Seizures are a common reason for me to evaluate cats and therefore (I assume) something you encounter frequently. Seizures originate from the forebrain (prosencephalon) and can be secondary to idiopathic epilepsy, structural epilepsy (such as congenital disease, neoplasia, meningoencephalitis, cerebrovascular events, head trauma, and on goes the list), or metabolic seizures (hypoglycemia, thiamine deficiency, etc.).
The mainstay treatment for cats has been phenobarbital for many years and not without good reason. Phenobarbital controls seizures in over 90% of cats, regardless of the etiology, and has predictable clinical side effects. When cats cannot tolerate phenobarbital, or have seizures in excess while on phenobarbital, other antiepileptic drugs (AED) are added; one of those is zonisamide. Very little data has been published to date about zonisamide use in cats. A recent study out of UW (go Bucky) was published recently. I thought it was worth a quick summary, TT style.
How many cats were enrolled in this retrospective, multicenter study? 57
The median age of seizure onset was 7 years (range 0.17-22) and the median age of onset for cats diagnosed with idiopathic epilepsy was 8 years. Note the difference between cats and dogs! Dogs have a lower median age at onset! Importantly, note that 30 cats did not have advanced diagnostics (52%) which means a final diagnosis was not reached and therefore their disease may have affected their response to zonisamide (but we don’t know!).
How did the cats on zonisamide respond to treatment?
There was a significant decrease in seizures/month AND seizures/day after starting zonisamide. Note: we don’t know how long the seizures were monitored before starting zonisamide.
70% of cats responded to zonisamide monotherapy (had less than 1 seizure per 3 months)
56% of cats responded to zonisamide as add-on therapy
Almost 70% of cats diagnosed with idiopathic epilepsy responded to zonisamide but it isn’t clear if these cats were on monotherapy or add-on therapy.
The median dose was 7.5 mg/kg/day with about 1/3 of the cats receiving the drug once daily and 2/3 receiving it twice daily.
More cats obtained seizure control on twice daily dosing than once daily dosing but the side effects were more profound on twice daily dosing.
Side effects were noted in 15/57 cats and they included inappetence (10 cats), sedation, ataxia and vomiting for most reports. The duration of the side effects was up to 4 weeks after starting the medication. This is surprising and warrants further investigation. Anorexia has been the side effect I note more commonly but
No clinically significant CBC or biochemistry changes were noted on the cats in this study
So what’s the take home message? Would I use zonisamide in a cat? A qualified, yes. Phenobarbital still has a better reported seizure control compared to this cohort of cats on zonisamide. BUT it is worth a try if the cat fails treatment with phenobarbital and the don’t have a history of a sulfa drug reaction (zonisamide is a sulfa derivative). Starting dose should be less than 10 mg/kg daily, and side effects appear dose dependent…but last awhile!
Thanks for reading this week’s, TidBit Tuesday! I hope you learned a little something – I know I did! Keep those consults rolling. Have a great week and stay warm and safe out there.
Reference: https://doi.org/10.1111/jvim.16984