Chronic_Neuropathic_Pain-Slides_Pierce-Smith.ppt

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  • Because of the inherent subjective nature of pain, and the fact that the word “pain” itself connotes multiple meanings, the International Association for the Study of Pain (IASP) has established a standardized definition of pain.
    The definition makes several important points:
    Pain is an unpleasant emotional experience as well as an unpleasant sensory experience. This distinction between the sensory aspects of pain and its emotional (or affective) component has had a significant influence on both research and the treatment of chronic pain.
    Also emphasized by the IASP in defining pain is that pain is always subjective. If patients regard their experience as pain and if they report it in the same ways as pain caused by tissue damage, it should be accepted as pain.
    IASP Task Force on Taxonomy. In: Merskey H, Bogduk N, eds. Classification of Chronic Pain. 2nd ed. Seattle, Wash: IASP Press; 1994:209-214.
  • The causes of acute pain are often known, but the causes of chronic pain and its associated symptoms are not well understood.1
    The pain experienced by patients with acute pain often can be alleviated. In general, the duration of acute pain is brief and has been well characterized.1 The time course of chronic pain, however, is usually indeterminate, and patients with chronic pain are often refractory to treatment.2
    One definition of chronic pain is pain that has persisted beyond the time of normal healing; for research purposes, however, chronic pain is often defined as pain that has persisted at least 3 (sometimes 6) months.3
    Because chronic pain can almost never be cured,4 optimal treatment usually involves helping the patient restore function and supporting a patient’s coping by utilizing approaches that minimize pain, maximize QOL, improve sleep, and enable patients to return to work and perform their regular activities.3,4
    1.Galer BS, Dworkin RH. A Clinical Guide to Neuropathic Pain. Minneapolis, Minn: The McGraw-Hill Companies, Inc; 2000:7-8.
    2.Rowbotham MC. Chronic pain: from theory to practical management. Neurology. 1995;45(suppl 9):S5-S10.
    3.Portenoy RK, Kanner RM. Definition and assessment of pain. In: Portenoy RK, Kanner RM, eds. Pain Management: Theory and Practice. Philadelphia, Pa: FA Davis Company. 1996:6.
    4.Woolf CJ, Mannion RJ. Neuropathic pain: aetiology, symptoms, mechanisms, and management. Lancet. 1999;353:1959-1964.
  • Chronic pain has a wide range of negative effects, not only for the individual patient but for families and society as well.
    Both physical and psychological aspects of a patient’s life may be impacted, including the ability to work or perform activities of daily living, sleep patterns, emotional state (depression, anxiety, anger), and self-esteem.
    Social, familial, marital, and/or sexual relations may be impaired, and patients may become socially isolated as they are no longer able to participate in their usual activities.
    The disability and lost workdays associated with chronic pain impose significant direct as well as indirect healthcare costs for society as a whole. The economic impact of chronic pain is staggering. Back pain, migraines, and arthritis alone account for medical costs of $40 billion annually; the total annual cost of pain from all causes is estimated to be more than $100 billion. Pain is the cause of 25% of all sick days taken yearly.1
    A growing scientific understanding of pain mechanisms has led to the evolving concept of pain as a disease state in its own right, one that may require ongoing treatment.
    However, do not expect analgesics to solve all these problems. A number of studies suggest that the best success in pain management relies on a multidisciplinary approach that includes patient education, medications, physical medicine, and psychological counseling. For example, when Becker et al compared the effect of multidisciplinary pain treatment (MPT) with that of treatment by a general practitioner after initial supervision by a pain specialist (GP group) in 189 patients with chronic, nonmalignant pain, they found that, after 6 months, the MPT group reported a statistically significant reduction in pain intensity (visual analog scale score, P<.001), improvement in psychological well-being (PGWB, P<.001), quality of sleep (P<.05), and physical functioning (Short Form-36–Physical Functioning, P<.05) compared with the GP group.2
    Thus, a coordinated approach to pain management often provides the most efficient and cost-effective approach, which leads to patient empowerment (improved perception of personal control over pain) and the best clinical outcome.
    1.U.S. News & World Report. Washington, DC: U.S. News & World Report L.P.; March 17, 1997:55-57, 60-62, 65, 67.
    2.Becker N, Sjogren P, Bech P, Olsen AK, Eriksen J. Treatment outcome of chronic non-malignant pain patients managed in a Danish multidisciplinary pain centre compared to general practice: a randomized controlled trial. Pain. 2000;84:203-211.
  • Nociceptive, or inflammatory, pain is pain resulting from activity in neural pathways caused by potentially tissue-damaging stimuli.1 Examples include postoperative pain, arthritis, mechanical low back pain, sickle cell crisis, and sports or exercise injuries.
    Neuropathic pain is pain caused by a primary lesion or dysfunction in the peripheral and/or central nervous systems.2 Examples of peripheral neuropathic pain syndromes include HIV sensory neuropathy, postherpetic neuralgia (PHN), and diabetic neuropathy. Examples of central neuropathic pain include central poststroke pain, spinal cord injury pain, trigeminal neuralgia, and multiple sclerosis pain.
    As indicated by the “mixed type” area on the slide, chronic pain can be of mixed etiology with both nociceptive and neuropathic characteristics.
    Two types of neuropathic pain—PHN and diabetic neuropathy—will be emphasized within this module. These types of pain are being stressed because the great majority of randomized controlled trials of treatments for neuropathic pain have examined these two disorders, and because our understanding of the mechanisms of neuropathic pain is largely derived from those studies.
    1.Portenoy RK, Kanner RM. Definition and Assessment of Pain. In: Portenoy RK, Kanner RM, eds. Pain Management: Theory and Practice. Philadelphia, Pa: FA Davis Company; 1996:4.
    2.Galer BS, Dworkin RH. A Clinical Guide to Neuropathic Pain. Minneapolis, Minn: The McGraw-Hill Companies Inc; 2000:8-9.
  • A variety of terms are used to describe neuropathic pain, including those listed on the slide: numbness, tingling, burning, paroxysmal, paresthetic, lancinating, electriclike, raw skin, shooting, deep, dull, and bonelike aching pain.
    Additional terms that are often used to describe neuropathic pain include squeezing, jabbing, broken-glass, cramping, spasms, icy cold, and frostbite.
    These terms are not perfectly sensitive or specific and are to be used only as a guide. Some patients with neuropathic pain will not use these terms to describe their pain experience, and some patients who use these terms have nonneuropathic pain.
    Terms used to describe pain are usually not helpful in differentiating among neuropathic conditions.1
    The primary signs and symptoms of neuropathic pain are allodynia and hyperalgesia, both of which are defined on the slide.2
    1.Galer BS. Neuropathic pain of peripheral origin: advances in pharmacologic treatment. Neurology. 1995;45(suppl 9):S17-S25.
    2.Backonja M-M, Galer BS. Pain assessment and evaluation of patients who have neuropathic pain. Neurol Clin. 1998;16:775-789.
  • Pain that manifests in diverse diseases may operate through common mechanisms. No pain mechanism is an inevitable consequence of a particular disease process. A given pain mechanism could be responsible for many different symptoms. More than one mechanism can operate in a single patient, and these may change over time.
    The main neurotransmitter in primary afferents is the excitatory amino acid glutamate. Activation of nociceptors causes the release of glutamate from central terminals; this release acts on the ionotropic glutamate receptor amino-3-hydroxy-5-methylisoxazole-4-proprionic acid postsynaptically to cause a rapid depolarization of dorsal horn neurones and, if threshold is reached, action potential discharge.
    Transduction: noxious stimuli cause ion channels in the membranes of thermal, mechanical, and chemical receptors located in the skin and tissue to open. Ions enter the receptor and depolarize it. Transmission: a wave of depolarization, or action potential, travels toward the spinal cord via A-beta (thinly myelinated) fibers and C (unmyelinated) fibers and up the ascending pathway. A-beta (light touch) fibers may become sensitized by CNS mechanisms to produce allodynia.
    Modulation/Perception: the ascending pain pathway carries impulses from the nociceptor to the sensory cortex; thus the sensation of pain is perceived.
    Interpretation: impulses are carried by 1st, 2nd, and 3rd order neurons. 1st order neurons carry impulses from the nociceptor to the dorsal horn of the spinal cord. 2nd order neurons carry impulses from the spinal cord to the thalamus, while 3rd order neurons carry the impulse from the thalamus to the primary sensory cortex.
    Crossman AR, Neary D. Neuroanatomy, 2nd ed. Churchill Livingstone, 2000.
    Galer B, Gammaitoni A, Alvarez N. 6. Immunology [XIV. Pain]. In: Dale DC, Federman DD, eds. WebMD Scientific American® Medicine. New York, NY:WebMD Corporation; 2003.
    Guyton AC, Hall J. Textbook of Medical Physiology, 10th Ed. Saunders, 2000.
    Woolf CJ, Mannion RJ. Neuropathic pain: aetiology, symptoms, mechanisms, and management. Lancet. 1999;353:1959-1964.
  • Many mechanisms have been proposed for neuropathic pain, but it is unknown which mechanisms are most relevant in humans. This slide lists the more widely accepted proposed mechanisms. In an individual patient, more than one mechanism is probably relevant. The ability to classify patients based on predominant pathophysiology may, hopefully, help target therapy.1
    Excitotoxicity: nerve damage results in a barrage of nociceptive input released into the spinal cord that can damage inhibitory cells and result in a disinhibited pain system.2
    Sodium channels: in damaged nerves, abnormal sodium channels may be produced that result in a hyperexcitable nerve.3
    Ectopic discharge: damaged nerves produce ectopic, or abnormal, nerve impulses that may promote pain perceptions.3
    Deafferentation: if the central nervous system (CNS) is deprived of normal nerve input, as in the case of amputation or plexus avulsion, pain may result. The classic picture is severe pain in an insensate (or absent) limb.4
    Central sensitization: with repeated sensory input, the CNS may become hyperresponsive (sensitized) to peripheral input, a so-called facilitated state. This state is caused by long-term or permanent changes in the anatomy or physiology of the CNS produced by pain.1-3
    1.Galer BS. Neuropathic pain of peripheral origin: advances in pharmacologic treatment. Neurology. 1995;45(suppl 9):S17-S25.
    2. Brookoff D. Chronic pain: 1. A new disease? Hosp Pract. July, 2000:45-52,59.
    3. Baron R. Peripheral neuropathic pain: from mechanisms to symptoms. Clin J Pain. 2000;16:S12-S20.
    4. Portenoy RK. Neuropathic pain. In: Portenoy RK, Kanner RM, eds: Pain Management: Theory and Practice. Philadelphia, Pa: FA Davis Company; 1996:94,97.
  • Neuropathic pain may result from the concatenation of a number of mechanisms: eg, sodium-channel accumulation, redistribution, and altered expression; increased expression of mRNA for specific neurotransmitters (eg, substance P); central sensitization; sprouting of sympathetic efferents into neuromas and dorsal horn and ganglion cells.
    Due to this multiplicity of mechanisms, it is unlikely that neuropathic pain corresponds to a unique entity. Each painful symptom may therefore correspond to a distinct mechanism and may only respond to a specific treatment.
    Spontaneous pain and paraesthesias associated with sodium channel activity, for example, may best respond to sodium channel blockers or antiepileptic agents. Increased transmission and reduced inhibition associated with hyperalgesia and allodynia may best respond to opioids or tricyclic antidepressants.
    Sensitive and specific diagnostic tools are needed to reveal the particular pathological processes involved in the pain experienced by the individual patient. But accurate diagnosis of pain mechanisms will only occur if the mechanisms can be adequately targeted with appropriate therapies.
    Attal N, Bouhassira D. Mechanisms of pain in peripheral neuropathy. Acta Neurol Scand Suppl. 1999;173:12-24.
    Woolf CJ, Manion RJ. Neuropathic pain: aetiology, symptoms, mechanisms, and management. Lancet. 1999;353:1959-1964.
  • The slide lists the various treatments for neuropathic pain in order of invasiveness.1 However, the efficacy of treatment does not necessarily match its invasiveness. For some patients, behavioral or physical therapy or a topical medication can be at least as effective as an interventional technique.2,3
    While there are many treatment options and combinations for neuropathic pain, this presentation will focus on those meeting three important criteria: 1) efficacy—demonstrated in controlled clinical trials; 2) safety—demonstrated in controlled clinical trials and subsequent clinical experience; 3) favorable tolerability profiles (ie, side effects, drug/drug interactions).
    Psychological/physical approaches to pain management include relaxation therapy and physical exercise programs.
    Topical medications consist of the lidocaine patch 5%, capsaicin, and a variety of custom-compounded topical agents of unknown effectiveness.2,4
    Oral medications include anticonvulsants such as gabapentin, tricyclic antidepressants (TCAs), opioids, and miscellaneous agents (eg, mexiletine, baclofen).1,4
    The two types of injections are nerve blocks and local infiltrations that are usually administered with local anesthetics and/or steroids.5
    The interventional techniques that require referral to a specialist are spinal cord stimulation, spinal analgesia, brain stimulation, and various neurosurgical procedures such as dorsal root entry zone lesions.2,6
    1.Mackin GA. J Hand Ther. 1997;10:96-109.
    2.Katz N. Clin J Pain. 2000;16:S41-S48.
    3.Leland JY. Geriatrics. 1999;54:23-37.
    4.Belgrade MJ. Postgrad Med. 1999;106:127-148.
    5.Galer BS et al. A Clinical Guide to Neuropathic Pain. 2000:97.
    6.Gonzales GR. Neurology. 1995:45(suppl 9):S11-S16.
  • Nonpharmacologic strategies may be useful in easing pain and improving function, especially if used adjunctively with pharmacologic remedies. However, nonpharmacologic strategies are rarely sufficient to replace pharmacotherapies, especially in the case of chronic neuropathic pain.1
    A number of trials have demonstrated that transcutaneous electrical nerve stimulation has efficacy in ameliorating chronic neuropathic pain. However, the apparatus may be difficult for some patients to operate and the treatment itself is time-consuming.2
    1.Ferrell B, Herr K, Epplin J, et al. The management of persistent pain in older persons. Programs and Abstracts of the American Geriatric Society 2002 Annual Scientific Meeting. May 8–12, 2002; Washington, DC.
    2.Kuman D, Marshall HJ. Diabetic peripheral neuropathy: amelioration of pain withtranscutaneous electrostimulation. Diabetes Care. 1997;20:1702-1705.
  • Although there are numerous medications for the treatment of neuropathic pain, those listed on this slide have demonstrated efficacy in multiple, consistent, randomized, controlled trials. As such, these agents provide an evidence-based treatment approach for neuropathic pain and will constitute the focus of this program’s discussion on pharmacologic treatment.
    When selecting a pharmacologic treatment regimen, consideration should also be given to safety and tolerability factors such as side-effect profile and potential for drug interactions. Controlled clinical trials and clinical experience document that the lidocaine patch, because of its nonsystemic mechanism of action, has the least potential for adverse side effects or drug interactions. Among systemic agents, gabapentin, which has no significant side effects, has demonstrated favorable safety and tolerability. Based on these factors, as well as being FDA-approved for the treatment of PHN, the lidocaine patch and gabapentin are often selected as initial treatments for neuropathic pain.1-6
    Nortriptyline, desipramine, tramadol, and controlled-release oxycodone also have demonstrated safety and tolerability profiles which are more favorable than those of earlier agents such as amitriptyline, phenytoin, carbamazepine, and others. 1,2,7-14
    1. Backonja M et al. JAMA. 1998;280:1831-1836.
    2. Rowbotham M et al. JAMA. 1998;280:1837-1842.
    3. Carter GT et al. Phys Med Rehabil Clin N Am. 2001;12:447-459.
    4. Rowbotham MC et al. Pain. 1996;65:39-44.
    5. Galer BS et al. Clin J Pain. 2002;18:297-301.
    6. Galer BS et al. Pain. 1999;80:533-538.
    7. Rice AS et al. Prostaglandins Leukot Essent Fatty Acids. 2002;66:243-256.
    8. Gorson DM. Diabetes Care. 1998; 21:2190-2191.
    9. Max MB et al. N Engl J Med. 1992;326:1250-1256.
    10. Watson CPN et al. Neurology. 1998;51:1166-1171.
    11. Watson CP. Clin J Pain. 2000;16(suppl 2):S49-S55.
    12. Watson CP et al. Neurology. 1998;50:1837-1841.
    13. Harati Y et al. Neurology. 1998;50:1842-1846.
    14. Sindrup SH et al. Pain. 1999;83:389-400.
  • Only five medications, pregabalin, duloxetine, lidocaine patch 5%, gabapentin, and carbamazepine, have been approved by the FDA for treatment of neuropathic pain—specifically, for treatment of diabetic peripheral neuropathy (DPN), postherpetic neuralgia (PHN), and trigeminal neuralgia.
    The approval of pregabalin was based on the results of six double-blind clinical trials involving more than 9,000 patients, which showed that treatment with pregabalin significantly reduced pain in patients with DPN and PHN. Pain relief was reported as early as the first week of treatment in some patients, and was sustained over the three-month trials.1
    The efficacy of duloxetine for the management of neuropathic pain associated with DPN was established in two randomized, 12-wk, double-blind, placebo-controlled, fixed-dose studies. Treatment with duloxetine at 60 mg qd or bid significantly reduced 24-hour average pain levels compared with placebo. In these trials, 58% of 1074 patient treated with duloxetine reported at least a 30% sustained reduction in pain.1 On the basis of this data, FDA approved duloxetine for DPN on September 7, 2004.
    In two double-blind, vehicle-controlled randomized clinical trials, lidocaine patch 5% provided statistically significantly greater pain relief to patients with PHN than did vehicle-control patches without lidocaine. On the basis of those studies, FDA approved lidocaine for treatment of PHN. Anecdotal evidence of a beneficial treatment in patients with other types of neuropathic pain have been published.2,3
    Eight double-blind, placebo-controlled, randomized clinical trials of gabapentin for chronic pain found that, at daily dosages up to 3600 mg, gabapentin significantly reduced pain compared with placebo in patients with PHN, painful diabetic neuropathy (PDN), mixed neuropathic pain syndromes, among other neuropathic disorders. On the basis of two large randomized trials, FDA approved gabapentin for treatment of PHN.,4,5
    Carbamazepine has a well-established beneficial effect in trigeminal neuralgia, and it is approved by the FDA for the treatment of this syndrome. Based on the results of the clinical trials of anticonvulsants in chronic neuropathic pain, carbamazepine can be recommended for patients who have not responded to an adequate trial of gabapentin when treatment with an anticonvulsant is sought. 6,7
    Cymbalta. Label and approval history. Available at: http://www.fda.gov/cder/foi/label/2004/21733lbl.pdf. Accessed Sept 10, 2004.
    Rowbotham MC, Davies PS, Verkempinck C, Galer BS. Lidocaine patch:double-blind controlled study of a new treatment method for post-herpetic neuralgia. Pain. 1996;65:39-44.
    Rowbotham MC, Perander J, Friedman E. Topical lidocaine patch relieves postherpetic meuralgia more effectively than a vehicle topical patch: results of an enriched enrollment study. Pain. 1999;80:533-538.
    Rowbotham MC, Harden N, Stacey B et al. Gabapentin for the treatment of postherpetic neuralgia: a randomized controlled trial. JAMA. 1998;280:1837-1842.
    Rice ASC, Maton S, Postherpetic Neuralgia Study Group: Gabapentin in postherpetic neuralgia: a randomised, double blind, placebo controlled study. Pain. 2001;94:215-224.
    McQuay HJ, Carroll D, Jadad AR, Wiffen P, Moore A. Anticonvulsant drugs for management of pain: a systematic review. BMJ. 1995;311:1047-1052.
    Loeser JD. Cranial neuralgias. In: Loeser JD, Buutler SH, Chapman CR, Turk DC, eds. Bonicas’s Management of Pain. 3rd ed. Philadelphia PA: Lippincott Williams & Wilkins;2001:855-866.
  • Available drug treatments for chronic pain currently include simple analgesics such as acetaminophen, salicylates and other nonsteroidal anti-inflammatory drugs, traditional opioid drugs, and adjuvant agents (eg, antidepressants, anticonvulsants). Typically, the choice of a drug is made by balancing the indications for treatment, the clinical efficacy of the drug, and its toxicity. An understanding of the mechanism of action of these drugs helps to establish their role in therapy.
    Better understanding of the pathophysiology of acute and chronic pain has led to numerous advances in pharmacologic management of painful disorders, including low back pain, migraine headache, fibromyalgia, postherpetic neuralgia, osteoarthritis, rheumatoid arthritis, and cancer-related neuropathic pain.
    Opioids mimic the actions of endogenous opioid peptides by interacting with mu, delta, or kappa opioid receptors. The opioid receptors are coupled to G1 proteins and the actions of the opioids are mainly inhibitory. They close N-type voltage-operated calcium channels and open calcium-dependent inwardly-rectifying potassium channels. This results in hyperpolarization and a reduction in neuronal excitability. They also decrease intracellular cAMP which modulates the release of nociceptive neurotransmitters (eg, substance P).
    Inhibition of prostaglandin synthesis by cyclooxygenase is the principal mode of the analgesic and anti-inflammatory actions of NSAIDs. Cyclooxygenase is inhibited irreversibly by aspirin and reversibly by other NSAIDs. The widespread inhibition of cyclooxygenase is responsible for many of the adverse effects of these drugs. NSAIDs also reduce prostaglandin production within the CNS. This is the main action of paracetamol.
    Argoff CE. Pharmacologic management of chronic pain. J Am Osteopath Assoc. 2002;102(suppl 3):S21-S27.
    Aronson MD. Nonsteroidal anti-inflammatory drugs, traditional opioids, and tramadol: contrasting therapies for the treatment of chronic pain. Clin Ther. 1997;19:420-32; discussion 367-8.
    Bovill JG. Mechanisms of actions of opioids and non-steroidal anti-inflammatory drugs. Eur J Anaesthesiol Suppl.1997;15:9-15.
  • Anticonvulsant medications have been used in the treatment of neuropathic pain for many years without FDA approval (except for carbamazepine’s indication for trigeminal neuralgia). Pregabalin, however, as of September 2004 has received approvable letters from the FDA for neuropathic pain associated with diabetic peripheral neuropathy and postherpetic neuralgia. The slide provides a summary of many of the controlled trials that have been conducted examining the efficacy of anticonvulsant drugs in the treatment of various neuropathic pain syndromes.1-7
    The studies of carbamazepine and phenytoin conducted in the 1960s and 1970s do not meet today’s standards of methodological rigor.8 The phenytoin studies have produced both successful and unsuccessful results.9
    The two studies of gabapentin are among the largest clinical trials of the treatment of neuropathic pain ever conducted.8,10 These studies have stimulated a great deal of clinical and research interest in the efficacy and mechanisms of action of anticonvulsant drugs in treating patients with neuropathic pain.
    First-generation anticonvulsant drugs, which include carbamazepine and phenytoin, sometimes provoke serious side effects and drug-drug interactions that do not occur with second-generation anticonvulsants.11 We will be focusing on gabapentin because it is the anticonvulsive most commonly used for neuropathic pain and for which there is the most clinical data.
    1. Rowbotham M et al. JAMA. 1998;280:1837-1842.
    2. Eisenberg E et al. Neurology. 2001;57:505-509.
    3. Simpson DM et al. Neurology. 2000;54:2115-2119.
    4. Campbell FG et al. J Neurol Neurosurg Psychiatry. 1966;29:265-267.
    5. Zakrzewska JM et al. Pain. 1997;73:223-230.
    6. Zakrzewska JM et al. J Neurol Neurosurg Psychiatry. 1989;52:472-476.
    7. Vestergaard K et al. Neurology. 2001;56:184-190.
    8. Rull J et al. Diabetologia. 1969;5:215-218.
    9. Chadda VS et al.J Assoc Physicians India. 1978;26:403-406.
    10. Backonja M et al. JAMA. 1998;280:1831-1836.
    11. Ross EL. Neurology. 2000;55:S41-S46.
  • Gabapentin is an anticonvulsant which has recently been approved for the treatment of PHN, but it does not have FDA approval for other neuropathic pain syndromes.1
    Its mechanism of action has not been completely identified.
    Gabapentin has limited intestinal absorption and is usually well tolerated. Among the more common adverse events associated with its use are dizziness and sedation. It has rare serious adverse effects.
    No clinically significant drug-drug interactions are known.
    The time to peak concentration is 2 to 3 hours, and the elimination half-life is 5 to 7 hours. Plasma clearance, however, decreases in older patients and in patients with impaired renal function.2
    The effective dose for adjunctive therapy of partial seizures with or without secondary generalization in adults with epilepsy is 900 to 1,800 mg/day, given in divided doses tid and titrated over 3 days.2 For pain, clinical experience has shown that much higher doses are often necessary and well tolerated; the usual dosage range is up to3,600 mg/day (tid-qid).1
    1.Backonja M-M. Anticonvulsants (antineuropathics) for neuropathic pain syndromes. Clin J Pain. 2000;16:S67-S72.
    2.Neurontin (gabapentin) [package insert]. Morris Plains, NJ: Parke-Davis; 1999.
  • Topical treatment is not the same as transdermal treatment. Topical treatment means the drug stays and acts primarily locally, with minimal systemic absorption and effects. Transdermal treatment attempts to have systemic effects by delivering the drug through the skin instead of orally, intravenously, or by other means.
    Because it is a topical agent, the lidocaine patch 5% achieves insignificant serum levels, even with chronic use. This enhances safety and makes drug interactions unlikely.1 Clinical trials have shown no statistical difference between lidocaine patch 5% and placebo patch with regard to side effects.2 The most common adverse event reported with the topical lidocaine patch 5% is transient minor local irritation of the skin.3
    Transdermal therapies for neuropathic pain include the fentanyl patch. Transdermal systems need to be applied to nonirritated skin. They deliver medication systemically, which means a slower onset of action. Patients are advised to use short-acting analgesics until analgesic efficacy with the patch is achieved.
    Because serum levels of the drug increase correlatively with duration of transdermal patch wear-time, side effects can be significant and problematic. Nausea, mental clouding, and skin irritation are commonly reported. More serious side effects include serious or life-threatening hypoventilation and bradycardia. Drug-drug interactions may also be a problem, especially concomitant use of the transdermal fentanyl patch and central nervous system (CNS) depressants (eg, benzodiazepines).4
    1.Argoff CE. New analgesics for neuropathic pain: the lidocaine patch. Clin J Pain. 2000;16(2 suppl):S62-S66.
    2.Galer BS, Rowbotham MC, Perander J, Friedman E. Topical lidocaine patch relieves postherpetic neuralgia more effectively than a vehicle topical patch: results of an enriched enrollment study. Pain. 1999;80:533-538.
    3.Galer BS, Dworkin RH. A Clinical Guide to Neuropathic Pain. Minneapolis, Minn: McGraw-Hill Companies Inc; 2000:61-64.
    4.Duragesic [package insert]. Titusville, NJ: Janssen Pharmaceutica; 1999.
  • The topical lidocaine patch, the first drug with an FDA-approved indication for PHN, provides an effective treatment option with minimal side effects.
    The topical lidocaine patch 5% is a pliable 10 cm x 14 cm patch. The lidocaine patch 5% can be affixed directly to the affected areas. Multiple patches may be used to treat multiple painful sites or the patch may be trimmed. Up to three patches may be applied to intact skin for up to 12 hours within a 24-hour period.1 In a recent study in which four patches were used for 3 days plasma lidocaine concentrations were well below those associated with either cardiac arrythmias or toxicity (mean Cmax at steady state with lidocaine patches applied QD and bid was 186 ng/mL and 225 ng/mL, respectively; AUCss was reported at 3,550 ng*h/mL and 2,253 ng*h/mL for the QD and bid dosing groups, respectively).2
    The efficacy of the lidocaine patch 5% has been demonstrated in three randomized vehicle-controlled trials.3-5 The patch is indicated for treatment of PHN.1
    Because it is a topical agent, the lidocaine patch 5% achieves insignificant serum levels, even with chronic use. This enhances safety.6-8 Clinical trials have shown no statistical difference between lidocaine patch 5% and placebo patch with regard to side effects.5 The most common adverse event reported with the topical lidocaine patch 5% is transient minor local irritation of the skin.7
    In one clinical trial of patients treated with the vehicle patches, data suggest that the patch provides a mechanical barrier to the stimuli that cause allodynia.4
    1Lidoderm (lidocaine patch 5%) [package insert].
    2.Alvarez NA et al. In: Programs and Abstracts of the IASP 10th World Congress on Pain. 2002. Abstract 175-P171.
    3.Rowbotham MC et al. In: Programs and Abstracts of the 8th World Congress on Pain - Abstracts. 1996. 274.
    4.Rowbotham MC et al. Pain. 1996;65:39-44.
    5.Galer BS et al. Pain. 1999;80:533-538.
    6.Argoff CE. Clin J Pain. 2000;16(2 suppl):S62-S66.
    7.Galer BS et al. A Clinical Guide to Neuropathic Pain. 2000:61-64; 8. Gammaitoni AR et al. Ann Pharmacother. 2002;36:236-240.
    8.Gammaitoni A et al. J Pain. 2002;3(suppl 1):52.
  • Opioid therapy entails a number of risks for patients, but these potential problems can be prevented or circumvented.
    Titration of opioid analgesics should be based on optimizing therapeutic efficacy while minimizing side effects. Regimens of fixed doses are generally preferred over prn regimens.1
    Documentation is critical and should include the initial evaluation, substance abuse history, psychosocial issues, pain/pain relief, side effects, functional outcomes, and continuing monitoring. Regular discussions with family members about the patient’s condition and use of opioids can improve the accuracy of monitoring.1
    The laws on patient monitoring vary from state to state, but the federal government regulates and legislates the use of controlled substances and drugs. Generally, federal laws have priority over state laws.2
    Most opioid side effects can be controlled with appropriate specific management (eg, prophylactic bowel regimen, use of stimulants).3
    Patients on opioids or those who appear to require them also have significant psychosocial rehabilitative issues and are generally best referred to a multidisciplinary center with experience managing chronic pain with opioids.1
    Addiction is referred to by many as psychological dependence.
    1. Pappagallo M. Aggressive pharmacologic treatment of pain. Rheum Dis Clin N Am. 1999;25:193-213.
    2. Clark HW. Policy and medical-legal issues in the prescribing of controlled substances.J Psychoactive Drugs. 1991;23:321-328.
    3. Zenz M. Morphine myths: sedation, tolerance, addiction. Postgrad Med J. 1991;67:S100-S102.
  • This slide addresses the issue of aberrant drug-taking behaviors.
    Before considering initiation of opioid treatment, it is important for the physician, patient, and family to understand the distinction between physical dependence, tolerance, and addiction.
    Physical dependence is a pharmacologic effect characterized by the development of a withdrawal syndrome when an opioid drug is discontinued, when the dose is substantially reduced, or if an antagonist is administered. Dependence occurs in almost all patients on opioids, and does not connote addiction.1
    Tolerance means that a greater amount of drug is needed over time to maintain a therapeutic effect. The number of patients who develop clinically relevant tolerance is unknown. Tolerance may also occur to side effects, and thus may be beneficial. Some patients who develop tolerance can have their pain managed by judicious dose increases;2 others who develop inexorable tolerance cannot have their pain managed by opioids. There is no evidence to support a role for analgesic tolerance in the development of drug addiction. Addiction is, however, often (though not always) associated with tolerance.
    Addiction is a psychiatric disorder consisting of continued, compulsive use of the substance despite harm.1 The Diagnostic and Statistical Manual of Mental Disorders provides nine categories of opioid use or opioid-induced disorders, including diagnostic criteria for opioid dependence or opioid abuse.3
    True addiction (patient loss of control) may become obvious only when the physician stops prescribing the medicine. There is, however, little evidence that addiction is common within the chronic pain population. In a study reviewing the available data, it was found that prevalence estimates for addiction in patients with chronic pain ranged from 3% to 19%.4
    1.American Academy of Pain Medicine, American Pain Society, American Society of Addiction Medicine. Definitions related to the use of opioids for the treatment of pain. 2001. Available at: http://www.ampainsoc.org/advocacy/opioids2.htm. Accessed October 2, 2002.
    2.Zenz M. Morphine myths: sedation, tolerance, addiction. Postgrad Med J. 1991;67:S100-S102.
    3.American Psychiatric Association. Diagnostic and Statistical Manual of Mental Disorders. 4th Ed. Rev Ed. Washington, DC: American Psychiatric Publishing, Inc.; 2000:269-277.
    4.Fishbain DA, Rosomoff HL, Rosomoff RS. Drug abuse, dependence, and addiction in chronic pain patients. Clin J Pain. 1992;8:77-85.
  • Tricyclic antidepressants (TCAs) act in part by inhibiting the reuptake of norepinephrine and serotonin into presynaptic neurons. They have been used to relieve neuropathic pain, although this indication has not been approved by the FDA.
    However, many controlled clinical trials and meta-analyses have demonstrated that TCAs (eg, imipramine, amitriptyline, desipramine, nortriptyline, clomipramine) can significantly reduce the pain of diabetic neuropathy and PHN.1-4
    Some, but not all, selective serotonin reuptake inhibitors (SSRIs) have also been shown to be effective for neuropathic pain. Paroxetine and citalopram (slightly) have shown benefit for diabetic neuropathy,1,2 while fluoxetine has proved to be no more effective than placebo.3 In general the SSRIs are felt to be, at best, inconsistently effective for neuropathic pain.4
    Some patients who receive antidepressants for neuropathic pain may experience improvement in insomnia, anxiety, and depression.4,5 Onset of analgesia with antidepressants generally occurs before the onset of the antidepressant effect. The pain-relieving effect of antidepressant agents appears to be independent of their antidepressant effect.4 Selective norepinephrine reuptake inhibitors (SNRIs) are to be explored for use in neuropathic pain.
    This module will focus on desipramine and nortriptyline because they are the two antidepressants most commonly used for treatment of neuropathic pain and for which there is the most clinical data.6-8
    1.Sindrup SH et al. Pain. 1990;42:135-144.
    2. Sindrup SH et al. Clin Pharmacol Ther. 1992;52: 547-552.
    3. Max MB et al. N Engl J Med. 1992;326:1250-1256.
    4. Galer BS et al. A Clinical Guide to Neuropathic Pain. 2000:71-72,93.
    5. Pappagallo M. Rheum Dis Clin N Am. 1999;25:193-213.
    6. Max MB et al. Neurology. 1988;38:1427-1432.
    7. Watson CP at al. Neurology. 1998;51:1166-1171.
    8. Kishore-Kumar R et al. Clin Pharmacol Ther. 1990;47:305-312.
  • Adverse effects commonly reported with TCAs are anticholinergic effects, which are listed on the left side of the slide. The adverse effects include blurred vision, cognitive changes (such as concentration, memory loss, and confusion), constipation, dry mouth, orthostatic hypotension, sedation, tachycardia, and urinary retention. All TCAs are reported to cause these adverse events in varying degrees of frequency and severity.1,2
    The TCA agents listed on the right side of the slide are organized in descending order of adverse effects, starting with desipramine (fewest adverse effects), nortriptyline, imipramine, doxepin, and amitriptyline (most adverse effects).2,3
    Because of the potential for adverse events and outcomes, amitriptyline should not be prescribed for people older than 65 years. Desipramine would be more appropriate for this population. Of all the drugs that are inappropriate for the elderly, amitriptyline is one of most frequently prescribed.4
    Because the TCAs appear to be almost equally efficacious, a rational approach for clinical practice is to start with the agents with the fewest adverse effects, unless a specific “side effect,” such as nighttime sedation, is desired.
    1.Rowbotham MC, Petersen KL, Davies PS, et al. Recent developments in the treatment of neuropathic pain. Proceedings of the 9th World Congress on Pain. Seattle, Wash: IASP Press; 2000:833-855.
    2.Mackin GA. Medical and pharmacologic management of upper extremity neuropathic pain syndromes. J Hand Ther. 1997;10:96-109.
    3.Tunali D, Jefferson JW, Greist JH. Depression and Antidepressants: A Guide. Madison, Wis: Information Centers, Madison Institute of Medicine; 1999.
    4.Piecoro LT, Browning SR, Prince TS, et al. Database analysis of potentially inappropriate drug use in an elderly Medicaid population. Pharmacotherapy. 2000;20:221-228.
  • Interventional treatments for neuropathic pain include neural blockade, neurolytic techniques, and stimulatory techniques.
    Neural blockade includes sympathetic blocks for complex regional pain syndrome type I (CRPS-I), which occurs without a definable nerve lesion and is also called reflex sympathetic dystrophy, and complex regional pain syndrome type II (CRPS-II), which occurs when a definable nerve lesion is present; both syndromes are also known as causalgia.1,2
    Neurolytic techniques are primarily employed for pain caused by cancer.3
    Pumps and stimulators are the main interventional techniques in routine clinical use.2 Stimulatory techniques encompass spinal cord and peripheral nerve stimulation.4 The main advantage of spinal cord stimulation is that it is a nonpharmacologic intervention that spares patients pharmacy visits, bills, and side effects.5
    Spinal analgesia is widely used for neuropathic pain but is a less conservative therapy than spinal cord stimulation. By acting directly on the spinal cord, spinal analgesia may provide improved pain control with fewer side effects than do systemic drugs.
    Among these techniques, only spinal analgesia has been shown to be effective in randomized controlled trials (and even this has been studied only short-term).4
    1.Galer BS, Dworkin RH. A Clinical Guide to Neuropathic Pain. Minneapolis, Minn: McGraw-Hill Companies Inc; 2000:120,135.
    2.MacFarlane BV, Wright A, O’Callaghan J, Benson HAE. Chronic neuropathic pain and its control by drugs. Pharmacol Ther. 1997;75:1-19.
    3.Katz N. Neuropathic pain in cancer and AIDS. Clin J Pain. 2000;16(suppl 2):S41-S48.
    4.Portenoy RK, Kanner RM, eds. Pain Management: Theory and Practice. Philadelphia, Pa: FA Davis Company; 1996:278,293-294,306-307.
    5.Gonzales GR. Central pain: diagnosis and treatment strategies. Neurology. 1995;45(suppl 9):S11-S16.
  • There are a number of potential new treatments for neuropathic pain in clinical trials and open-label studies, and results from many of these were presented at the August 2002 World Congress on Pain, the 5th International Conference on the Mechanisms and Treatment of Neuropathic Pain held in November 2002, the 2003 American Pain Society Annual Scientific Meeting, and the 2004 Joint Scientific Meeting of the American Pain Society and the Canadian Pain Society. Several of these emerging treatments are listed above and will be discussed in detail on the following slides.
  • Surgical blocking procedures are very risky and seldom result in long-term pain relief. Indeed, not uncommon for the pain to recur in a more severe form.
    Once again, not good results long-term from spinal fusion, and so used only when patients have failed all other treatment methods and are severely disabled by the pain.
  • Biofeedback for the treatment of chronic pain appears to be no more effective than relaxation methods. Relaxation may work in two ways: 1) reducing muscle tension; and 2) helping the patient better manage stress and anxiety. Relaxation exercises are frequently used in preparing a pregnant women for the delivery of her child. Relaxation may also stimulate the release of endogenous opioids, as well as boosting immune function. Evidence suggests that its effects are modest but useful in combination with other methods.
    Mechanism by which hypnosis works for some pain conditions, particularly acute pain such as that during surgery, is not well understood. Cognitive methods of pain control appear to be as effectives as hypnosis.
  • Counter-irritation is where you stimulate one area of the body to inhibit pain in another area – works up to a point.
  • When speak of pain program models, are talking of clinics or programs developed specifically for patients with pain. Does not mean this is the only place pts with pain receive treatment. As you all are aware, any clinic or inpatient program is likely to treat pts that have pain as one of their problems. Difference is that here we are talking about specialty treatment approaches specifically for pain. By definition, the referred patients will be those with more complex pain problems that have not responded to treatment in other clinics or programs.
    Basically are 5 models of pain treatment programs. Are in order from the least complex to the most complex. Generally, as move down the list, required resources increase as do the training needs of staff and the complexity of referred patients.
    Type of model implemented in a particular setting depends on:
    1) patient needs (CA, non-CA, or both; CP vs CPS; local vs regional)
    2) facility needs ( frequency of CP ER/Ambulatory care visits; narcotic analgesics use)
    3) available resources (staff FTE and space)
    4) staff expertise in pain
  • Pain Consultation Team - 2 or more individuals who serve in an advisory capacity regarding the treatment of difficult pain cases. May simply meet and review the record or may see the pt collectively or individually.
  • Multidisciplinary outpatient pain programs vary considerably in their scope and focus. May involve as few as 2 disciplines or as many as 8-10 or more. Goal is to provide more intensive pain treatment services which cut across specific discipline lines. Program framework typically is cognitive-behavioral, with the goal of reducing the frequency of maladaptive behaviors and increasing frequency of appropriate behaviors. At this level major goal no longer is limited to pain relief. Instead, interested primarily in increasing activity levels and muscle strength, and decreasing pain behaviors, eliminating reliance on certain medications such as narcotic analgesics or muscle relaxants, and reducing depression, and social isolation.
  • Table of the typical roles associated with disciplines represented on chronic pain teams. Programs vary by which disciplines are represented on each team. Some treatments may be done by one of several disciplines, if training is provided. Example: Relaxation training and OT at Tampa VAMC.
  • Most patients can obtain clinically meaningful relief with appropriate treatment.
    Given the multiple mechanisms of neuropathic pain, polypharmacy may be required for patients who do not respond adequately to treatment with a single agent.
    Drugs should be titrated aggressively either to the point where significant pain relief is achieved or intolerable side effects occur.
    New treatments for neuropathic pain that target specific pathways may help address the underlying mechanisms involved in pain.
    Treatment should balance efficacy, safety, and tolerability, and progress from the least to the most invasive treatments. More invasive treatments are not necessarily more effective than less invasive ones. The goals of treatment should include not only reducing pain as much as possible but also improving the patient’s QOL.1
    Patients with inadequate pain relief may benefit from referral to multidisciplinary pain treatment centers.2
    1.Galer BS, Dworkin RH. A Clinical Guide to Neuropathic Pain. Minneapolis, Minn:
    The McGraw-Hill Companies, Inc; 2000:53-55.
    2.Cunningham AL, Dworkin RH. The management of post-herpetic neuralgia. BMJ.
    2000;321:778-779.
  • Chronic_Neuropathic_Pain-Slides_Pierce-Smith.ppt

    1. 1. Chronic Pain Management Beverly Pearce-Smith, MD Clinical Assistant Professor Department of Anesthesiology UPMC-McKeesport Hospital July 2008
    2. 2. IASP Definition of Pain “Pain is an unpleasant sensory and emotional experience associated with actual or potential tissue damage or described in terms of such damage.”
    3. 3. Acute vs Chronic Pain Characteristic Acute Pain Chronic Pain Cause Generally known Often unknown Duration of pain Short, well-characterized Persists after healing, ≥3 months Treatment approach Resolution of underlying cause, usually self-limited Underlying cause and pain disorder; outcome is often pain control, not cure
    4. 4. What is Acute Pain?  Physiologic response to tissue damage  Warning signals damage/danger  Helps locate problem source  Has biologic value as a symptom  Responds to traditional medical model  Life temporarily disrupted (self limiting)
    5. 5. What is Chronic Pain?  Chronic pain is persistent or recurrent pain, lasting beyond the usual course of acute illness or injury, or more than 3 - 6 months, and adversely affecting the patient’s well-being  Pain that continues when it should not
    6. 6. What is Chronic Pain?  Difficult to diagnose & perplexing to treat  Subjective personal experience  Cannot be measured except by behavior  May originate from a physical source but slowly it “out-shouts” and becomes the disease  It has no biologic value as a symptom  Life permanently disrupted (relentless)
    7. 7. Domains of Chronic Pain Social Consequences  Marital/family relations  Intimacy/sexual activity  Social isolation Socioeconomic Consequences  Healthcare costs  Disability  Lost workdays Quality of Life Physical functioning Ability to perform activities of daily living Work Recreation Psychological Morbidity Depression Anxiety, anger Sleep disturbances Loss of self-esteem
    8. 8. Mixed Type Caused by a combination of both primary injury and secondary effects Nociceptive vs Neuropathic Pain Nociceptive Pain Caused by activity in neural pathways in response to potentially tissue-damaging stimuli Neuropathic Pain Initiated or caused by primary lesion or dysfunction in the nervous system Postoperative pain Mechanical low back pain Sickle cell crisis Arthritis Postherpetic neuralgia Neuropathic low back pain CRPS* Sports/exercise injuries *Complex regional pain syndrome Central post- stroke pain Trigeminal neuralgia Distal polyneuropathy (eg, diabetic, HIV)
    9. 9. Possible Descriptions of Neuropathic Pain  Sensations  numbness  tingling  burning  paresthetic  paroxysmal  lancinating  electriclike  raw skin  shooting  deep, dull, bonelike ache  Signs/Symptoms  allodynia: pain from a stimulus that does not normally evoke pain  thermal  mechanical  hyperalgesia: exaggerated response to a normally painful stimulus
    10. 10. Physiology of Pain Perception  Transduction  Transmission  Modulation  Perception  Interpretation  Behavior Injury Descending Pathway Peripheral Nerve Dorsal Root Ganglion C-Fiber A-beta Fiber A-delta Fiber Ascending Pathways Dorsal Horn Brain Spinal Cord Adapted with permission from WebMD Scientific American® Medicine.
    11. 11. Pathophysiology of Neuropathic Pain  Chemical excitation of nonnociceptors  Recruitment of nerves outside of site of injury  Excitotoxicity  Sodium channels  Ectopic discharge  Deafferentation  Central sensitization  maintained by peripheral input  Sympathetic involvement  Antidromic neurogenic inflammation
    12. 12. Multiple Pathophysiologies May Be Involved in Neuropathic Pain  More than one mechanism of action likely involved  Neuropathic pain may result from abnormal peripheral nerve function and neural processing of impulses due to abnormal neuronal receptor and mediator activity  Combination of medications may be needed to manage pain: topicals, anticonvulsants, tricyclic antidepressants, serotonin-norepinephrine reuptake inhibitors, and opioids  In the future, ability to determine the relationship between the pathophysiology and symptoms/signs may help target therapy
    13. 13. Neuropathic Pain  “Pain initiated or caused by a primary lesion or dysfunction in the nervous system” Merskey & Bogduk 1994  Central & peripheral sites  Acute & chronic pain states  CRPS I: consequent of acute, often minor trauma  CRPS II: consequence of nerve injury  Sympathetically maintained Pain (SMP) or independent of the SNS
    14. 14. Neuropathic Pain  Burning, stabbing, paraesthesia, allodynia, hyperalgesia  Threshold for activation of injured 1o afferents is lowered  Ectopic discharges may arise from the injury site or the DRG  2o to changes in Na+ channel expression  Central Sensitisation in the cord  2o to peripheral inputs  2o to central changes  Reduced inhibition  Functional (neurotransmitter) & anatomical (sprouting) changes in Aβ fibres  tactile allodynia (pain induced by light touch)
    15. 15. Acute Neuropathic Pain  Acute causes  iatrogenic, traumatic, inflammatory, infective  Acute neuropathic pain = 1-3%  Based on cases referred to an acute pain service  Majority still present at 12 months  May be a risk factor for chronic pain  Prompt diagnosis & Rx may prevent chronic pain
    16. 16. Examples of Acute NP  Phantom Limb Pain (PLP)  Complex Regional Pain Syndrome (CRPS)  Spinal Cord Injury Pain  Peripheral nerve injury  Post-surgical (eg thoracotomy, mastectomy)
    17. 17. NEUROPATHIC PAIN LESION IN THE NERVOUS SYSTEM  EXPERIENCED IN PARTS OF BODY THAT APPEAR NORMAL  CHRONIC  SEVERE  RESISTANT TO OVER THE COUNTER ANALGESICS  AGGRAVATED BY ALLODYNIA
    18. 18. Chronic Pain Syndromes  Neuralgias  Causalgia  Complex Regional Pain Syndrome (aka: RSD  Hyperesthesias  Myofascial pain syndromes  Hemiagnosia  Phantom limb pain
    19. 19. COMMON TYPES OF NEUROPATHIC PAIN PERIPHERAL • Polyradioculopathy • Alcoholic polyneuropathy • Entrapment neuropathies (carpal tunnel) • Nerve compression by tumor • Diabetic neuropathy • Phantom limb pain • Postherpetic neuralgia • Trigeminal neuralgia CENTRAL  Compressive myelopathy from spinal stenosis  HIV myelopathy  MS  Parkinson disease  Post ischemic myelopathy  Poststroke pain  Posttraumatic spinalcord injury
    20. 20. CLINICAL EVALUATION OF PTS WITH SUSPECTED NEUROPATHIC PAIN  HISTORY- pain intensity (0 to 10), sensory descriptors, temporal variation, functional impact, attempted treatments, alcohol  PHYSICAL- gross motor, DTRs, skin, sensory- light touch, pin prick, vibration, dynamic /thermal allodynia, hyperalgesia, tinel`s  SPECIAL TESTS- CT, MRI, EMG, nerve conduction, clinical biochemistry
    21. 21. NEUROPATHIC PAIN  SPONTANEOUS- CONTINOUS OR INTERMITTENT -Burning, Shooting, Shock-like  STIMULUS EVOKED- ALLODYNIA AND HYPERALGESIA -Extension of allodynia above and below the originally affected dermatomes is a feature of central sensitization.
    22. 22. Copyright ©2006 Canadian Medical Association or its licensors Gilron, I. et al. CMAJ 2006;175:265-275 Neuropathic pain arises following nerve injury or dysfunction
    23. 23. PATHOPHYSIOLOGY  PERIPHERAL MECHANISMS Peripheral nerve injury 1. Sensitization by spontaneous activity by neuron, lowered threshold for activation, increased response to given stimulus. 2. Formation of ectopic neuronal pacemakers along nerve and increased expression of sodium channels and voltage gated calcium channels. (α 2 delta subunit- where gabapentin acts) 3. Adjacent demyelinated axons can have abnormal electrical connections channels and increased neuronal excitability.
    24. 24. PATHOPHYSIOLOGY CENTRAL MECHANISMS Sustained painful stimuli results in spinal sensitization (neurons within dorsal horn) 1. Increased spontaneous activity of dorsal horn neurons, reduced activation thresholds and enhanced responsiveness to synaptic inputs. 2. Expansion of receptive fields, death of inhibitory interneurons (intrinsic modulatory systems). 3. Central sensitization mediated by NMDA receptors that further release excitatory amino acids and neuropeptides. 4. Sprouting of sympathetic efferents into neuromas and dorsal root and ganglion cells.
    25. 25. Pain Treatment Continuum Least invasive Most invasive Psychological/physical approaches Topical medications *Consider referral if previous treatments were unsuccessful. Systemic medications* Interventional techniques* Continuum not related to efficacy
    26. 26. Nonpharmacologic Options  Biofeedback  Relaxation therapy  Physical and occupational therapy  Cognitive/behavioral strategies  meditation; guided imagery  Acupuncture  Transcutaneous electrical nerve stimulation
    27. 27. Pharmacologic Treatment Options  Classes of agents with efficacy demonstrated in multiple, randomized, controlled trials for neuropathic pain  topical analgesics (capsaicin, lidocaine patch 5%)  anticonvulsants (gabapentin, lamotrigine, pregabalin)  antidepressants (nortriptyline, desipramine)  opioids (oxycodone, tramadol)  Consider safety and tolerability when initiating treatment
    28. 28. FDA-Approved Treatments for Neuropathic Pain  Carbamazepine  trigeminal neuralgia  Duloxetine  peripheral diabetic neuropathy  Gabapentin  postherpetic neuralgia  Lidocaine Patch 5%  postherpetic neuralgia  Pregabalin*  peripheral diabetic neuropathy  postherpetic neuralgia *Availability pending based upon controlled substance scheduling by the DEA.
    29. 29. BRAIN Pharmacologic Agents Affect Pain Differently Descending Modulation Central SensitizationPNS Local Anesthetics Topical Analgesics Anticonvulsants Tricyclic Antidepressants Opioids Anticonvulsants Opioids NMDA-Receptor Antagonists Tricyclic/SNRI Antidepressants Anticonvulsants Opioids Tricyclic/SNRI AntidepressantsCNS Spinal Cord Peripheral Sensitization Dorsal Horn
    30. 30. Anticonvulsant Drugs for Neuropathic Pain Disorders  Postherpetic neuralgia  gabapentin*  pregabalin *  Diabetic neuropathy  carbamazepine  phenytoin  gabapentin  lamotrigine  pregabalin *  HIV-associated neuropathy  lamotrigine  Trigeminal neuralgia  carbamazepine*  lamotrigine  oxcarbazepine  Central poststroke pain  lamotrigine *Approved by FDA for this use. HIV = human immunodeficiency virus.
    31. 31. Gabapentin in Neuropathic Pain Disorders  FDA approved for postherpetic neuralgia  Anticonvulsant: uncertain mechanism  Limited intestinal absorption  Usually well tolerated; serious adverse effects rare  dizziness and sedation can occur  No significant drug interactions  Peak time: 2 to 3 h; elimination half-life: *Not approved by FDA for this use.
    32. 32. Gabapentin  Action: ↓ NT release from hyper-excited neurones  variable oral absorption, no interactions, completely renally excreted  Indication:  Protective analgesia  Neuropathic pain treatment (NNT = 4.7)  SE: sedation, dizziness, ataxia, tremor  NNH minor = 4, NNH major =12-18  COST!  Doses:  Pre-op: 600-1200mg (1-2 hours pre-op)  Post-op “prophylaxis”: 100-300mg TDS (? 2 weeks)  Post-op “treatment”: 100-900mg TDS (usu 300-600mg tds) Dahl JB, Mathiesen O, Moiniche S. ‘Protective premedication’: an option with gabapentin and related drugs? A review of gabapentin and pregabalin in the treatment of post-operative pain. Acta Anaesthesiol Scand 2004; 48: 1130—1136 Hurley RW, Cohen SP, Williams KA, Rowlingson AJ, Wu CL. The Analgesic Effects of Perioperative Gabapentin on Postoperative Pain: A Meta-Analysis. Reg Anesth Pain Med 2006;31:237-247.
    33. 33. Pregabalin  Very similar to gabapentin  More reliable oral absorption  Slightly different side effect profile  Doses: 75-300mg BD
    34. 34. Other Anticonvulsants  Effective (NNT 2-3) but less “user friendly”  Most have uncommon but serious SE (eg. aplastic anaemia, hepatotoxicity, Stevens- Johnson syndrome etc)  NNH minor = 3, NNH major = 16 - 24  Consider  Carbamazepine 100mg BD (↑ to 400mg bd/tds)  Valproate 200mg BD (↑ to 1000-2000mg/d)  Phenytoin 100mg nocte (↑ to 500mg/d) Finnerup NB, Otto M, McQuay HJ, Jensen TS, Sindrup SH. Algorithm for neuropathic pain treatment: An evidence based proposal. Pain 118 (2005) 289–305
    35. 35. Topical vs Transdermal Drug Delivery Systems Systemic activity Applied away from painful site Serum levels necessary Systemic side effects Peripheral tissue activity Applied directly over painful site Insignificant serum levels Systemic side effects unlikely Topical (lidocaine patch 5%) Transdermal (fentanyl patch)
    36. 36. Lidocaine Patch 5%  Lidocaine 5% in pliable patch  Up to 3 patches applied once daily directly over painful site  12 h on, 12 h off (FDA-approved label)  recently published data indicate 4 patches (18–24 h) safe  Efficacy demonstrated in 3 randomized controlled trials on postherpetic neuralgia  Drug interactions and systemic side effects unlikely  most common side effect: application-site sensitivity  Clinically insignificant serum lidocaine levels  Mechanical barrier decreases allodynia
    37. 37. Lidocaine  Action: Na+ channel block  Indication: peripheral NP, ? others  Useful IV or topical (NNT = 4.4)  No reliable oral equivalent (mexiletine NNT = 10)  SE: similar rates to placebo for sedation, N/V, pruritis etc  CNS toxicity at plasma levels > 5 mcg/ml  Dose: IV 1-2 mg/kg/hr (??duration)  Patches available in USA, ?EMLA here Challapalli V, Tremont-Lukats IW, McNicol ED, Lau J, Carr DB. Systemic administration of local anesthetic agents to relieve neuropathic pain. Cochrane Database of Systematic Reviews 2005, Issue 4.
    38. 38. Ketamine  Action: NMDA receptor antagonist  ‘anti-hyperalgesic', 'anti-allodynic' and 'tolerance- protective' agent  Indication: Protective analgesia, NP treatment, opioid- tolerant patients  SE: Dysphoria, nightmares, “psychedelic” effects  Dose: Low doses usually well tolerated  Intra-op: 0.5mg/kg bolus then 0.25-0.5 mg/kg/hr (beware prolonged recovery)  Post-op: 0.1-0.2 mg/kg/hr (?duration) Himmelseher S, Durieux ME. Ketamine for Perioperative Pain Management. Anesthesiology 2005; 102:211–20 Australian and New Zealand College of Anaesthetists and Faculty of Pain Medicine. Acute Pain Management: Scientific Evidence, 2nd Ed. Australian and New Zealand College of Anaesthetists, Melbourne, Australia, 2005; 143-144
    39. 39. Principles of Opioid Therapy for Neuropathic Pain  Opioids should be titrated for therapeutic efficacy versus AEs  Fixed-dose regimens generally preferred over prn regimens  Document treatment plan and outcomes  Consider use of opioid written care agreement  Opioids can be effective in neuropathic pain  Most opioid AEs controlled with appropriate specific management (eg, prophylactic bowel regimen, use of stimulants)  Understand distinction between addiction, tolerance, physical dependence, and pseudoaddiction
    40. 40. Opioids  A select group of pain patients benefits from opioids, with resultant pain reduction and improved physical and psychological functioning  They have minimal side effects & show increased activity levels & less pain  Other patients do poorly with opioids, experiencing tolerance and side effects, especially with escalating doses
    41. 41. Distinguishing Dependence, Tolerance, and Addiction  Physical dependence: withdrawal syndrome arises if drug discontinued, dose substantially reduced, or antagonist administered  Tolerance: greater amount of drug needed to maintain therapeutic effect, or loss of effect over time  Pseudoaddiction: behavior suggestive of addiction; caused by undertreatment of pain  Addiction (psychological dependence): psychiatric disorder characterized by continued compulsive use of substance despite harm
    42. 42. Opioids  Action: ↓ NT release, ↓ cell excitability  Indications: Any NP  Oxycodone, morphine (NNT = 2.5)  Tramadol (NNT = 3.9)  SE: usual, and ?OIH  Doses: usual  ? Stay below 100-200mg/d PO Morphine equivalent (ie. 30-60mg/d IV)  ? methadone & buprenorphine less hyperalgesic Finnerup NB, Otto M, McQuay HJ, Jensen TS, Sindrup SH. Algorithm for neuropathic pain treatment: An evidence based proposal. Pain 118 (2005) 289–305
    43. 43. Antidepressants in Neuropathic Pain Disorders*  Multiple mechanisms of action  Randomized controlled trials and meta-analyses demonstrate benefit of tricyclic antidepressants (especially amitriptyline, nortriptyline, desipramine) for postherpetic neuralgia and diabetic neuropathy  Onset of analgesia variable  analgesic effects independent of antidepressant activity  Improvements in insomnia, anxiety, depression  Desipramine and nortriptyline have fewer adverse effects *Not approved by FDA for this use.
    44. 44. Tricyclic Antidepressants: Adverse Effects  Commonly reported AEs (generally anticholinergic):  blurred vision  cognitive changes  constipation  dry mouth  orthostatic hypotension  sedation  sexual dysfunction  tachycardia  urinary retention  Desipramine  Nortriptyline  Imipramine  Doxepin  Amitriptyline Fewest AEs Most AEs AEs = adverse effects.
    45. 45. Tricyclic Antidepressants  Action: Mixed (↑ 5-HT &/ Norad at synapse)  Indication:  All NP treatment (except SCI, PLP, HIV)  NNT: overall = 3.1, central = 4.0, periph = 2.3  PHN prevention: 50% ↓ if used for 90days  SE: dizzy, sedation, anticholinergic  NNH minor = 5, NNH major = 16  Doses  Amitriptylline 10-25mg nocte, max 100mg  Nortriptylline (?less sedating) same doses Finnerup NB, Otto M, McQuay HJ, Jensen TS, Sindrup SH. Algorithm for neuropathic pain treatment: An evidence based proposal. Pain 118 (2005) 289–305
    46. 46. Venlafaxine  Action: SNRI  Indication:  mastectomy pain prophylaxis,  peripheral NP treatment (NNT=5.5)  SE: sedation/insomnia, ataxia, ↑BP, nausea  NNH major = not significant  Doses  Protective 75mg/d (pre-op then for 2wks)  Treatment: 37.5-375mg/d Reuben SS, Makari-Judson G, Lurie SD. Evaluation of efficacy of the perioperative administration of venlafaxine XR in the prevention of postmastectomy pain syndrome. J Pain Symptom Manage. 2004; 27: 133-39.
    47. 47. Calcitonin  Action: uncertain  Indication: PLP, CRPS, ?other NP  SE: N/V, flushing, dizzy, allergy  Skin prick test advised  Dose: 100 IU in 100ml saline over 1hr  Pre-treat with anti-emetics  Repeat daily for 3 days Visser EJ. A review of calcitonin and its use in the treatment of acute pain. Acute Pain 2005;7 :185-189.
    48. 48. Interventional Treatments for Neuropathic Pain  Neural blockade  sympathetic blocks for CRPS-I and II (reflex sympathetic dystrophy and causalgia)  Neurolytic techniques  alcohol or phenol neurolysis  pulse radio frequency  Stimulatory techniques  spinal cord stimulation  peripheral nerve stimulation  Medication pumps CRPS = complex regional pain syndrome.
    49. 49. Summary of Advances in Treatments for Neuropathic Pain*  Botulinum toxin: low back pain  Lidocaine patch 5%: low back pain, osteoarthritis, diabetic and HIV-related neuropathy, with gabapentin  CR oxycodone: diabetic neuropathy  Gabapentin: HIV-related neuropathy, diabetic peripheral neuropathy, others  Levetiracetam: neuropathic pain and migraine  Oxcarbazepine: neuropathic pain; diabetic neuropathy  Bupropion: neuropathic pain  Transdermal fentanyl: low back pain *Applications not approved by FDA.
    50. 50. CURRENT MANAGEMENT NON PHARMACOLOGIC  EXERCISE  TENS  PENS  GRADED MOTOR IMAGERY  CBT
    51. 51. World Health Organization (WHO) Analgesic Ladder.
    52. 52. INTERVENTIONAL PAIN MANAGEMENT  Epidural or Perineural injections of local anesthetics or cortico steroids.  Implantations of epidural and intrathecal drug delivery systems.  Neural ablative procedures.  Insertion of spinal cord stimulators.  Sympathetic nerve blocks.
    53. 53. Treatment Goals - I  Reduce and manage pain  Decreased subjective pain reports  Decreased objective evidence of disease  Optimize medication use  Increase function & productivity  Restore life activities  Increase psychological wellness  Reduce level of disability
    54. 54. Treatment Goals - II  Stop cure seeking  Reduce unnecessary health care  Prevent iatrogenic complications  Improve self-sufficiency  Achieve medical stabilization  Prevent relapse / recidivism  Minimize costs - maintain quality  Return to gainful employment
    55. 55. Chronic Pain Evaluations  Comprehensive multidisciplinary evaluations offers a means of developing an appropriate treatment plan  This can help identify factors which may prolong complaints of pain and disability despite traditional medical care  Such an evaluation can also identify who would benefit from a more structured and intensive functional restoration program
    56. 56. Measuring Opioid Usefulness  Each individual with chronic pain should be viewed as unique and the ultimate outcome of the use of opioid medication must be viewed in terms of  Pain relief  Objective gains (function or increased activity)  Does taking an opioid allow the person to be happier and do more things without unacceptable side effects or do the medications only create more problems and no observable change in activity level?
    57. 57. Adjunctive Treatment Modalities  Joint, bursal & trigger point injections  Botulinum toxin injections  Nerve root and sympathetic blocks  Peripheral and plexus blocks  Facet and medial branch injections  Lidocaine infusions  Epidurals  Neuroablative techniques  Chemical, Thermal, & Surgical  Neuromodulation  Spinal cord stimulators & Implanted spinal pumps
    58. 58. Physical & Occupational Therapy  Active  Improved body mechanics  Spine stabilization  Stretching & strengthening  Aerobic conditioning  Aquatics therapy  Work hardening  Self-directed fitness program
    59. 59. Psychological Approaches  Non-drug pain management skills  Anxiety & depression reduction  Biofeedback, relaxation training, stress reduction skills, mindfulness meditation, & hypnosis  Cognitive restructuring  Improve coping skills  Learn activity pacing  Habit reversal  Maintenance and relapse prevention
    60. 60. Functional Restoration  Locus of control issues  Timely and accurate diagnosis  Assessment of psychosocial strengths and weaknesses including analysis of support system  Evaluation of physical and functional capacity  Treatment planning and functional goal setting for return to life and work activities  Active physical rehabilitation  Cognitive behavioral treatment  Patient and family education  Frequent assessment of compliance and progress
    61. 61. 61 Facial Nerves and Pain  Trigeminal nerve  Largest of 12 cranial nerves  Three major branches  Ophthalmic nerve  Sensory information (tactile, thermoception, nociception, proprioception) from green areas, nasal mucosa,and frontal sinuses  Maxillary nerve  sensory information from pink areas, nasal mucosa, palate, ethmoid and sphenoid sinuses  Mandibular nerve  Sensory input from yellow areas, floor of the mouth, and anterior 2/3 of tongue  Motor control of muscles involved in biting, chewing, and swallowing
    62. 62. 62 Neural Mechanisms of Pain  Gate Control Theory  Melzack and Wall (1965)  Perception of pain mediated by a “gate” located in the dorsal horn of the spinal cord SG- SG+ T cell L-fibers mediating tactile perception (A-α and A-β) S-fibers mediating pain perception (A- δ and C) PAIN Central Control closes gate opens gate - + + + + +
    63. 63. 63 Experimental Evidence for the Gate  Selective inactivation of L- fibers results in greater pain perception from noxious stimuli (Price, Hi, and Dubner, 1977)  Phantom Limb Pain may result from reduced L-fiber input (Melzack, 1970) SG- SG+ T cell L-fibers mediating tactile perception (A-α and A-β) S-fibers mediating pain perception (A- δ and C) PAIN Central Control closes gate opens gate - + + + + +
    64. 64. 64 Endorphins and Pain  Endorphins: neurotransmitters that act as endogenous (naturally-occurring) morphine-like substances  Endorphins bind to same receptor sites in brain stem as opiates  SPA works best when endorphin sites are stimulated— may release endorphins into the nervous system (Hosubuchi et al., 1977)
    65. 65. 65 Endorphins and Pain  Concentration of endorphins is generally less for people suffering from chronic pain (Akil et al., 1978)  Opiate inhibitors (e.g., naloxone) decrease the analgesic effects of acupuncture, SPA, and placebos  Stress-induced analgesia may result from increased release of endorphins during stress
    66. 66. Nociceptors in Skin Epidermi s Dermis Free Nerve Endings
    67. 67. Pain Pathways  Lots of effort to id neural pathways  Found distinct categories of nerve fibres  A δ : mylinated, carry rapidly sharp pains (20- 30 ms-1 )  C : unmylinated, carry slowly burning pain (0.5- 2 ms-1 )
    68. 68. Associated Area of Brain Fibres pass signals up spinal cord as electrical impulses then onto the thalamus Thalamus relays messages to cortex Proved difficult to id. specific area of the cortex that produce pain
    69. 69. Gate Control Theory (Melzack & Wall, 1965) A gate in the substantia gelatinosa of the dorsal horn can be open or closed, blocking pain information. The gate can be closed by descending signals from the brain, or by the balance of activity in A-beta fibres (large myelinated) and C fibres (small non-myelinated)  A-beta fibres produce touch sensations  C fibres produce dull diffuse pain. Greater activity in A-beta fibres closes the gate, greater activity in C fibres opens it.  Other factors influencing the gate include  Attention  Emotional & Cognitive factors  Physical factors Some forms of analgesia, e.g. TENS & acupuncture, might be accommodated within gate control theory.
    70. 70. Gate Control Theory - Melzack & Hall (1965) Experience Behaviour Tissue damage Gate – amplifies or attenuates signal Pain Perception Emotion
    71. 71. Opening & Closing the Gate Factor Opens Closes Physical injury agitation medication Emotional anxiety stress frustration depression tension relaxation optimism happiness Behavioural (Cognitive) rumination boredom enjoyable activities complex tasks distraction social interaction
    72. 72. Problems for Gate Control Theory  Evidence for propsed moderators, but no physical evidence of gate  Still organic basis for pain (phantom limb?)  Not truly integrative re: psyche & soma  Still improvement on stimulus-response paradigm
    73. 73. Subsequent Pain Theories  Reflect trends in general psychology  Fordyce (1976) - pain as behaviour  Reinforcement contingencies  +ve reinforcement (e.g. attention / affection for pain behaviours)  -ve reinforcement (e.g. avoid unpleasant events such as work, school)  Recently, growth in cognitive behaviour models
    74. 74. Fear-Avoidance Theory  (-ve) appraisals (catastrophising) → fear of pain (illness cognitions) & re-injury  Fear of pain → avoidance of potentially painful events (illness behaviour)  Little opportunity to disconfirm beliefs  Avoidance → disuse syndrome & ↑ p (mood problems)
    75. 75. Treatments  Mirror pain theories  Medical (especially acute pain)  Non-anti-inflammatory non-steroid (paracetamol)  Anti-inflammatory non-steroids (eg ibprofen)  Opioids (eg morphine)  Psychological  Behavioural initially
    76. 76. Treatment of Chronic Pain  Surgical procedures to block the transmission of pain from the peripheral nervous system to the brain.  Synovectomy – Removing membranes that become inflamed in arthritic joints.  Spinal fusion – joins two or more adjacent vertebrae to treat chronic back pain.
    77. 77. Psychological Pain Control Methods  Biofeedback – provides biophysiological feedback to patient about some bodily process the patient is unaware of (e.g., forehead muscle tension).  Relaxation – systematic relaxation of the large muscle groups.  Hypnosis – relaxation + suggestion + distraction + altering the meaning of pain.
    78. 78. Psychological Pain Methods  Acupuncture – not sure how it works. Could include:  Counter-irritation – may close the spinal gating mechanism in pain perception.  Expectancy  Reduced anxiety from belief that it will work.  Distraction  Trigger release of endorphins
    79. 79. Phantom Limb pain  Affects the majority of amputees  For most the sensation fades, but a minority experience lasting discomfort.  Theories  Neuroma  Deafferented spinal neurons  Melzack (1992) Neuromatrix innate linkage between sensation, emotion and self-recognition.  Merzenich (1998) Cortical remapping & unmasking Ramachandran (1992) phantom leg sensations often referred from the chest, phantom arm sensations from the face.
    80. 80. Phantom limb pain: during amputation under general anesthesia the spinal cord can still “experience” the insult produced by the surgical procedure and central sensitization occurs. To try to prevent it, local infiltration of anesthetics in the site of surgery. But studies show also rearrangement of cortical circuits (cortical region of the missing limb receives afferents from other site of the skin) Phantom Pain intensity as a function of Cortical Reorganization.
    81. 81. Analgesia Peripheral via prostaglandin synthesis inhibition (e.g. Asprin) Central via receptors for endogenous opioids. Bind to receptors in the periaqueductal gray, which activate descending serotoninergic fibres. These inhibit pain transmission Endogenous opioids also underlie some psychological influences. Naloxone blocks both TENS and placebo analgesia
    82. 82. PAIN TREATMENT CONTINUUM Diagnosis Oral Medications PT, Exercise, Rehabilitation Behavioral Medicine Corrective Surgery Therapeutic Nerve Blocks Oral Opiates Implantable Pain Management Devices Neurostimulation Intrathecal Pumps Neuroablation
    83. 83. Multi-Disciplinary Pain Program Models  Pain Consultation Team  Multidisciplinary Programs  Multidisciplinary Outpatient Programs  Multidisciplinary Inpatient Programs  Pain Service
    84. 84. Pain Consultation Team  Multidisciplinary group  Provides consultation services only  not ongoing treatment Anesthesiology Psychology Neurology Pharmacy Nursing Consultation Team Referral Recommendatio n
    85. 85. Multidisciplinary Clinics  Comprised of 2 or more disciplines  Goal is to provide coordinated and more comprehensive care to patients for more complex chronic pain problems  3 general subtypes  Psychoeducational clinic (mild and motivating)  Problem-based clinic (e.g. headache, LBP, FM)  Comprehensive multidisciplinary clinic  Inpatient or outpatient
    86. 86. Chronic Pain Disciplines and Roles (Core) Anesthesiology – nerve blocks Kinesiotherapy – pool therapy; activity Neurology – eval. treatment Nursing – patient care Physical Medicine – exercise; modalities Physical Therapy – exercise; modalities Psychology – eval. and treatment Occupational Therapy – UE eval and treatment
    87. 87. •Rheumatoid and Osteo-arthritis •Back pain •Menstrual Pain •Labour Pain •Peripheral Nerve Injuries •Shingles •Headache and Migraine •Cancer Pain •Trigeminal Neuralgia •Phantom Limb Pain •Sports Injuries •Sciatica •Aching Joints •Post Operative Pain •Muscular Pain •Whiplash and Neck Injury and many others                                                   
    88. 88. Decrease Inflammatory Response NSAIDs, Local Anesthetics, Steroids Decrease Conduction Gabapentin, Carbamazepine, Local Anesthetics, Opioids Prevent Centralization COX 2, Opioids, Ketamine, α-2 Agonists. Increase Inhibition TCA’s, SSRI’s, Clonidine Modify Expression Anxiolytics Mechanistic Approach To Pain Therapy
    89. 89. Summary  Chronic neuropathic pain is a disease, not a symptom  “Rational” polypharmacy is often necessary  combining peripheral and central nervous system agents enhances pain relief  Treatment goals include:  balancing efficacy, safety, and tolerability  reducing baseline pain and pain exacerbations  improving function and QOL  New agents and new uses for existing agents offer additional treatment options
    90. 90. Further Reading  Rosenzweig et al. cover pain in the second half of chapter eight.  Horne, S. & Munafo, M. (1997). Pain, theory, research and intervention. Oxford University Press  Wall, P. & Melzack, R. (1988). The challenge of Pain. Penguin.
    91. 91. REFERENCES  Review Neuropathic pain: a practical guide for the clinician ; Ian Gilron, C. Peter N. Watson, Catherine M. Cahill and Dwight E. Moulin  Dworkin RH, Backonja M, Rowbotham MC, et al. Advances in neuropathic pain. Arch Neurol 2003;60:1524-34.  Gilron I, Bailey JM, Tu D, et al. Morphine, gabapentin, or their combination for neuropathic pain. N Engl J Med 2005;352:1324-34.  Stephen Macres, Understanding Neuropathic Pain  Eisenberg E, McNicol ED, Carr DB. Efficacy and safety of opioid agonists in the treatment of neuropathic pain of nonmalignant origin. JAMA 2005;293:3043-52.
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