Pathogenesis of bladder calculi in the presence of urinary stasis

Purpose: Although minimal evidence exists, bladder calculi in men with benign prostatic hyperplasia are thought to be secondary to bladder outlet obstruction induced urinary stasis. We performed a prospective, multi-institutional clinical trial to determine whether metabolic differences were present in men with and without bladder calculi undergoing surgical intervention for benign prostatic hyperplasia induced bladder outlet obstruction. Materials and Methods: Men who elected surgery for bladder outlet obstruction secondary to benign prostatic hyperplasia with and without bladder calculi were assessed prospectively and compared. Men without bladder calculi retained more than 150 ml urine post-void residual urine. Medical history, serum electrolytes and 24-hour urinary metabolic studies were compared. Results: Of the men 27 had bladder calculi and 30 did not. Bladder calculi were associated with previous renal stone disease in 36.7% of patients (11 of 30) vs 4% (2 of 27) and gout was associated in 13.3% (4 of 30) vs 0% (0 of 27) (p <0.01 and 0.05, respectively). There was no observed difference in the history of other medical conditions or in serum electrolytes. Bladder calculi were associated with lower 24-hour urinary pH (median 5.9 vs 6.4, p = 0.02), lower 24-hour urinary magnesium (median 106 vs 167 mmol, p = 0.01) and increased 24-hour urinary uric acid supersaturation (median 2.2 vs 0.6, p <0.01). Conclusions: In this comparative prospective analysis patients with bladder outlet obstruction and benign prostatic hyperplasia with bladder calculi were more likely to have a renal stone disease history, low urinary pH, low urinary magnesium and increased urinary uric acid supersaturation. These findings suggest that, like the pathogenesis of nephrolithiasis, the pathogenesis of bladder calculi is likely complex with multiple contributing lithogenic factors, including metabolic abnormalities and not just urinary stasis.