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

OBJECTIVE: To report the occurrence of rebound phenomenon in a dog with phenobarbital and zonisamide toxicosis treated with intermittent hemodialysis. CASE SUMMARY: A 6-year-old neutered female Australian Shepherd was presented after ingesting a toxic dose of phenobarbital and zonisamide, resulting in a comatose state. Intermittent hemodialysis was performed to remove both phenobarbital and zonisamide, without any complications. Immediately following treatment, the dog's mentation normalized. Over the next 10 h, the patient's neurologic status declined and it became comatose once again. Based on this clinical change, it was elected for the patient to undergo a second dialysis treatment, after which neurologic status normalized. Subsequently, there was no return of clinical signs, and the dog was discharged into home care with further therapy for previously diagnosed idiopathic epilepsy. Therapeutic drug monitoring obtained throughout the treatments demonstrated significant redistribution of both phenobarbital and zonisamide, resulting in the observed recurrence of obtundation. NEW OR UNIQUE INFORMATION: This is the first report documenting the occurrence of redistribution phenomenon in a patient undergoing extracorporeal removal of toxins. In addition, this is the first case report documenting the use of hemodialysis to treat zonisamide toxicosis.