STONE DISEASE

Metabolic risk factors and the impact of medical therapy on the management of nephrothiasis in obese patients
Ekeruo WO, Tan YH, Young MD, Dahm P, Maloney ME, Mathias BJ, Albala DM, Preminger GM
Comprehensive Kidney Stone Center, The Division of Urology, Department of Surgery, Duke University Medical Center, Durham, North Carolina, 27710, USA
J Urol. 2004; 172: 159-163

• Purpose: Previous studies have demonstrated that obesity can increase the risk of stone formation as well as recurrence rates of stone disease. Yet appropriate medical management can significantly decrease the risk of recurrent stone disease. Therefore, we analyzed our obese patient population, assessing the risk factors for stone formation and the impact of selective medical therapy on recurrent stone formation.
• Materials and Methods: A retrospective chart review was performed to identify obese patients with stone disease from our Stone Center. Metabolic risk factors for stones were identified as well as patient response to medical therapy. A similar analysis was performed on a group of age and sex matched nonobese stone formers.
• Results: Of 1,021 patients 140 (14%) were identified as obese (body mass index greater than 30). Of these patients complete metabolic evaluations were available in 83 with an average follow-up of 2.3 years. The most common presenting metabolic abnormalities among these obese patients included gouty diathesis (54%), hypocitraturia (54%) and hyperuricosuria (43%), which presented at levels that were significantly higher than those of the nonobese stone formers (p <0.05). Stone analysis was available in 32 obese patients with 63% having uric acid calculi. After initiating treatment with selective medical therapy obese and nonobese patients’ demonstrated normalization of metabolic abnormalities, resulting in an average decrease in new stone formation from 1.75 to 0.15 new stones formed per patient per year in both groups.
• Conclusions: Obesity, as a result of dietary indiscretion, probable purine gluttony and possible type II diabetes, appears to have a significant role in recurrent stone formation. Appropriate metabolic evaluation, institution of medical therapy and dietary recommendations to decrease animal protein intake can significantly improve the risk of recurrent stone formation in these often difficult to treat patients.
  
  
• Editorial Comment
  With an increase in the proportion of obese individuals, interest in medical evaluation and treatment of problems unique to or overrepresented in this patient population has expanded. Stone disease is no exception, and the unique challenges posed by the surgical treatment of morbidly obese individuals have encouraged efforts to reduce the risk of stone occurrence. Ekeruo and colleagues reviewed the outcomes of medical evaluation and treatment of 83 obese stone formers at an average follow-up of 2.3 years, and found that gouty diathesis, hypocitraturia and hyperuricosuria were the most common metabolic abnormalities identified, and that these abnormalities were more pronounced than those identified in a group of matched non-obese stone formers. Moreover, uric acid stone composition was overrepresented in this patient group (63%) compared with the non-obese group in whom uric acid stones comprised only 11% of stones. Some of these finding are expected based solely on overindulgent eating patterns (elevated urinary calcium, uric acid and oxalate). However, the finding of low urine pH is particularly interesting given the recent report showing that insulin resistance (commonly seen in obese patients) is associated with a defect in ammoniagenesis, thereby leading to an acid urine and subsequent promotion of uric acid stones (1). Although a high acid ash diet (from overindulgence in animal protein) can itself cause a decrease in urinary pH, the findings seen above persisted even when patients were maintained on a controlled metabolic diet, suggesting that the effect is, at least in part, diet-independent.
  Of note, the initiation of directed medical and dietary therapy aimed at correcting the underlying metabolic abnormalities resulted in normalization of urinary parameters and a reduction in the rate of stone formation. As such, metabolic evaluation and medical and dietary therapy should be encouraged in these patients, with a good expectation of reduced stone recurrence and consequently less frequent need for surgical intervention.

REFERENCE
1. Sakhaee K, Adams-Huet B, Moe OW, Pak CY: Pathophysiologic basis for normouricosuric uric acid nephrolithiasis. Kidney Int. 2002; 62: 971-9.

Dr. Margaret S. Pearle
Associate Professor of Urology
University of Texas Southwestern Med Ctr
Dallas, Texas, USA