Pain Management in Small Animals With Lameness

Control of pain in small animals with lameness, including postoperative patients, involves a broad range of analgesics, including NSAIDs and opioids. Analgesic agents may be administered via oral, parenteral (including CRI), epidural, local, or transdermal routes.

Nonpharmacological pain management strategies include acupuncture, massage, physical rehabilitation, environmental modification, dietary modification, and owner education. The combination of drug and nondrug pain control modalities typically provides superior analgesia and allows lower medication dosages.

All owners should be counseled to maintain pets with lameness at a lean body weight and provide regular, low-impact exercise. Physical therapy can include therapeutic exercises and therapeutic modalities to help control pain, build and maintain muscle mass, and improve joint range of motion. Examples of therapeutic exercises include joint-range-of-motion, cavaletti, sit-to-stand, and balancing exercises.

Additional therapeutic modalities can include laser therapy, extracorporeal shock wave therapy, and therapeutic ultrasonography.

When pain due to lameness is not controlled by environmental factors alone, oral pain medications, particularly NSAIDs, can be an important next step in treatment.

Commonly administered NSAIDs include COX-2 inhibitors, aspirin, and several other drugs (see the table NSAIDs Used for Pain Management in Dogs and Cats).

The use of NSAIDs is contraindicated in animals with gastroenteritis, or coagulopathy, and in patients undergoing concurrent corticosteroid treatment. NSAIDs should be used cautiously in patients with hepatic or renal insufficiency; other pain control medications and modalities should be considered.

Opioid analgesics bind to mu, kappa, and/or delta receptors in the CNS; full mu agonists provide superior pain relief. Commonly administered full mu agonists administered via bolus injection include the following (1):

• morphine: in dogs, 0.1–0.5 mg/kg, IM, SC, or slow IV

• oxymorphone: in dogs and cats, 0.05–0.2 mg/kg, IV, IM, or SC, every 3–4 hours

• hydromorphone: in dogs and cats, 0.05–0.2 mg/kg, IV, IM, or SC, every 2–4 hours

Epidural administration of preservative-free morphine (0.1 mg/kg) in the lumbosacral space is also a useful adjunct for postoperative pain relief in the hindlimbs and to decrease overall anesthetic requirements (2).

Fentanyl is a short-acting opioid often administered via either CRI or transdermal patch (3, 4). In dogs, fentanyl patches are applied for 72 hours, at the following dosages: for dogs with body weight (BW) < 10 kg, 25 mcg/hour; for BW 10–20 kg, 50 mcg/hour; for BW 20–30 kg, 75 mcg/hour; for BW 30–40 kg, 100 mcg/hour (5). Fentanyl patches are not immediately effective, so they must be in place for at least 12 hours before they can be used as the sole analgesic modality.

Methadone is a synthetic opioid that works via mu receptors and NMDA receptors to control pain. It is administered at 0.2–0.4 mg/kg SC, IV, or IM, every 4–6 hours (6, 7).

Buprenorphine is a partial mu agonist with a longer dosing interval that can also be administered transmucosally. Buprenorphine has the longest duration of action and is now available in a higher-concentration injection (for dogs: 0.24 mg/kg, SC, every 24 hours for up to 3 days) (8, 9) and an extended-release transdermal formulation (for cats: 2.7–6.7 mg/kg, applied to clean, dry skin on the dorsal cervical region near the base of the skull) that lasts up to 4 days (10). Transdermal buprenorphine is appropriate only for cats; it should not be used in dogs, where it has low bioavailability.

Oral opioids administered for pain relief are likely not as helpful as other oral medications in dogs and cats, because of limited bioavailability after oral administration. In cats with osteoarthritis, tramadol (2 mg/kg, PO, every 12 hours) has been shown to improve activity and mobility (11).

Local administration of analgesics via intra-articular injections of morphine, bupivicaine, or lidocaine preoperatively, as a preemptive block of intracapsular pain receptors, has been described (12). However, intra-articular local anesthetics have been associated with cartilage damage.

Local nerve blocks (femoral, saphenous, and sciatic) can also be performed for pain control after orthopedic surgery (13). A bupivicaine liposome suspension has been developed for intraoperative local infiltration in dogs (5.3 mg/kg) aimed at decreasing postoperative pain for several days (14, 15).

Corticosteroids are considered weak analgesic adjuncts because they indirectly decrease pain via their primary mode of action as local anti-inflammatory agents at the injury site. They are not typically used to control pain associated with lameness in dogs and can be associated with substantial adverse effects, including catabolic effects on muscle and ligaments leading to long-term worsening of lameness. Other, safer anti-inflammatory alternatives are preferred. Steroid use is also contraindicated during concurrent NSAID treatment.

Gabapentin is a calcium channel blocker used to inhibit neurons stimulated by pain; it is useful for treating animals with chronic or neuropathic pain and is a generally safe medication. A typical starting dosage is 10–15 mg/kg, PO, every 8 hours (16). The present author generally titrates up to 20–30 mg/kg as needed, depending on clinical response.

Dexmedetomidine and medetomidine with vatinoxan hydrochlorides are analgesic sedatives useful for facilitating examinations or diagnostic evaluations while providing some pain relief (17).

Joint fluid modifiers (eg, glucosamine, chondroiton sulfate, hyaluronan, pentosan polysulfate, omega-3 fatty acids) have received extensive attention for their 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.

There is evidence that omega-3 fatty acids, however, can improve weight bearing in dogs with osteoarthritis (18). Stem cell and platelet-rich plasma therapies to alleviate pain and discomfort that is due to diseased joints are also newer modalities for which scientific validation is pending.

Anti–nerve growth factor antibodies are therapeutic monoclonal antibodies that are used to treat pain due to osteoarthritis in dogs and cats. Adverse events associated with these medications have been reported, and more studies might be needed to determine the long-term effects of the medications and to help with appropriate patient selection.