Horseshoe kidneys are a complex anatomic variant of fused kidneys, with a 20% reported incidence of associated calculi. Anatomic causes such as high insertion of the ureter on the renal pelvis and obstruction of the ureteropelvic junction are thought to contribute to stone formation via impaired drainage, with urinary stasis, and an increased incidence of infection. In this multi-institutional study, we evaluated whether metabolic factors contributed to stone development in patients with horseshoe kidneys. A retrospective review of 37 patients with horseshoe kidneys was performed to determine if these patients had metabolic derangements that might have contributed to calculus formation. Stone compositions as well as 24-hour urine collections were examined. Specific data points of interest were total urine volume; urine pH; urine concentrations of calcium, sodium, uric acid, oxalate, and citrate; and number of abnormalities per patient per 24-hour urine collection. These data were compared with those of a group of 13 patients with stones in caliceal diverticula as well as 24 age-, race-, and sex-matched controls with stones in anatomically normal kidneys. Eleven (9 men and 2 women) of the 37 patients (30%) with renal calculi in horseshoe kidneys had complete metabolic evaluations available for review. All patients were noted to have at least one abnormality, with an average of 2.68 abnormalities per 24-hour urine collection (range 1-4). One patient had primary hyperparathyroidism and underwent a parathyroidectomy. Low urine volumes were noted in eight patients on at least one of the two specimens (range 350-1640 mL/day). Hypercalciuria, hyperoxaluria, hyperuricosuria, and hypocitraturia were noted in seven, three, six, and six patients, respectively. No patients were found to have gouty diathesis or developed cystine stones. Comparative metabolic analyses of patients with renal calculi in caliceal diverticula or normal kidneys revealed a distinct profile in patients with horseshoe kidneys, with a higher incidence of hypocitraturia. All patients with renal calculi in horseshoe kidneys were noted to have metabolic abnormalities predisposing to stone formation. In this initial series of 11 patients, hypovolemia, hypercalcuria and hypocitraturia were most common metabolic defects. These findings suggest that metabolic derangements play a role in stone formation in patients with a horseshoe kidney. Patients with calculi in anatomically abnormal kidneys should be considered for a metabolic evaluation to identify their stone-forming risk factors in order to initiate preventative selective medical therapy and reduce the risk of recurrent calculus formation.
