Control of pain in small animals with lameness, including those in the postoperative period, involves a broad range of analgesics including NSAIDs and opioids. Analgesic agents may be administered via oral, parenteral (including constant-rate infusions), epidural, local, or transdermal routes. Nonpharmacologic pain management strategies include acupuncture, massage, physical rehabilitation, and diet.
Commonly administered NSAIDs include COX-2 inhibitors, aspirin, and several other drugs (see Table: NSAIDs Used for Pain Management in Dogs and Cats). The use of NSAIDs is contraindicated in animals with hepatic or renal insufficiency, gastroenteritis, or coagulopathy, and in patients receiving concurrent corticosteroid treatment.
NSAIDs Used for Pain Management in Dogs and Cats
Opioid analgesics bind to mu, kappa, and delta receptors in the CNS to provide pain relief. Commonly administered opioids include morphine (0.1 mg/kg, IV, IM, SC, every 3–4 hours), oxymorphone (0.05 mg/kg, IV, IM, SC, every 3–4 hours), hydromorphone (0.1 mg/kg, IV, IM, SC, every 2–4 hours), butorphanol (0.1 mg/kg, IV, IM, SC, every 2–4 hours in dogs and cats), and buprenorphine (0.01 mg/kg, IV, IM, SC, every 8 hours in dogs and cats, also transmucosal in cats).
Opioid analgesics can be administered with sedatives such as acepromazine (0.5 mg/kg, IV, IM, SC, every 4–6 hours) for enhanced efficacy of analgesia and sedation. Oxymorphone, hydromorphone, and butorphanol are more potent than morphine. Buprenorphine has the longest duration of action and is now available in a sustained-release formulation (dogs: 0.03–0.06 mg/kg, SC, single injection; cats: 0.12 mg/kg, SC, single injection). Another opioid, fentanyl, is most frequently administered via a transdermal patch applied at 3 day intervals on shaved skin. Oral opioids administered for pain relief include tramadol (5 mg/kg, 3 times a day), butorphanol (1 mg/kg, 3 times a day), hydromorphone (0.5 mg/kg, every 8 hours), codeine (1 mg/kg, every 8 hours), and oxycodone (0.3 mg/kg, every 8 hours).
Local administration of analgesics involves intraarticular injections of morphine (1 mg diluted in 5 mL of saline), bupivicaine (1 mL/20 kg body wt), or lidocaine (1 mL/20 kg body wt) preoperatively, as a preemptive block of intracapsular pain receptors. Recently, a bupivicaine liposome suspension was developed for intraoperative local infiltration in dogs (0.4 mL/kg) aimed at reducing postoperative pain. Administration of epidural morphine (0.1 mg/kg) in the lumbosacral space is also a useful adjunct for postoperative pain relief in the hind limbs and to reduce overall anesthetic requirements. Corticosteroids are considered weak analgesic adjuncts, because they indirectly reduce pain via their primary mode of action as local anti-inflammatory agents at the injury site. Drugs administered include prednisone or prednisolone (1–2 mg/kg/day, PO) or dexamethasone (1–2 mg/kg/day, IV). Their use is contraindicated during concurrent NSAID treatment.
Gabapentin (10 mg/kg, PO, every 12 hours) is a calcium channel blocker used to inhibit neurons stimulated by pain; it is useful for treatment of animals with chronic or neuropathic pain. Dexmedetomidine (5 mcg/kg/day, IM) and medetomidine (10 mcg/kg/day, IM) are newer analgesic-sedative, alpha-2-receptor blocking agents useful to facilitate examinations or diagnostic evaluations.
Joint fluid modifiers (glucosamine, chondroiton sulfate, hyaluronan, pentosan polysulfate, omega-3 fatty acids) have received extensive attention for the possible effectiveness in treating degenerative joint disease and alleviating associated discomfort. Although contraindications and adverse effects are few, robust evidence in support of these compounds is limited, and most reports are regarded as anecdotal. Stem cell and platelet-rich plasma therapies to alleviate pain and discomfort due to diseased joints are also newer modalities for which scientific validity is pending.