Today's Veterinary Practice

NOV-DEC 2018

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PEER REVIEWED 26 NOVEMBER/DECEMBER 2018 todaysveterinarypractice.com its lack of a role in tissue protection and healing: "pain that has persisted beyond the normal tissue healing time … pain without apparent biological value." 1 As with acute pain, chronic pain can cause the pain-mediated adverse effects previously mentioned and thus requires analgesic treatment. Components of Chronic Pain The most common cause of chronic maladaptive pain in mammals is osteoarthritis, or degenerative joint disease, and the second most common cause is cancer. The overall sensation of pain from any cause is multifactorial, and pain from either osteoarthritis or cancer generally has both inflammatory (which can be protective if not excessive) and neuropathic (which is always maladaptive) components. 3 Inflammatory Pain Inflammatory pain is a very common component of most pain syndromes, making the inclusion of anti-inflammatory drugs (e.g., nonsteroidal anti- inflammatory drugs [NSAIDs] or piprants [grapiprant]) an integral component of chronic pain therapy for all patients, unless use of these drugs is contraindicated (e.g., gastrointestinal ulceration, renal dysfunction, or hepatic dysfunction). However, anti-inflammatory drugs alone may not completely alleviate pain that is moderate to severe and/or longstanding. It isn't that anti-inflammatories don't work; rather, it is that they can't control pain from other sources, like neuropathic pain. For adequate control of moderate to severe chronic pain, multimodal analgesia (i.e., use of more than 1 mode of treatment) is almost always necessary. Neuropathic Pain Neuropathic pain is commonly defined as "pain caused by a disease or lesion which leads to damage or dysfunction of the somatosensory (pain) system." 4 Neuropathic pain is maladaptive and can be fairly intense; pain impulses are often described by people as "lightning bolts" or "stabbing pain." Components of neuropathic pain include peripheral sensitization (increased pain stimuli from peripheral tissues) and central sensitization (increased activity of the pain pathway at the dorsal horn neurons of the spinal cord, often referred to as windup). Neuropathic pain also often includes ectopic firing of nociceptors, which normally do not fire unless stimulated, and down-regulation of opioid receptors, making drugs in this class largely ineffective for treating this pain syndrome. Pain is naturally controlled to some extent by the descending inhibitory limb of the pain pathway; however, this limb is often dysfunctional in patients with neuropathic pain. These pathologies in the pain pathway can lead to abnormal pain sensations such as hyperalgesia (exaggerated pain sensation from a mildly painful stimulus) and allodynia (pain sensation from a nonpainful stimulus). Examples of neuropathic pain commonly encountered in veterinary medicine include nerve entrapment, nerve damage after surgery or trauma, amputations, tumors associated with or impinging on nerves, lumbosacral disc disease/ degeneration, discospondylitis, feline lower urinary tract disease, chronic changes associated with osteoarthritis (degenerative joint disease) ( FIGURE 1 ), and many others. 3,5,6 It is intuitive that neurologic injury or damage can lead to neuropathic pain but FIGURE 1. Degenerative joint disease in the cubital joint of a cat (A) and the coxofemoral joints of a dog (B). B A

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