STONE DISEASE

Biochemical and physicochemical presentations of patients with brushite stones
Pak CY, Poindexter JR, Peterson RD, Heller HJ
Center for Mineral Metabolism and Clinical Research, University of Texas Southwestern Medical Center, Dallas, Texas, USA
J Urol. 2004; 171: 1046-9

• Purpose: We determined whether the biochemical and physicochemical backgrounds of patients with brushite stones differ from those with hydroxyapatite and calcium oxalate stones.
• Materials and Methods: From a computer data base of patients completing ambulatory evaluation 19 with brushite stones, 24 with hydroxyapatite stones and 762 with calcium oxalate stones were identified with the specified composition in greater than 70% of stones.
• Results: Absorptive hypercalciuria type I was present in 63% of patients with brushite, 17% with hydroxyapatite and 30% with calcium oxalate stones. Distal renal tubular acidosis was noted in 32% of patients with brushite, 42% with hydroxyapatite and 3% with calcium oxalate stones. Mean urinary calcium in the brushite group was significantly higher than in the hydroxyapatite and calcium oxalate groups (265 +/- 125 vs 186 +/- 103 and 187 +/- 95 mg daily, respectively). Urinary pH in the brushite group was slightly but significantly higher than in the calcium oxalate group (6.15 +/- 0.30 vs 5.91 +/- 0.42). The brushite relative saturation ratio in the brushite group was marginally higher than in the hydroxyapatite group and significantly higher than in the calcium oxalate group (3.25 +/- 2.03 vs 2.34 +/- 1.51 and 1.83 +/- 1.66, respectively).
• Conclusion: Patients with predominantly brushite stones could be distinguished from those with predominantly hydroxyapatite and calcium oxalate stones by higher urinary saturation with respect to brushite due mainly to hypercalciuria from absorptive hypercalciuria.
  
  
• Editorial Comment
  Brushite stone formers constitute a particularly aggressive and difficult-to-treat subset of calcium stone formers. The low fragility of brushite stones observed in vitro is consistent with the clinical finding that they are relatively SWL-resistant; consequently, these stones typically require endoscopic treatment. Moreover, brushite stone formers tend to be highly metabolically active, with high recurrence rates even when patients have been rendered stone free after surgery (1). Therefore, insight into the physicochemical causes of brushite stone formation may facilitate management of these patients, who in my practice constitute a most challenging group of patients to manage medically.
  Pak and colleagues searched their stone registry to identify 19 patients with predominantly brushite stones and 24 with predominantly hydroxyapatite stones, then compared them with a control group of 762 calcium oxalate stone formers to discern differences in urinary physicochemisty. Brushite stone formers were found to have urine that is significantly more supersaturated with respect to brushite than the other 2 groups, primarily as a result of higher urinary calcium. Indeed, absorptive hypercalciuria was overrepresented in the brushite group (63%) and underrepresented in the hydroxyapatite group (17%) compared with the calcium oxalate group (30%). Not surprisingly, urinary pH was also higher in the brushite and hydroxyapaptite groups than the calcium oxalate group.
  The authors raised the question as to why brushite, which represents a relatively unstable form of calcium phosphate that forms at lower urine pH than hydroxyapatite, fails to undergo conversion to hydroxyapatite during normal periods of urinary alkalinization. They speculate that perhaps urinary inhibitors prevent the transformation. Equally important in my mind, is why these patients are so difficult to treat medically. Anecdotally, correction of their hypercalciuria often fails to result in a corresponding reduced rate of stone recurrence as is typically seen with calcium oxalate stone formers. Whether pH manipulation (i.e., lowering of urine pH) can prevent brushite stone formation requires further clinical investigation, but it is likely to be a difficult and potentially dangerous (risk of bone loss) maneuver. For now, clinical studies such as these, may help shed some light on this difficult group of patients.

Reference
1. Klee LW, Brito CG, Lingeman JE: The clinical implications of brushite calculi. J. Urol. 1991, 145: 715-8.

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