Treating cancer pain in dogs and cats - Veterinary Medicine
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Treating cancer pain in dogs and cats
No matter the type of cancer, pain is common at various stages, causing not only suffering but also other adverse physiological effects. Make sure you're aware of and are using the best management options—from surgery to radiation to drugs.


VETERINARY MEDICINE


Opioids


Table 2. Principal Opioids Used to Treat Cancer Pain in Dogs and Cats
Opioids are considered the mainstay of cancer pain therapy in human oncology, and no good reason exists for it to be any different in veterinary oncology (Table 2).1-3 One reason opioids are so helpful is that their dosage can be gradually and safely increased as needed to provide the desired level of analgesia. Side effects can be more important at higher dosages but are relatively predictable and tolerable, except for dysphoria in cats and Nordic canine breeds. Other main side effects include sedation, constipation, bradycardia, respiratory depression, panting, mydriasis (cats), cough suppression, altered laryngeal reflexes, vomiting, and histamine release (mostly morphine, especially with rapid intravenous dose).1-3,9 These side effects are encountered more commonly at higher dosages and with long-term use, and some can be treated symptomatically (constipation, vomiting).2,10,48,49 Opioids are used mostly for moderate to severe pain, and their effects are potentiated when used concurrently with other analgesics, including NSAIDs. The three main types of opioid receptors are mu, delta, and kappa, and they are located primarily in the superficial dorsal horn. The clinically used and best analgesic opioids have mu-receptor agonistic activity.2


Figure 2. This 8-year-old Doberman pinscher is being treated with full-course radiation therapy for a nasal tumor and has early radiation side effects (mucositis). An esophageal feeding tube was placed for enteral nutrition, and the dog was also treated with a transdermal fentanyl patch, an oral NSAID, and a mouthwash solution (viscous lidocaine, magnesium aluminum hydroxide, diphenhydramine) given once or twice a day.
The standard opioid remains morphine, a pure mu-receptor agonist. Morphine is relatively inexpensive, has predictable and tolerable side effects, and can be administered by various routes (subcutaneous, intramuscular, intravenous, oral, epidural). Other mu agonists useful in treating cancer pain include fentanyl (transdermal, intravenous infusion), hydromorphone, and oxymorphone. The use of transdermal fentanyl in companion animals has been recently reviewed (Figure 2).50 The partial agonist buprenorphine has a long duration of action and can be useful, especially with the effective transmucosal (mouth) route in cats, for treating breakthrough pain.9,11,14,45 Butorphanol, a mixed mu antagonist and kappa agonist, is not considered adequate for treating cancer pain, especially when pain is moderate to severe.2,3 In addition to butorphanol's lesser analgesic potency when compared with pure mu agonists, its duration of analgesia is short (20 to 90 minutes) when compared with the duration of sedation (hours), often leading to a false interpretation of comfort in sedated but painful patients.9,51-53 An opioid that is increasingly used for cancer pain in people is methadone.1-3 This interesting opioid not only is a mu-receptor agonist but also produces additional analgesic effects through noncompetitive antagonism of the N-methyl-D-aspartate (NMDA) receptors.1-3,9 Methadone has recently been described for treating pain in companion animals.9,14,45


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Source: VETERINARY MEDICINE,
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