Antidepressants and Anxiolytics

Chapter 26

Antidepressants and Anxiolytics

Prescription antidepressants and anxiolytic drugs routinely rank among the most commonly prescribed agents in the United States. Additionally, they are commonly used in veterinary medicine for a variety of behavioral disorders including separation anxiety, storm phobias, inappropriate urine marking, stereotypic behaviors, and psychogenic alopecia (see Chapter 117). Although mild adverse effects may be noted at therapeutic doses, severe toxicosis and death may result following overdose, especially if these drugs are ingested with other drugs with serotonergic properties (such as monoamine oxidase inhibitors or 5-hydroxytryptophan). Because of their frequent use, the palatability of some flavored veterinary formulations, and the potential for severe intoxication, unintentional overdoses of antidepressants rank among the most commonly reported cases to Pet Poison Helpline.

Antidepressants and anxiolytics encompass several drug classes, the most common of which include selective serotonin reuptake inhibitors (SSRIs), serotonin and norepinephrine reuptake inhibitors (SNRIs), tricyclic antidepressants (TCAs), benzodiazepines (BZDs), and nonbenzodiazepine (non-BZD) hypnotic agents. The pharmacologic and pharmacokinetic properties of these different classes vary greatly and account for a wide range of toxicities and mechanisms of action. Although some drugs, such as many SSRIs or TCAs, may cause severe intoxication in smaller dosages, others, such as BZDs, have a wider margin of safety and are less likely to result in severe toxicosis or death. Thus obtaining the exact name of the medication ingested by the pet is crucial to determine a proper course of treatment and guide prognosis. Due to the wide variability in clinical signs and treatments available for these drugs, along with the potential for severe intoxication, consultation with an animal poison control center is recommended (see Chapters 20 and Web Chapter 9).

Considerations for Decontamination

Appropriate decontamination procedures are paramount to successful treatment for most poisonings. Because many antidepressants and anxiolytics are rapidly absorbed, resulting in central nervous system (CNS) depression within 15 to 30 minutes of ingestion, decontamination must be judicious. For agents discussed in this chapter, the induction of emesis at home is not always advisable due to the increased risk of aspiration secondary to CNS depression. Additionally, emesis should never be induced in a symptomatic animal. Often, decontamination is most safely performed in a veterinary setting. Therefore educating receptionists, technicians, and other “front-line” agents about contraindications to emesis induction is imperative.

In cases of very recent ingestion (less than 5 minutes), the induction of emesis may be attempted at home in dogs (not cats) by administering fresh hydrogen peroxide, 3% (1 to 5 ml/kg, PO). Pet Poison Helpline typically recommends administering 1 ml/kg as the first dosage. If the dog has not vomited within 5 to 10 minutes and remains asymptomatic, a second dosage of 2 ml/kg may be administered. Offering a small amount of food prior to the administration of hydrogen peroxide may increase its effectiveness. Unfortunately, there are currently no safe and effective at-home emetic agents for cats. Products such as table salt, mustard, and syrup of ipecac are no longer recommended in any veterinary species.

If emesis cannot be safely induced in the dog at home, apomorphine (0.03 to 0.04 mg/kg) should be administered IV, IM, or by placing the tablet directly into the subconjunctival sac. If subconjunctival apomorphine is used, the subconjunctival sac must be flushed thoroughly after emesis or protracted vomiting may occur. Apomorphine use in cats is not recommended due to poor efficacy and the potential for CNS stimulation. Instead, xylazine (0.44 mg/kg, IM) may be administered. Reversal with yohimbine (0.1 mg/kg, IM, SC, or slowly IV) or atipamezole (Antisedan, 25 to 50 µg/kg, IM or IV) should be performed if severe CNS and/or respiratory depression develop from this treatment.

If the patient is symptomatic, the induction of emesis is contraindicated, but gastric lavage may still be effective provided the ingestion was recent (<60 minutes), a large number of pills were ingested and are likely located within the stomach, or the ingested drug results in delayed gastric emptying (e.g., TCAs).

Activated charcoal (1 to 5 g/kg, PO) with a cathartic such as sorbitol also may be administered in effort to adsorb the toxicants. In order to reduce the risk of aspiration, administration via an orogastric tube is advised in any symptomatic patient. Magnesium-based cathartics are not typically recommended due to the risk that ileus may result with subsequent hypermagnesemia (e.g., in cases of TCA ingestion). If the ingested toxicant undergoes enterohepatic recirculation (e.g., TCAs) or is a sustained-release formulation, administration of activated charcoal up to 6 hours after ingestion may still be beneficial. Additionally, multiple doses of activated charcoal (1 to 2 g/kg without a cathartic q4-6h for 24 hours) may be administered.

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Jul 18, 2016 | Posted by in PHARMACOLOGY, TOXICOLOGY & THERAPEUTICS | Comments Off on Antidepressants and Anxiolytics
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