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Pain management in horses: What are our alternatives? (Proceedings)

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Aug 01, 2008

Nonsteroidal anti-inflammatory drugs (NSAIDs) are the most widely used analgesic agents in equine medicine. They are quite effective in controlling pain and inflammation in a wide variety of conditions. However, there are occasions where NSAIDs alone are insufficient or when their potential adverse effects are unacceptable for a patient. The risk of adverse effects can be minimized or eliminated in some patients if alternative drugs, combinations of drugs or alternative routes of administration are considered. An understanding of available analgesic drugs, their mechanisms of action, potential adverse effects, possible routes of administration, and the type of pain being treated enables the practitioner to develop a patient-specific analgesic protocol.

Opioids

Opioids are often used in the control of perioperative pain in other species but have been less widely used in horses because of concerns regarding possible adverse effects. The combination of an opioid drug with a NSAID provides synergistic analgesia and represents a relatively straightforward approach to multimodal analgesia. Butorphanol has been the most widely used of the opioid drugs in equine medicine. It is predominantly a kappa agonist and therefore has fewer and less severe adverse effects as compared to morphine and other mu agonists. Butorphanol is not sedative in adult horses when used on its own. It is approved for use IV but is often administered IM for analgesic effects. Absorption of butorphanol after IM administration is very rapid (half life of absorption of 6 min) but systemic availability after IM injection is surprisingly low (37%) in adult horses. Terminal half-life after IM administration is 0.57 hrs. To maintain targeted plasma butorphanol concentrations above 10 ng/mL, administration of 0.08 mg/kg IM every 3 hours may be necessary. High intravenous dosages of butorphanol (0.1 - 0.5 mg/kg) are associated with excitatory behavior, increased locomotion and inhibition of gastrointestinal activity. These effects are less frequently observed with lower doses or when the drug is administered IM or by continuous rate infusion (CRI). When used as a CRI, butorphanol may decrease gastrointestinal motility and fecal production should be monitored appropriately. If a decrease in fecal production is observed, administration of mineral oil via nasogastric tube and/or decreasing infusion rates is recommended. Butorphanol CRI should rarely be continued for more than 12-24 hours. Epidural butorphanol may potentiate lidocaine-induced analgesia but is unlikely to provide significant analgesia when used as the sole agent. Unlike adult horses, neonatal foals receiving butorphanol by IV or IM injection appear profoundly sedated. This may represent differences in opioid receptor distribution or differences in P glycoprotein distribution within the central nervous system of neonates as compared to adults.

Other opioids that have been used as analgesic medications in horses include fentanyl, morphine and methadone. Transdermal fentanyl patches have been used in horses of all ages without significant reported adverse effects. However, recent experimental data demonstrates that absorption of drug is erratic at best, peak drug concentrations are often below the predicted therapeutic range, and even when administered intravenously, fentanyl may not be an effective analgesic agent for horses (Sellon et al, 2007, unpublished data). Therefore, use of fentanyl patches in adult horses cannot be recommended at this time. Morphine may be used intravenously as an analgesic agent in combination with alpha 2 agonists. When used intravenously as a sole analgesic agent, it may result in profound excitation. Anecdotally, IM morphine use is not associated with CNS excitation and this may represent an underappreciated analgesic option. Morphine may be administered epidurally on its own or in combination with detomidine to provide effective analgesia of the caudal half of the body. Epidural morphine administered at 0.05 to 0.1 mg/kg produces segmental analgesia; dorsal nerve branches of the lumbosacral plexus appear to be preferentially affected. Morphine is typically used at dosages of 0.1 to 0.2 mg/kg diluted to 10-20 ml with 0.9% saline (total volume of 0.04 ml/kg body weight). Analgesic effects are seen within 20-30 minutes and may last 8-24 hours without adverse effects on motor function. Some surgeons have anecdotally reported efficacy of intra-articular morphine for control of pain associated with arthroscopy or arthrotomy. Oral opioids are rarely used in horses; however, there are anecdotal reports that oral methadone may be efficacious.