Influence of BMI in Nephrolithiasis in an Appalachian Pediatric Population: A Single Center Experience
Margaret O. Murphy, PhD, RD, LD, Scott Erpelding, MD, Aftab S. Chishti, MD, Ali Ziada, MD, Stefan G. Kiessling, MD.
University of Kentucky Healthcare, Lexington, KY, USA.
BACKGROUND: The prevalence of pediatric nephrolithiasis is increasing in addition to pediatric obesity particularly in Appalachia. Abnormalities predisposing individuals to nephrolithiasis in adult patients include increased urinary sodium, uric acid excretion and low urine pH; however, limited data are available in the pediatric population. We investigated whether obese pediatric patients presenting with nephrolithiasis have a metabolic profile similar to reported findings in obese adults.
METHODS: A retrospective chart review was performed in children aged 1-18 seen in the Pediatric Kidney Stone Disease Clinic at Kentucky Children's Hospital between 2010 and 2016. Inclusion criteria included all patients with documented stones confirmed via ultrasound or computed tomography techniques and undergoing surgical or medical intervention.
RESULTS: Retrospective chart review identified 113 patient charts which were reviewed in the study with a mean age of 11.9 years. 72 patients (64%) had a normal BMI and 41 patients (37%) were considered overweight/obese. Normal BMI includes a BMI below the 85th percentile on the CDC growth chart while overweight is classified as a BMI between 85th-95th percentiles and obese is greater than 95th percentile. 54 patients underwent surgical intervention with an average of 2.6 procedures and 1.1 stone episodes. Stone analysis was available in 40 of 113 patients (36%). 28 children had calcium-oxalate-based stones, 7 had carbonite apatite-based stones, 3 had cystine-based stones, 1 had ammonia hydrogen urate-based stone and 1 was matrix stone. In obese patients, 65% presented with CaOx stones and 35% carbonite apatite-based stones while 74% patients with normal BMI presented with CaOx stones, 5% with carbonite apatite-based stones, and 13% with cystine. Recurrence rates of nephrolithiasis, defined as new kidney stone on ultrasound were found in 33 patients (30%). 40 patients (36%) were treated chronically with medication; 65% were prescribed potassium citrate, 13% thiazide diuretics, and 23% received a combination of potassium citrate and thiazide diuretics due to documented hypercalciuria. 74 patients (66%) underwent 24-hour urine collection through Litholink laboratories. The mean urine volume was 1.20 ± 0.09 L with no differences found between normal (n=43) and obese patients (n=31). Obese patients had significantly elevated urinary sodium, uric acid, phosphate, urea nitrogen, sulfate, chloride, ammonia, and low urine pH. Interestingly, obese patients have a significantly lower Protein Catabolic Rate while patients with normal BMI had significant hypercalciuria (4.1±0.4 vs 2.8 ± 0.3, p<0.05). No differences were found between obese and normal BMI patients in urine volume, supersaturation CaOx , or citrate. CONCLUSIONS: We identified a high rate of carbonite apatite-based stones in obese patients and found differences in several urinary mineral excretion rates including sodium, uric acid, phosphate, sulfur, ammonia, and lower pH. Additionally, obese patients have a significantly lower PCR suggestive of an inadequate daily protein intake. Our study demonstrates differences in types of stones and urinary metabolites in an obese pediatric population suggestive of different metabolic profiles contributing to pediatric stone disease. Dietary management could play a role in pediatric obese stone formers particularly in therapies to lower urinary sodium, uric acid while increasing citrate, PCR, and urine pH.
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