Predictive value of kidney stone composition in the detection of metabolic abnormalities

Can ureteral stent encrustation analysis predict urinary stone composition?

OBJECTIVES

To determine the value of mid-infrared spectroscopy (MIRS) of ureteral stent encrustations in predicting urinary stone composition.

METHODS

A retrospective study analyzed the composition of stent encrustations and urinary stones by MIRS in patients who had had a stent for ureteral obstruction between 2001 and 2003. The overall correlation was evaluated. The correlation coefficient kappa for agreement between the proportions of each component was calculated.

RESULTS

A total of 72 stents and 72 stones from 72 patients were analyzed. The mean stent indwelling time was 55.5 days (range 14 to 102). The stents had been placed for fever (52 cases, 72%), pain refractory to analgesics (15 cases, 21%), and impairment of kidney function (5 cases, 7%). The overall correlation between stone composition and stent encrustation was 71.4%, excluding biofilm analysis. The kappa value was 0.78 for the main component (n = 72; P < 0.0005), 0.61 for the secondary component (n = 72; P < 0.0005), and 0.91 for the agreement between the composition of encrustations at each end of a stent (n = 30; P < 0.0005).

CONCLUSIONS

MIRS analysis of stent encrustations is a reliable method of predicting stone composition when the stone cannot be retrieved. Systematic MIRS analysis of stent encrustations is not recommended but can be very useful in clinical situations in which no stone is available.

Clinical value of crystalluria and quantitative morphoconstitutional analysis of urinary calculi

Morphoconstitutional analysis of urinary calculi, i.e. morphologic examination combined with Fourier transform infrared spectroscopy (FTIR), is of decisive interest for the diagnosis of rare but severe inherited or acquired stone diseases such as cystine, 2,8-dihydroxyadenine, xanthine, struvite, ammonium urate or drug-containing calculi as well as primary hyperoxalurias. In the absence of early diagnosis and proper management, these diseases may lead to progressive loss of renal function. Among common forms of calcium oxalate (CaOx) stones, predominant CaOx monohydrate (whewellite) is mainly associated with hyperoxaluric conditions whereas predominant CaOx dihydrate (weddellite) is mainly associated with hypercalciuria, and this distinction is of interest to orient metabolic evaluation and preventive measures. Crystalluria examination, also based on morphology and FTIR, is a valuable diagnostic method when no stone is available for analysis. Presence of specific crystals (cystine, 2,8-dihydroxyadenine, struvite, ammonium urate) is diagnostic by itself. In all types of nephrolithiasis, serial crystalluria determination appears as a simple, cheap and reliable method to evaluate the risk of stone formation and assess the effectiveness of preventive measures. Determination of urinary crystal volume was in our experience a useful tool in the management of patients with cystinuria or primary hyperoxaluria in the post-transplantation period. In conclusion, both accurate morphologic and FTIR analysis of stones and serial crystalluria determination should be more largely used, in view of their value in the diagnosis and management of renal stone formers.

High incidence of kidney stones in Icelandic children

All children less than 18 years of age who were diagnosed with a first episode of kidney stones at pediatric referral centers in Iceland during the years 1995-2000 were studied retrospectively. The diagnosis was based on clinical features and results of imaging studies. Patients were invited for evaluation at the end of the study period. Twenty-six patients (15 females, 11 males), median age 9.4 (range 0.2-14.9) years, experienced 34 episodes of kidney stones. The annual incidence was 5.6 and 6.3 per 100,000 children less than 18 and 16 years of age, respectively. Abdominal pain was the most common symptom (N=17; 69%) and urinalysis revealed hematuria in 21 patients (80.8%), sterile pyuria in 17 (65%), and 2,8-dihydroxyadeninuria in two. Six patients (23%) had positive urine cultures at the time of diagnosis and five (20%) had urinary tract anomalies. Family history of kidney stones was positive in one third of patients. Metabolic risk factors for stone formation were identified in 22 of 23 patients (96%) who underwent evaluation. Hypercalciuria, the most common metabolic risk factor, was identified in 18 patients (78%). Stones passed spontaneously in nine patients (35%) and six patients had recurrent stone episodes. The incidence of kidney stones in Icelandic children is high compared with other Western populations, affecting females more than males. Underlying metabolic risk factors were identified in most patients.

PHYSICOCHEMICAL METABOLIC CHARACTERISTICS FOR CALCIUM OXALATE STONE FORMATION IN PATIENTS WITH GOUTY DIATHESIS

PURPOSE

We determined why calcium oxalate stones instead of uric acid stones form in some patients with gouty diathesis.

MATERIALS AND METHODS

Gouty diathesis was diagnosed from absence of secondary causes of uric acid stones or low urinary pH, and reduced fractional excretion of urate with discriminant score of the relationship between urinary pH and fractional excretion of urate less than 80. From the stone registry 163 patients with gouty diathesis were identified, including 62 with uric acid stones (GD + UA) and 101 patients with calcium oxalate stones (GD + Ca). Metabolic data and 24-hour urinary chemistry study were compared between the 2 groups.

RESULTS

Compared with GD + UA, GD + Ca had significantly greater urinary calcium (196 +/- 96 mg per day vs 162 +/- 82 mg per day, p <0.05) and significantly lower urinary citrate (430 +/- 228 vs 519 +/- 288 mg per day, p <0.05), resulting in higher urinary saturation of calcium oxalate. Both groups had low urinary pH (less than 5.5) and high urinary undissociated uric acid (greater than 100 mg/dl). Urinary calcium post-oral calcium load was significantly higher in GD + Ca than in GD + UA (0.227 vs 0.168 mg/dl glomerular filtrate, p <0.001).

CONCLUSIONS

Calcium oxalate stones may form in some patients with gouty diathesis due to increased urinary excretion of calcium and reduced excretion of citrate. Relative hypercalciuria in GD + Ca may be due to intestinal hyperabsorption of calcium.

ELUCIDATION OF FACTORS DETERMINING FORMATION OF CALCIUM PHOSPHATE STONES

PURPOSE

In this study we compared the effect of the urinary concentration of calcium with that of oxalate on the urinary relative saturation ratio (RSR) of calcium oxalate and brushite in patients with pure or mixed calcium phosphate stones.

MATERIALS AND METHODS

A retrospective data analysis was performed of urinary stone risk analysis in 133 patients with pure or mixed calcium phosphate stones (30% or greater). Correlation was sought between stone events and urinary stone risk factors, and between brushite and urinary constituent RSRs before and after adjustment for variables.

RESULTS

The formation rates of total events and stone surgeries weakly but significantly correlated with RSR urinary brushite (total events r = 0.22, p = 0.01) but not with RSR calcium oxalate. Urinary RSR brushite significantly correlated positively with urinary calcium (r = 0.67, p <0.0001), phosphorus and pH. After adjustment RSR brushite inversely correlated with citrate (r = -0.3, p = 0.0006). However, it did not correlate with oxalate.

CONCLUSIONS

In stone formers with calcium phosphate or mixed calcium oxalate-calcium phosphate stones the stone formation rate depends on the urinary saturation of brushite but not of calcium oxalate. RSR brushite is determined by the urinary concentration of calcium, phosphate, citrate and pH but not by oxalate. Hypercalciuria should be controlled in patients with idiopathic calcareous renal calculi who form stones of pure or mixed calcium phosphate.

Association between vitamin D receptor gene polymorphisms and fasting idiopathic hypercalciuria in recurrent stone-forming patients

OBJECTIVES

To investigate the association between fasting idiopathic hypercalciuria (IHc), defined as IHc in the fasting state associated with normal parathyroid function, and ApaI, BsmI, and FokI polymorphisms of the vitamin D receptor (VDR) gene in 159 hypercalciuric recurrent stone formers. IHc contributes to the formation of calcium kidney stones in more than one half of reported cases.

METHODS

We examined 62 patients with fasting IHc (24 women, mean age 42.8 +/- 11.1 years, body mass index 25.7 +/- 4.8 kg/m2), 97 patients with absorptive IHc (41 women, mean age 43.5 +/- 10.8 years, body mass index 26.1 +/- 4.4 kg/m2), and 124 healthy control subjects (52 women, mean age 41.9 +/- 10.4 years, body mass index 25.4 +/- 5.1 kg/m2) without a history of nephrolithiasis and without IHc. The bone mass density and VDR genotype and haplotype frequencies were determined in the studied populations.

RESULTS

A reduced bone mass density was observed in fasting IHc patients compared with absorptive IHc patients (P = 0.009) and control subjects (P = 0.006). The prevalence of ApaI and BsmI VDR genotypes and alleles in patients with fasting IHc was significantly different statistically (P <0.05) from that observed in patients with absorptive IHc and control subjects, and the ba haplotype was overrepresented in these patients. No statistically significant difference in the distribution of FokI VDR genotypes and alleles was found between the studied groups.

CONCLUSIONS

Our results suggest a genetic association between 3' VDR alleles, fasting IHc, and reduced bone mass density in patients with recurrent stone formation.

Novel insights into the pathogenesis of uric acid nephrolithiasis

PURPOSE OF REVIEW

The factors involved in the pathogenesis of uric acid nephrolithiasis are well known. A low urinary pH is the most significant element in the generation of stones, with hyperuricosuria being a less common finding. The underlying mechanism(s) responsible for these disturbances remain poorly characterized. This review summarizes previous knowledge and highlights some recent developments in the pathophysiology of low urine pH and hyperuricosuria.

RECENT FINDINGS

Epidemiological and metabolic studies have indicated an association between uric acid nephrolithiasis and insulin resistance. Some potential mechanisms include impaired ammoniagenesis caused by resistance to insulin action in the renal proximal tubule, or substrate competition by free fatty acids. The evaluation of a large Sicilian kindred recently revealed a putative genetic locus linked to uric acid stone disease. The identification of novel complementary DNA has provided an interesting insight into the renal handling of uric acid, including one genetic cause of renal uric acid wasting.

SUMMARY

The recognition of metabolic, molecular, and genetic factors that influence urinary pH, and uric acid metabolism and excretion, will provide novel insights into the pathogenesis of uric acid stones, and open the way for new therapeutic strategies.

Serum uric acid-lowering therapies: Where are we heading in management of hyperuricemia and the potential role of uricase

Although allopurinol has been available for approximately 50 years, hyperuricemia and its sequelae are not only prevalent, but the incidence and costs associated with this disorder continue to increase. However, several new therapies have been developed. Recombinant urate oxidase has been useful in the treatment of tumor lysis hyperuricemia, and pegylated urate oxidase shows promise in patients with hyperuricemia and gout. Febuxostat and Y-700 are new oral xanthine oxidase inhibitors that are in human clinical trials. Tailoring of antilipid therapy in selected hyperuricemic and hyperlipidemic patients with fenofibrate may be of benefit in lowering blood cholesterol and uric acid levels. Similarly, treatment of selected hyperuricemic patients who also are hypertensive with losartan or amlodipine may be beneficial in lowering blood pressure and hyperuricemia. Despite these advances, new treatments for hyperuricemia are needed.

Biochemical distinction between hyperuricosuric calcium urolithiasis and gouty diathesis

OBJECTIVES

To determine whether the biochemical presentation and urinary physicochemical environment of patients with hyperuricosuria presenting with calcium stones (hyperuricosuric calcium urolithiasis [HUCU]) differs from those of patients with gouty diathesis (GD) or idiopathic uric acid urolithiasis.

METHODS

A total of 122 patients with HUCU and 68 patients with GD were identified from our "stone registry" of patients who underwent a full ambulatory evaluation. All patients with HUCU had urinary uric acid greater than 800 mg/day in men and greater than 750 mg/day in women and presented with calcium stones. Those with GD had pure uric acid stones or mixed uric acid-calcium stones and did not have secondary causes of uric acid urolithiasis. Data derived from the fasting serum and 24-hour urine samples collected on a random diet and on a diet restricted in calcium, sodium, and oxalate were compared between the two groups.

RESULTS

Compared with patients with HUCU, those with GD had significantly higher serum uric acid and lower urinary uric acid and pH levels (mean value 5.38 and 5.35 on random and restricted diets versus 6.09 and 6.14, respectively). The fractional excretion of urate and the discriminant score of the relationship between urinary pH and the fractional excretion of urate were significantly lower in those with GD than in those with HUCU. Patients with HUCU displayed a greater urinary saturation of sodium urate and calcium oxalate compared with those with GD, and those with GD had a higher urinary content of undissociated uric acid and lower urinary saturation of brushite (calcium phosphate).

CONCLUSIONS

Patients with HUCU presented with normal urinary pH and hyperuricosuria, accompanied sometimes by hypercalciuria, which produced increased urinary saturation of sodium urate and calcium oxalate. In contrast, those with GD had a low fractional excretion of urate (that contributed to hyperuricemia) and low urinary pH (that led to increased amount of undissociated uric acid). The varying biochemical and physicochemical presentations of the two conditions can be ascribed to overindulgence with purine-rich foods in those with HUCU and underlying primary gout in those with GD.