Today's Veterinary Practice

NOV-DEC 2018

Today's Veterinary Practice provides comprehensive information to keep every small animal practitioner up to date on companion animal medicine and surgery as well as practice building and management.

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FEATURES NOVEMBER/DECEMBER 2018 31 on postoperative pain in dogs undergoing mastectomy, although pain scores did not differ, dogs receiving NSAIDs plus gabapentin required fewer opioid rescue doses than dogs receiving NSAIDs alone; thus, the gabapentin did seem to have an effect. 16 When looking at chronic conditions, among dogs with Chiari-like malformation and syringomyelia, pain scores did not differ among those receiving gabapentin and those receiving topiramate (an anticonvulsant and treatment for migraines in humans), but quality of life scores were better for the dogs receiving gabapentin. 17 However, dogs receiving gabapentin immediately after surgery for herniated disc extrusion, which would be a mix of chronic pain from the disc and acute pain from the surgery, demonstrated no significant pain relief with gabapentin compared with placebo. 18 The fact that the results between these studies are mixed is probably explained by 2 factors: 1) chronic pain has many facets, and pain from sources other than neuropathic were probably present; and 2) doses of gabapentin were low (5 to 10 mg/kg q12h) and perhaps ineffective, which actually makes the fact that any improvement was seen quite encouraging. More appropriate dosing, as defined by patient-specific dose and administration-interval adjustments, could potentially provide analgesia. In a thermal research model, gabapentin seems to provide minimal benefit for control of acute pain caused by heat. 19 However, because acute pain is not often neuropathic pain, the lack of evidence does not mean that gabapentin is not effective. Even in chronic pain studies, results for gabapentin may be mixed in terms of provision of analgesia because chronic pain is complicated and no one drug will work for all types of chronic pain in all patients. However, gabapentin is a common addition to analgesic protocols and, based on its mechanism of action, effectiveness can be anticipated if the dose and diagnosis (that the source of pain is actually neuropathic pain) are correct. What Are the Adverse Effects of Gabapentin? The primary side effect (not necessarily an adverse effect!) of gabapentin therapy in dogs and cats is sedation. This side effect is generally dose-related and usually alleviated by decreasing the dose. If sedation occurs and the dose is reduced, sedation can often be avoided if the dose is slowly titrated back up to its original high/therapeutic level over several weeks. Sedation is more common in patients receiving other sedating drugs (e.g., tramadol). Dose-dependent ataxia and weakness can occur, especially in older patients with decreased muscle strength. In rats and humans, gabapentin is primarily cleared by the kidneys; in dogs, it is also metabolized by the liver. 20 In cats, the route of clearance is unknown but is presumed to be primarily renal. Thus, gabapentin may undergo more rapid accumulation, and thus more rapid onset of adverse effects, in cats and dogs with renal disease and dogs with hepatic disease. For these patients, a lower starting dose and slower dose escalation is recommended. How Is Gabapentin Supplied and What Does It Cost? Gabapentin is available as a generic drug in tablets and capsule of various strengths. It is also available as a liquid, but some liquid formulations contain a low concentration of xylitol. The liquid can be compounded without xylitol. Gabapentin is not a DEA-controlled drug and is not expensive. Are There Any Other Drugs in This Class (Anticonvulsants) That Are Used for Pain Relief? Pregabalin is FDA-approved for the treatment of numerous neuropathic pain syndromes in humans. Dosing at 1 to 2 mg/kg PO q12h is supported by pharmacokinetic studies in dogs 21 and cats. 22 Pregabalin is DEA class V and fairly expensive. NOTE: Gabapentin is also commonly used for behavior modification, especially in cats, before stressful events such as trips to a veterinary clinic. 23 The dosage for this indication is 50 to 150 mg/cat PO at least 2 hours before the scheduled stressor will occur. In very anxious or fractious cats, the same dose is often administered the night before the stressor will occur. AMANTADINE What Is Amantadine? Amantadine is a very interesting drug with multiple uses stemming from varied mechanisms of action. Amantadine is FDA-approved for the treatment of influenza virus A infection and Parkinson's disease in humans. The mechanisms of action by which amantadine treats those 2 conditions differ, and the mechanism by which it contributes to analgesia differs yet again. In its role in pain management, amantadine antagonizes N-methyl-D-aspartate (NMDA) receptors

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