ultimately cause activity in the dorsal horn of the spinal cord become sen- both the somatotopically appropriate portion sitized by prior noxious stimuli is often of the sensory cortex and the limbic system.14 referred to as “windup” or “central sensitiza- The response to noxious stimuli can be tion.”Much less is known about pain-induced modulated by their repeated application.12 For sensitization of the supraspinal components example, peripheral nociceptors become of the CNS. Collectively, however, the above more responsive with the repeated application mechanisms enhance sensitivity to noxious of noxious stimuli. Their sensitivity can be stimuli and may increase the level of pain further enhanced by many tissue factors and experienced following surgery. inflammatory mediators released in the course of tissue injury. The response of neu- Preemptive Analgesia rons in the dorsal horn of the spinal cord of One of the most critical observations con- experimental animals has been found to be cerning central sensitization is the role played biphasic. The initial response to a noxious by the first phase of the pain response. Opiates stimulus is brief and correlates with the sharp, administered before the first phase and well-localized initial pain. The second phase of reversed with the opiate antagonist naloxone the response is more prolonged and correlates (Narcan) before the expected onset of the sec- with the dull, diffuse pain experienced after ond phase were capable of preventing this late stage of the pain response.15 Thus, preventing the initial neural cascade could lead to long- Pain Sensitization term benefits by eliminating the hypersensi- tivity produced by noxious stimuli. 10 Animal experiments demonstrated the ben- Hyperalgesia efits of preventing central sensitization by . 16 8 . infiltrating with local anesthetics, an Normal pain approach that was particularly effective with response pain associated with deafferentation, as might 6 Injury occur with amputation.17 Collectively, results like these led to the concept of preemptive 4 analgesia—initiating an analgesic regimen Pain intensity before the onset of the noxious stimulus to 2 . Allodynia prevent central sensitization and limit the subsequent pain experience.18 0 Surgery may be the clinical setting where ILLUSTRATIONS BY DAVID KLEMM Stimulus intensity preemptive analgesia techniques will be the most effective because the onset of the intense noxious stimulus is known (Figure 3).19 To FIGURE 1. Noxious stimuli can sensitize the nervous system response to appreciate the design of clinically effective subsequent stimuli. The normal pain response as a function of stimulus strategies in this setting, it is essential to recog- intensity is depicted by the curve at the right, where even strong stim- uli are not experienced as pain. However, a traumatic injury can shift the nize that otherwise adequate levels of general curve to the left. Then, noxious stimuli become more painful (hyperal- anesthesia with a volatile drug such as isoflu- gesia) and typically painless stimuli are experienced as pain (allodynia). rane (Forane) do not prevent central sensiti- 1980 AMERICAN FAMILY PHYSICIAN www.aafp.org/afp VOLUME 63, NUMBER 10 / MAY 15, 2001 Preoperative Pain Control zation.20 Thus, the potential for central sensi- pain than either spinal anesthesia or general tization exists even in unconscious patients anesthesia alone, and these benefits appeared to who appear to be clinically unresponsive to last many days.23 surgical stimuli. Although spinal anesthesia clearly provides a better intraoperative block of the surgical Efficacy stimulus, the more effective postoperative Despite solid demonstrations of its effects analgesia produced by infiltration with a long- in some animal models, considerable contro- acting local anesthetic may have been an versy surrounds the use of preemptive analge- important factor in preventing central sensiti- sia in clinical settings. This controversy exists zation. During routine laparoscopy with gen- because not all clinical trials of preemptive eral anesthesia, infiltrating with local anes- analgesia have resulted in clear demonstra- thetic before incision was found to be more tions of its efficacy. In evaluating clinical trials effective than infiltrating with saline before of preemptive analgesia, the timing of the intervention is only one factor. It is also essen- tial to consider the ability of the intervention Providing Postoperative Pain Relief to prevent central sensitization and whether other aspects of the perioperative pain experi- ence may be of sufficient duration and inten- sity to mask any intraoperative benefits from Pain Opioids the preemptive analgesia. Alpha2 agonists Many clinical protocols have mirrored the laboratory studies using animals that gave birth to the concept of preemptive analgesia. Ascending Descending modulation However, these animal experiments employed input painful stimuli of intensity, duration and Local anesthetics somatotropic extent that were generally far less Dorsal Opioids horn than that experienced during even relatively Alpha2 agonists minor surgery in human patients.21 Therefore, . it should not be surprising that interventions Dorsal root with a limited capacity for preventing central . ganglion sensitization,
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