Zonisamide add-on for drug-resistant partial epilepsy

Background The majority of people with epilepsy have a good prognosis and their seizures can be well controlled with the use of a single antiepileptic agent, but up to 30% develop refractory epilepsy, especially those with partial seizures. In this review we summarise the current evidence regarding zonisamide, when used as an add-on treatment for drug-resistant partial epilepsy. Objectives To evaluate the efficacy and tolerability of zonisamide when used as an add-on treatment for people with drug-resistant partial epilepsy. Search methods We searched the Cochrane Epilepsy Group Specialized Register (12 February 2013), the Cochrane Central Register of Controlled Trials (The Cochrane Library 2013, Issue 1) (January 2013), MEDLINE (Ovid, 1946 to 12 February 2013), SCOPUS (13 February 2013), ClinicalTrials.gov (12 February 2013) and the WHO International Clinical Trials Registry Platform ICTRP (13 February 2013). In addition, we contacted Eisai Limited (makers and licensees of zonisamide) and experts in the field to seek any ongoing/unpublished studies. Selection criteria Randomised, placebo-controlled, add-on trials of zonisamide in people with drug-resistant partial epilepsy. Data collection and analysis Two review authors independently selected trials for inclusion and extracted data. Outcomes were: (1) 50% or greater reduction in total seizure frequency; (2) treatment withdrawal; (3) adverse effects. Primary analyses were intention-to-treat. We estimated summary risk ratios (RRs) for each outcome. All studies were assessed for risk of bias using the Cochrane risk of bias tool and the quality of evidence was assessed using the GRADE approach and presented in a summary of findings table. Main results Five trials (949 participants) were included. The overall RR with 95% confidence interval (CI) for 50% reduction in seizure frequency compared to placebo for 300 to 500 mg/day of zonisamide was 2.00 (95% CI 1.58 to 2.54). The RR for 50% reduction in seizure frequency compared to placebo for any dose of zonisamide (100 to 500 mg per day) was 1.92 (95% CI 1.52 to 2.42). The number needed to treat (NNT) was 6 for this outcome. Two trials provide evidence of a dose response relationship for this outcome. The RR for treatment withdrawal for 300 to 500 mg/day of zonisamide compared to placebo was 1.64 (95% CI 1.20 to 2.25) and for 100 to 500 mg per day was 1.47 (95% CI 1.07 to 2.01). NNT for this outcome was 21. The CIs of the following adverse effects indicate that they are significantly associated with zonisamide: ataxia 3.77 (99% CI 1.28 to 11.11); somnolence 1.83 (99% CI 1.08 to 3.11); agitation 2.35 (99% CI 1.05 to 5.27) and anorexia 2.71 (99% CI 1.29 to 5.69). Across the 5 studies, risk of bias domains were rated as low is bias or unclear. None of the evidence for outcomes was downgraded for quality. Authors' conclusions Zonisamide has efficacy as an add-on treatment in people with drug-resistant partial epilepsy. In this review minimum effective and maximum tolerated doses cannot be identified. The trials reviewed were of a maximum stable-dose phase of 18 weeks in duration and results cannot be used to confirm longer periods of effectiveness in seizure control. The results cannot be extrapolated to monotherapy or to people with other seizure types or epilepsy syndromes.