Anticonvulsants for Emergency Treatment of Seizures in Foals and Horses
Diazepam for Emergency Treatment of Seizures in Foals and Horses
Diazepam is the most commonly used drug to treat seizures and status epilepticus in horses.
For specific therapeutic recommendations, including dosages, refer to the relevant disease chapter.
The elimination half-life of this benzodiazepine in horses is long (7–22 hours).
Horses with seizures induced by toxins or adverse effects of drugs (eg, xylazine) can be administered diazepam as the first-line treatment.
Diazepam should not be left in a plastic container or in syringes, because it becomes inactivated after a few minutes. It is also inactivated by exposure to light.
To prevent further seizures after initial diazepam injection, a loading dose of phenobarbital can be started as a follow-up and then continued at a maintenance dosage. If sedation occurs, the dose should be decreased.
Midazolam for Emergency Treatment of Seizures in Foals and Horses
In adult horses, midazolam (combined with ketamine and xylazine) has been used as a sedative; it might also be considered for emergency seizure treatment.
The half-life of midazolam in horses is 2–5.8 hours at 0.05 mg/kg, IV, and 3.2–15 hours at 0.1 mg/kg, IV, with a plasma clearance of 10 mL/kg/minute.
Midazolam has also been used as a first-line treatment for neonatal seizures.
Phenytoin for Emergency Treatment of Seizures in Foals and Horses
Phenytoin has been used more often in horses than in small animals, not only for its anticonvulsant effect but also for ventricular arrhythmias, myotonia, rhabdomyolysis, hyperkalemic periodic paralysis, and stringhalt. Because oral absorption of this drug is highly variable in horses, maintaining a consistent plasma concentration is difficult.
At high doses of phenytoin, horses might develop adverse effects such as recumbency and excitement. Sedation can occur initially at high plasma concentrations, and monitoring the plasma concentration can prevent adverse effects.
Effective plasma concentrations of phenytoin are 5–20 mcg/mL (mean 8.8 mcg/mL).
For specific therapeutic recommendations, including dosages, refer to the relevant disease chapter.
Other Drugs for Emergency Treatment of Seizures in Foals and Horses
If injectable anticonvulsants are not available, standard doses of alpha-2-agonist drugs have been suggested as an alternative.
Other reported anecdotal ways to control ongoing seizure activity include the administration of sodium pentobarbital, a mixture of 12% chloral hydrate and 6% magnesium sulfate, or a mixture of 5% guaifenesin and thiamylal. However, no controlled studies establishing the efficacy and safety of these treatments have been reported.
If these treatments are not effective, general anesthesia must be considered. At this stage, however, euthanasia should also be considered, given the poor prognosis.
If cerebral edema is suspected, antiedemic drugs should be considered.
Drugs for Emergency Treatment of Seizures in Equine Neonates
Diazepam
Diazepam has been used routinely to treat neonatal seizures in foals. High doses can be fatal to neonates.
Midazolam
Midazolam has been administered in foals to treat neonatal seizures and can be considered as a first-line treatment with an initial IV bolus followed by CRI.
Phenytoin
As an alternative to other anticonvulsants, phenytoin can be considered for initial treatment of seizures in foals. However, erratic plasma concentrations can limit the usefulness of phenytoin. The dose might need to be decreased if sedation occurs.
Anticonvulsants for Emergency Treatment of Seizures in Ruminants
Published data on seizure management in ruminants are scarce and sometimes outdated.
Diazepam can be administered intravenously and, theoretically, intrarectally in sheep and goats. However, no study has evaluated the intrarectal route of administration.
In sheep and goats, pentobarbital has been reported to induce anesthesia. The duration of effect is 5–30 minutes.
Midazolam has been administered intravenously or intramuscularly in sheep (1).
For specific therapeutic recommendations, including dosages, refer to the relevant disease chapter.
Anticonvulsants for Emergency Treatment of Seizures in Swine
Midazolam has been administered in pigs, mainly for sedative effect.
Anticonvulsants for Emergency Treatment of Seizures in Exotic Mammals
One study used guinea pigs as animal models to evaluate the use of midazolam to treat organophosphate-induced seizures. Treatment with midazolam was effective if administered within 10 minutes after onset of a seizure. A higher dose was found efficacious after 30 minutes (2).
In contrast, nasal administration of midazolam was found to be most effective at 10 and 30 minutes after seizure onset. Nasal midazolam should be further evaluated to treat naturally occurring seizure activity in guinea pigs.
For specific therapeutic recommendations, including dosages, refer to the relevant disease chapter.
Anticonvulsants for Emergency Treatment of Seizures in Birds
For specific therapeutic recommendations, including dosages, refer to the relevant disease chapter.
As in other species, in avian species benzodiazepines (diazepam or midazolam) are the first-line treatment to stop seizures in emergency situations.
Diazepam can be administered intravenously or intracloacally; midazolam can be administered intramuscularly, intravenously, intracloacally, or intranasally. The dose can be repeated every 2 minutes via the same route up to 3 times if seizures continue. Intravenous or intraosseous routes of administration are preferred, and a catheter should be placed to enable additional drug administration. If seizure activity continues, a CRI of diazepam can be started.
Phenobarbital can be administered as a CRI alone or with a diazepam CRI if there is ongoing seizure activity.
Currently, longterm treatment of idiopathic seizures in birds involves the use of phenobarbital or potassium bromide. Few studies or case reports have evaluated the use of newer antiseizure drugs in birds, and reported doses have been extrapolated from dogs. None of these drugs have been studied specifically for their effects in avian species.
Anticonvulsants for Emergency Treatment of Seizures in Reptiles
One article reported the use of diazepam and propofol to treat status epilepticus associated with marijuana toxicosis in three green iguanas (Iguana iguana). Diazepam was administered intravenously via the coccygeal vein in boluses given 15 minutes apart for up to 3 doses. One iguana, which continued to seize despite receiving three diazepam doses, was given propofol intravenously via the coccygeal vein to induce anesthesia, and anesthesia was maintained with isoflurane delivered in oxygen (3).
References
Simon BT, Scallan EM, O O, et al. Pharmacokinetics and pharmacodynamics of midazolam following intravenous and intramuscular administration to sheep. Am J Vet Res. 2017;78(5):539-549. doi:10.2460/ajvr.78.5.539
Reddy SD, Reddy DS. Midazolam as an anticonvulsant antidote for organophosphate intoxication—a pharmacotherapeutic appraisal. Epilepsia. 2015;56(6):813-821. doi:10.1111/epi.12989
Girling SJ, Fraser MA. Cannabis intoxication in three Green iguanas (Iguana iguana). J Small Anim Pract. 2011;52(2):113-116. doi:10.1111/j.1748-5827.2010.01017.x
