THE MERCK VETERINARY MANUAL
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Chronic Pain

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The treatment of chronic pain relies on pharmacologic and nonpharmacologic methods. Commonly treated chronic pain syndromes include osteoarthritis, nonsurgical intervertebral disc disease, and laminitis. Some chronic pain responds to drugs used to treat acute pain, such as opioids and NSAIDs; however, other types of chronic pain require the addition of novel drugs such as gabapentin, tramadol, acetaminophen, and amantadine. Regardless of the cause or the species, chronic pain is itself a dynamic disease process requiring careful assessment and frequent reassessment. Therapy is rarely monomodal, and adjustments in therapy are necessary over time.

The nonpharmacologic treatment of chronic pain depends on the underlying cause and the species. Among these therapies are acupuncture, rehabilitation, nutraceutical supplements, low-level laser, massage, transcutaneous electric nerve stimulation, and herbal supplements. The body of literature critically examining these is relatively small. (Also see Complementary and Alternative Veterinary Medicine).

Osteoarthritis is the most common reason for persistent pain in people, dogs, cats, and horses. The nonpharmacologic goals to manage osteoarthritis pain include increasing mobility, limiting disease progression, and possibly facilitating tissue repair within the joint. Weight control or reduction and mild to moderate daily exercise are beneficial. Excessive and strenuous exercise should be avoided, because it may further strain the joints and exacerbate pain, thus limiting the ability to exercise routinely. Providing warmth during cold and damp weather and extra bedding or padding may also improve comfort. Surgical removal of bone fragments and osteochondritic lesions and the restoration of joint stability is often necessary to slow the progression of disease and reduce pain. Joint replacement or arthrodesis may be indicated in severe cases. Orthotic bracing may be beneficial in small animals when surgical intervention is not appropriate. A number of studies using chondroprotective agents such as polysulfated glycosaminoglycans, chondroitin sulfate, glucosamine, and hyaluronic acid suggest a benefit through stimulation of cartilage matrix synthesis and inhibition of enzymatic degradation of cartilage. However, the efficacy of these agents may vary with specific product used, route of administration, and underlying conformational, pathologic, and neurologic issues. Recent work in regenerative medicine, including the use of autologous adult stem cells, may prove highly effective as well. Finally, acupuncture and rehabilitation therapy have been used to treat chronic osteoarthritis pain in a number of veterinary species with promising results.

Cancer pain presents a unique clinical challenge. Components of cancer pain are tumorigenic (tumor-related products), inflammatory, and neuropathic. Similar to other forms of chronic pain, it does not always respond adequately to common therapies. Opioids remain a cornerstone of treatment and are usually one part of a multimodal plan. Other analgesics include NSAIDs, tramadol, acetaminophen (in dogs), and amantadine for its ability to decrease wind-up. An opioid and an NSAID are routinely used in combination because of their synergistic analgesic effect. Recently, bisphosphonates have been used to treat osteolytic pain associated with bony metastasis. Pamidronate is the synthetic analog of inorganic pyrophosphate. It has proved useful in pain palliation because of its ability to adsorb to bone mineral matrix, inhibit pathologic lysis induced by osteoclasts, and delay progression of bony metastatic lesions.

Similar to all chronic pain syndromes, cancer pain management requires frequent assessment and reassessment combined with appropriate adjustments in therapy.

Last full review/revision August 2013 by Sandra Allweiler, DVM, DACVA

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