Nephrolithiasis after bariatric surgery for obesity

Abstract

Surgical intervention has become an accepted therapeutic alternative for the patient with medically complicated obesity. Multiple investigators have reported significant and sustained weight loss after bariatric surgery that is associated with improvement of many weight-related medical comorbidities, and statistically significant decreased overall mortality for surgically treated as compared with medically treated subjects. Although the Roux-en-Y gastric bypass (RYGB) is considered an acceptably safe treatment, an increasing number of patients are being recognized with nephrolithiasis after this, the most common bariatric surgery currently performed. The main risk factor appears to be hyperoxaluria, although low urine volume and citrate concentrations may contribute. The incidence of these urinary risk factors among the total post-RYGB population is unknown, but may be more than previously suspected based on small pilot studies. The etiology of the hyperoxaluria is unknown, but may be related to subtle and seemingly subclinical fat malabsorption. Clearly, further study is needed, especially to define better treatment options than the standard advice for a low-fat, low-oxalate diet, and use of calcium as an oxalate binder.

Figure 1. Commonly-used weight loss procedures

Panel A: Laporoscopic adjustable gastric band (LAGB). Panel B: Vertical banded gastroplasty (VBG). Panel C: Proximal Roux-en Y gastric bypass (RYGB). Panel D: Distal RYGB. Panel E: Biliopancreatic diversion with the duodenal switch (BPD-DS). See text for discussion.

Figure 2. Urine oxalate excretion and calcium oxalate supersaturation amongst those stone formers who present early (<6 months) or late (> 6 months) after RYGB

Hyperoxaluria was more prominent amongst those that presented later (Panel A), although calcium oxalate supersaturation was equally high in the <6 months group largely due to lower urine volumes (Panel B). Adapted from [42].

Figure 3. Urine chemistries amongst a random sampling of patients before (n=20), 6 months after (n=8) and 12 months after (n=13) RYGB

Hyperoxaluria was common at 12 but not 6 months (Panel A) with a corresponding increase in urinary calcium oxalate supersaturation (Panel D). Urine volume (Panel B), citrate and calcium excretions (Panel C) also fell postoperatively. These data suggest that many patients may be at risk for calcium oxalate stones after this procedure. Adapted from [42].