STONE DISEASE

Durability of the medical management of cystinuria
Pietrow PK, Auge BK, Weizer AZ, Delvecchio FC, Silverstein AD, Mathias B, Albala DM, Preminger GM
Division of Urology, Department of Surgery, Comprehensive Kidney Stone Center, Duke University Medical Center, Durham, NC, USA
J Urol. 2003; 169: 68-70

• Purpose: Cystinuria is an autosomal recessive disorder of dibasic amino acid transport in the kidney that leads to an abundance of cystine in the urine. This molecule is poorly soluble in urine and it is prone to crystallization and stone formation at concentrations above 300 mg./l. Medical treatment in these patients has incorporated increasing urine volumes, alkalinization and thiol medications that decrease the availability of free cystine in urine. Despite a reasonable prognosis for reduced stone formation we and others have noted difficulties in patients complying with medical management recommendations. Therefore, we evaluated the durability of treatment success in our patients with cystinuria.
• Materials and Methods: A retrospective chart review was performed in all patients with cystinuria referred to the comprehensive kidney stone center at our institution for an 8-year period. Medical therapy, stone recurrence rates, compliance with medications and scheduled followup, and the results of metabolic evaluations via 24-hour urine collections were reviewed. The average concentrations of urinary cystine in initial and followup 24-hour samples were compared in patients compliant and noncompliant with medical treatment. In addition, each patient was mailed a 1-page questionnaire to assess the self-perception of medical compliance.
• Results: We identified 26 patients with a mean age of 32 years at referral (range 13 to 67) who were followed an average of 38.2 months (range 6 to 83). Females represented 58% of those with cystinuria. Overall compliance with medical recommendations was poor with a short duration of success. Of the 26 patients followed at our stone center only 4 (15%) achieved and maintained therapeutic success, as defined by urine cystine less than 300 mg./l. An additional 11 patients (42%) achieved therapeutic success but subsequently had failure at an average of 16 months (range 6 to 27). Of these patients 7 (64%) regained therapeutic success at an average of 9.4 months (range 4 to 20). Five patients (19%) never achieved therapeutic success, while an additional 6 (23%) failed to present to followup appointments or provide subsequent 24-hour urine studies despite referral to a tertiary care center. Patient self-assessment of medical compliance was uniformly high regardless of physician perceptions or treatment results.
• Conclusions: The durability of medically treating patients with cystinuria is limited with only a small percent able to achieve and maintain the goal of decreasing cystine below the saturation concentration. Greater physician vigilance in these complicated stone formers is required to achieve successful prophylactic management. Furthermore, these patients require better insight into the own disease to improve compliance.
  
  
• Editorial Comment
  Despite the relative simplicity of the pathophysiology of cystinuria compared with calcium oxalate nephrolithiasis, stone prevention in cystinurics remains a frustratingly difficult problem. Once hydration and alkalinization fail to prevent stone recurrence in cystinuria, the addition of chelating agents becomes necessary. Unfortunately, the choice of available agents is strikingly few, the medication is expensive and the side effects are often prohibitive. Consequently compliance with medication regimens is uniformly poor.
  This sobering article by Pietrow and colleagues reviews the outcomes and perceptions of 26 cystinuric patients at a tertiary stone center. Although just over half the patients initially achieved therapeutic success as determined by urinary cystine levels, only 15% of patients maintained levels below cystine solubility. Moreover, nearly one quarter of patients was lost to follow-up. Interestingly, patients achieving and not achieving therapeutic success had similar perceptions of their compliance with medication and dietary regimens.
  This article underscores the importance of close monitoring of cystinuric patients to maximize compliance and ultimately therapeutic success. Furthermore, it emphasizes the need for pharmaceutical companies to simplify drug regimens through higher dose pills to reduce the unwieldy number of pills required daily (12 on average in this series) and to make an effort to develop new medications with fewer side effects and lower cost. Unfortunately, the relatively low incidence of this disorder has discouraged pharmaceutical companies from pursuing active research and development in this area. For now, close patient follow-up is the best way to monitor and encourage these patients to follow a prescribed medical regimen that has proven efficacy in reducing stone recurrence.

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