Abstracts

Nephrolithiasis in association with topiramate therapy is a well known but an uncommon association. Its incidence is 2-4 times that expected in the background population and thought to be due to topiramate[apos]s carbonic anhydrase inhibiting effect which predisposes to calcium nephrolithiasis. Maintaining adequate hydration decreases its occurrence.
We assessed the incidence of nephrolithiasis with topiramate therapy at Hattie Larlham Center, a foundation that institutionalizes Ohio[apos]s children and young adults with severe mental retardation and developmental disabilities. A retrospective chart review was performed. Recorded parameters included sex, age, duration and dosage of topiramate, concomitant medication, radiological studies, calcium and fluid intake and treatment. Of the 126 individuals (ages 4-45 years) housed at Hattie Larlham, 26 (20%) were treated with topiramate. Of these 26 individuals, 9 (35%) developed grit in the urine. Age at onset of stones ranged from 11-29 years. Duration of topiramate therapy before onset of side effect averaged 29 months (range 1-84 months). Topiramate dosage varied from 150-900 mg/day. Four of 9 patients were on topiramate monotherapy. With the exception of one individual who was on an oral diet, patients had a fixed fluid intake of 32-58 cc/kg/day. Total daily calcium intake including diet and supplementation varied between 1500-2100 mg (average 1746 mg).
All patients were noted to have stone fragments in urine. One of 9 had a horseshoe kidney without evidence of urinary outflow obstruction. Of those stones that were analyzed, all were 90% calcium phosphate. One patient redeveloped stones 3 years later on zonisamide monotherapy. Stone composition then showed 60% struvite and 49% calcium phosphate and oxalate.
Seven of 9 patients were weaned off topiramate and 3 of 9 were treated with citrate supplementation. Incidence of nephrolithiasis with topiramate in the sedentary institutionalized patients may be higher than the 1.5% risk reported in the literature. This may in part be due to increased calcium supplementation used to help prevent bone loss. Inadequate hydration was not an issue in our patients since the majority had fixed fluid intake.
Prospective studies with urinary chemistries including calcium on topiramate therapy in this population will help substantiate our observations.