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Chronic Pain: Pathophysiology and Treatment Implications

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An examination of the current understanding of the processes and related therapies aimed at treatment of chronic pain in animals is presented. Discussion focuses on mechanisms involved in the neural pathways of chronic pain, differences between acute and chronic pain, and pharmacologic options for chronic pain as they relate to inflammatory, neoplastic, and neuropathic processes.

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What Is Chronic Pain?

Chronic pain has been defined as aberrant somatosensory processing in the peripheral or central nervous system (CNS) that is sustained beyond the normally expected time course relative to the stimulus. This definition, although helpful, provides only part of the story. Chronic pain is often insidious, vague, and difficult to pinpoint. Chronic pain may arise from a primary dysfunction within the nervous system. Chronic pain is difficult to diagnose by health care professionals, and its diagnosis

Therapeutic Implications

Better understanding of the pathophysiology associated with chronic pain will introduce the challenges to finding effective treatments and plant the seeds to nurture novel approaches in this quest. Drugs that are effective for acute pain may have little or no benefit when used for chronic pain syndromes. Conversely, drugs with little demonstrated benefit in treatment of operative pain may have significant efficacy in the treatment of chronic pain. The long-term therapies typically instituted

Chronic Inflammatory Pain

Use of nonsteroidal antiinflammatory analgesics has become the hallmark of therapeutic interventions for mild to moderate chronic inflammatory pain in animals. By inhibiting production of COX enzymes, these agents decrease prostanoid production. Prostanoids are mediators of inflammation and amplify nociceptive input.9 Hyperalgesic responses to tissue injury are primarily ascribed to the effects of COX-2. Some drugs of this class, for example aspirin, may inhibit other significant promoters for

Cancer Pain

Pain in patients with cancer may be related to processes associated with the tumor itself or to the treatment of the disease. Nonsteroidal antiinflammatory analgesics aid in the reduction of inflammatory mediators that sensitize the pain pathways. Opioids constitute the backbone of treatment of moderate to severe cancer pain. Side effects associated with chronic administration of opioids include vomiting, dysphoria, diarrhea, or constipation. Dose-dependent analgesia from opioids is achieved

Neuropathic Pain

Neuropathic pain is pain originating in the nervous system. Traumatic damage to nervous tissue is a significant initiating factor in neuropathic pain. Additionally, ingredients in the “sensitizing soup” may activate macrophages at the site of nerve injury, resulting in production of tumor necrosis factor or interleukin-1β, substances that promote neuropathic pain. Features of neuropathic pain include: 1) central sensitization; 2) central disinhibition (imbalance in excitatory and inhibitory

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