Recurrent Neurologic Signs After Hemodialysis for Phenobarbital and Zonisamide Intoxication in a Dog.

Dogs treated with hemodialysis for antiepileptic drug toxicosis may experience rebound neurologic signs due to drug redistribution, necessitating extended post-treatment monitoring.

This case report describes a 6-year-old neutered female Australian Shepherd presenting in a comatose state following ingestion of a toxic dose of phenobarbital and zonisamide. Intermittent hemodialysis was performed without complications, resulting in immediate normalization of mentation post-treatment. However, over the subsequent 10 hours, the dog's neurologic status progressively declined, returning to a comatose state. A second hemodialysis session was performed, after which neurologic status again normalized with no further recurrence of signs. The dog was discharged with ongoing management for previously diagnosed idiopathic epilepsy. Therapeutic drug monitoring throughout both treatment sessions revealed significant redistribution of both phenobarbital and zonisamide from peripheral tissue compartments back into the systemic circulation, explaining the rebound neurologic deterioration. This redistribution phenomenon — well recognized in human toxicology (e.g., digoxin, lithium) but previously undocumented in veterinary extracorporeal therapy — underscores the importance of prolonged post-dialysis monitoring in veterinary patients treated for lipophilic or highly protein-bound drug toxicoses. This report is also the first to document hemodialysis as an effective treatment for zonisamide toxicosis in any species. Clinicians utilizing extracorporeal removal techniques for antiepileptic drug toxicosis should be aware of the potential for rebound toxicity and plan monitoring protocols accordingly.

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