Today's Veterinary Practice

MAR-APR 2018

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19 MARCH/APRIL 2018 ● TVPJOURNAL.COM ESSENTIALS Pharmacological influences Numerous therapeutic drugs can impact appetite, most often by triggering feelings of nausea (eg, various antibiotics) or by causing ileus or delayed gastric emptying (opioids). If a patient is hyporexic, it can be very helpful to look critically at its current medication list and see if anything can be discontinued or switched. In people, opioid-induced constipation is a major issue. We do not tend to pay that much attention to our patients' stools (or lack thereof!) unless we note diarrhea, but I believe that iatrogenic constipation can affect comfort and appetite. Drugs are a common cause, but lack of activity and reluctance to defecate in a strange environment also play a role. Physiological influences This category includes sensations of nausea, gastric discomfort, gastroesophageal reflux and constipation. All of these processes are likely to impact food intake, and effective management may make a substantial difference to appetite. Maropitant (1 mg/kg SQ q24h; 2 mg/kg PO q24h) is my first choice for nausea, as it is well tolerated and may reduce visceral pain. Persistent gastric ileus may be mitigated with a prokinetic agent; metoclopramide has some effect on gastric emptying, but erythromycin (1 mg/kg IV or PO q8-12h) is generally superior. As our patients cannot describe their symptoms, it can be hard to identify gastric discomfort or gastroesophageal reflux. Lip smacking and exaggerated swallowing motions are suggestive but are usually associated with severe compromise and may not be noted in animals with milder changes. Although there is appropriate concern about overuse of proton pump inhibitors in critically ill animals, a short trial with pantoprazole (1 mg/kg IV q24h) or omeprazole (0.7 mg/kg PO q12h) may be worthwhile. If imaging or physical examination indicates fecal retention, a glycerin enema (3-5 mls) may trigger defecation, improve patient comfort and increase food intake. There is a strong neurological relationship between the stomach and colon, and studies in children have shown that constipation can suppress appetite. PHARMACOLOGIC INTERVENTIONS FOR INAPPETENCE Although numerous drugs have been used off-label for appetite stimulation in both dogs and cats, their effects are unpredictable and often disappointing. However, the ghrelin agonist class of drugs shows tremendous promise in this regard and are used for this purpose in people. The FDA has recently approved a ghrelin receptor agonist for use as an appetite stimulant in dogs (capromorelin; Entyce®). A feline-specific version is currently in development. We do not have time to discuss all the agents used to stimulate food intake, so the list below covers only the most widely used. Mirtazapine This is a noradrenergic/serotonergic agent and is licensed for use in people as an antidepressant. It stimulates 5-HT1 receptors and strongly antagonizes 5-HT2 and 5-HT3 receptors. It has been shown to be a potent appetite stimulant in some cats. The therapeutic window is small, and overdose can result in hyperexcitability, crying and tremors. It should not be combined with cyproheptadine (see below). Based on pharmacokinetic studies, it can be given daily in cats, although many do well with 2 mg PO or transdermally q2-3d. In addition to promoting food intake, mirtazapine may also decrease nausea and have a modest prokinetic effect. Cyproheptadine This is an antihistamine drug with antiserotonergic effects. Increased food intake is a recognized side effect, likely through antagonism if 5-HT receptors in the hypothalamus. I have found it to be less predictable than mirtazapine with respect to appetite stimulation, and it must be given twice daily. It can take a few days to see an effect, and the drug should be tapered rather than discontinued abruptly. Side effects can be worrisome, and include CNS depression, paradoxical excitement, anticholinergic effects and a lowering of the seizure threshold. Diazepam This is a benzodiazepine and enhances GABA neuro-transmission. It was routinely used in cats for appetite stimulation until an association between oral diazepam and acute hepatotoxicity emerged. A single IV dose may be considered, in an effort to 'kick start' intake but long-term use in cats is unwise. The effect in dogs is often less dramatic.

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