Use of Zonisamide in Cats
Zonisamide has been recommended for use in cats and dogs with seizures for almost 20 years despite a lack of efficacy studies. Not that this is surprising -we love to extrapolate from human and canine medicine to cats – but it was frustrating to say the least. Well, that has been improved recently after a study evaluating the effectiveness of add-on zonisamide OR single agent zonisamide in a population of cats with seizures. (https://doi.org/10.1111/jvim.16984). This was a retrospective study of 57 cats, with seizures of any etiology that were prescribed zonisamide as part of their treatment plan. Of this group, 16/57 were diagnosed with idiopathic epilepsy (Tier II level confidence), 4/57 neoplasia, 4/57 had congenital brain disease, 2/57 head trauma and 1/57 with metabolic encephalopathy. The remainder (30 cats) did not under go diagnostic testing to obtain a diagnosis and were treated for presumptive disease based on the clinician’s assessment.
Results
Prior to treatment, the majority of cats were reported to have generalized seizures with both focal and generalized seizures taking second place. Focal only seizures were a distant third place with only 12 cats. The cats in this study demonstrated a significant reduction in seizure days (number of days having a seizure) and seizures (total count) per month following administration of zonisamide. Within the confirmed idiopathic epilepsy group, this group had a reduction in seizures/month of 1, with a total of 69.2% of cats with idiopathic epilepsy responding to zonisamide treatment. This equates to 9 of 13 cats for whom they had efficacy data. Add-on zonisamide was noted in 33 cats for whom 56% responded to treatment. Lastly, for cats whom zonisamide was the only AED, a 70% response rate was noted. A few cats had an increase in seizures after starting zonisamide. This may have been related to disease progression or resistant epilepsy. The retrospective nature, along with the lack of diagnostic testing in all cats, makes assumptions difficult.
The median daily dose of zonisamide was 7.55 mg/kg (range 3.8-17.7 mg/kg) and it was given q12h most commonly (36/57 cats). This drug can be given q12 or q24h based on the PK available.
Adverse effects were noted in 15/57 (26%) which is relatively low compared to the reported adverse effects for, say, phenobarbital. The adverse effects included inappetence (n=10), transient sedation (n=6), ataxia (n=4), vomiting (n=3). The adverse effects appeared dose related therefore higher doses resulted in more frequent adverse effects. The adverse effects lasted for variable amounts of time but typically 2-4 weeks. Mild changes to CBC were noted but significance was unknown due to concurrent disease. Liver enzyme elevation including ALT, ALP and ASK was noted but increases were mild.
What’s the take home message?
Zonisamide appears to control 56- 70% of seizures for cats, regardless of etiology. This number is closer to 70% for idiopathic epileptic cats.
Zonisamide has a low incidence of adverse effects, however the nausea/vomiting/anorexia adverse effects should be closely monitored.
Zonisamide is metabolized through the liver and is a sulfa derivative so use caution in patients with a known sensitivity.
I hope you enjoyed this week’s TidBit and will join me in celebrating the publication of solid data about zonisamide! I hope you have a great week and look forward to working with you soon.