Prophylaxis of uric acid stone in patients with inflammatory bowel disease following extensive colonic resection

Epidemiology and clinical pathophysiology of uric acid kidney stones

There is global diversity in the prevalence of uric acid (UA) nephrolithiasis. UA nephrolithiasis comprises 8-10 % of all kidney stones in the United States. However, its prevalence is higher in patients with type 2 diabetes mellitus and those with obesity. Three significant urinary abnormalities have been described as the main etiologic factors for the development of UA nephrolithiasis; low urinary pH, hyperuricosuria and low urinary volume. However, an unduly acidic urine below the ionization constant of uric acid (pKa < 5.5) increases the urinary content of undissociated uric acid and thereby uric acid precipitation. Previous studies have shown the two major pathogenic mechanisms for unduly urinary pH are increased net acid excretion (NAE) and reduced renal ammonium (NH4 (+)), with a combination resulting in overly acidic urine. The impaired ammonium excretion has been demonstrated in a steady state in 24-hour urine and also following an oral ammonium chloride (NH4Cl) challenge to amplify ammoniogenic defects in this population. Similar abnormalities have been disclosed in normal populations and also in T2DM populations without kidney stones. To date, the underlying mechanism of increased acid production, source and nature of putative organic acid anions have not been fully elucidated. One plausible mechanism is the production of organic acid by intestinal and aerobic metabolism. This may occur in obese, diabetic and uric acid stone formers due to the differences in gut microflora.

Clinical evaluation of urolithiasis in Crohn's disease

OBJECTIVES

To investigate the characteristics of urolithiasis associated with Crohn's disease in a Japanese population.

METHODS

We studied 98 patients with Crohn's disease: 39 with urolithiasis and 59 without urolithiasis. Patients were treated at the Social Insurance Central General Hospital, or at the Toho University Omori, Ohashi, or Sakura Medical Centers.

RESULTS

Calculi were more frequent in men (n = 30) than women (n = 9). Mean time from diagnosis of Crohn's disease to diagnosis of calculi was 8.8 years (range 0 to 22 years). Calculi were present on the right side in 19 patients and the left side in 19 patients. Stone were composed of calcium oxalate in nine patients, calcium oxalate and calcium phosphate in two patients, and ammonium urate in five patients. The rate of concurrent calculi was significantly higher in ileostomates. The probability of developing calculi was approximately eight times higher for patients with a urine pH of <or=6.0 than for those with a urine pH of >or=6.5.

CONCLUSIONS

The rate of concurrent urolithiasis was higher in patients with a urine pH of <or=6.0, ileostomy, or two or more bowel resections. To prevent formation of calculi, Crohn's disease patients require regular urological examination including urinalysis, ultrasonography, and kidney ureter bladder X-ray.

Stones from bowel disease

Kidney stones are increased in patients with bowel disease, particularly those who have had resection of part of their gastrointestinal tract. These stones are usually CaOx, but there is a marked increase in the tendency to form uric acid stones, as well, particularly in patients with colon resection. These patients all share a tendency to chronic volume contraction due to loss of water and salt in diarrheal stool, which leads to decreased urine volumes. They also have decreased absorption, and therefore diminished urinary excretion, of citrate and magnesium, which normally act as inhibitors of CaOx crystallization. Patients with colon resection and ileostomy form uric acid stones, as loss of bicarbonate in the ileostomy effluent leads to formation of an acid urine. This, coupled with low urine volume, decreases the solubility of uric acid, causing crystallization and stone formation. Prevention of stones requires treatment with alkalinizing agents to raise urine pH to about 6.5, and attempts to increase urine volume, which increases the solubility of uric acid and prevents crystallization. Patients with small bowel resection may develop steatorrhea; if the colon is present, they are at risk of hyperoxaluria due to increased permeability of the colon to oxalate in the presence of fatty acids, and increased concentrations of free oxalate in the bowel lumen due to fatty acid binding of luminal calcium. EH leads to supersaturation of urine with respect to CaOx, in conjunction with low volume, hypocitraturia and hypomagnesuria. Therapy involves a low-fat, low-oxalate diet, attempts to increase urine volume, and agents such as calcium given to bind oxalate in the gut lumen. Correction of hypocitraturia and hypomagnesuria are also helpful.