Continued Use of Zonisamide Following Development of Renal Calculi

The reported incidence of renal calculi complicating zonisamide (ZNS) therapy for epilepsy ranges from 0.2 to 4.4%.1–3 Typically this complication leads to discontinuation of the drug. For patients uniquely responsive to zonisamide, the benefits of continued treatment may outweigh the risk of calculus recurrence. We report three patients who developed nephrolithiasis but continued ZNS treatment without recurrence of calculi. A 43-year-old man with symptomatic generalized epilepsy, mental retardation, and cerebral palsy since infancy took ZNS 900 mg/day, added to ongoing therapy with valproate and diazepam. His seizures decreased from four to five per week to rare events. One year later he developed flank pain, dysuria, and gross hematuria. An abdominal CT showed multiple calculi in the left kidney and ureter. His pain resolved with hydration, but discontinuation of ZNS led to recurrence of frequent seizures. ZNS was restarted after alternative anticonvulsants failed, with calcitrate to decrease risk of stone recurrence. Two renal ultrasounds 3 and 29 …