abdominal CT kidney stones computed tomography
Figure 1. CT abdomen of this patient shows three radio-opaque stones in the left kidney.
A 60-year-old Asian man with history of recurrent nephrolithiasis, congenital solitary left kidney, gout, hypertension, pre-diabetes was seen in nephrology clinic. Since his 20’s, he has had multiple lithotripsy and ureteral stenting procedures to remove stones and to relieve hydronephrosis. He has a strong family history of kidney stones and gout.
His creatinine was 1.5 mg/dL at the initial visit. It increased to 2.0 mg/dL on repeat testing 3 months later. Computed tomography (CT) of the abdomen showed moderate left hydronephrosis with at least 3 stones measuring 1.4-1.7 cm in the renal collecting system (Figure 1). He underwent percutaneous nephrolithotomy to remove stones with left ureteral stent placement. Analysis of retrieved stones showed stones to be 60% calcium oxalate and 40% calcium phosphate. After nephrolithotomy and stent placement, creatinine improved to 1.5 mg/dL. He then underwent a second-look ureteroscopy and stent removal. Subsequently, creatinine increased again to 1.9 mg/dL. A repeat CT scan showed recurrent left hydronephrosis with ureteropelvic junction narrowing. There is a 0.6 cm calculus of the lower pole of the left kidney. He is scheduled to undergo ureteroscopy and possible ureteral stent placement.
He would like to know what causes recurrent stone formation and how to prevent future stones. We ordered additional tests including 24-hour urine collection.
Here are the findings:
Serum sodium 135 mEq/L; potassium 3.2 mEq/L; chloride 100 mEq/L; CO2 18 mEq/L; BUN 60 mg/dL; creatinine 1.8 mg/dL; urine sodium 100 mEq/L; potassium 40 mEq/L; and chloride 80 mEq/L. Arterial blood gas (ABG) pH is 7.29.
The 24-hour urine testing showed urine volume 1.2 L/day; urine pH 6.127; calcium 280 mg/day (reference range: woman <200 mg/day, man <250 mg/day); oxalate 18 mg/day (reference range: 20-40 mg/day); uric acid 0.155 g/day (reference range: woman <0.8 g/day, man <0.75 g/day); urine citrate <23mg/day (reference range: woman >550 mg/day, man >450 mg/day); and sodium 166 mg/day.
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This case was prepared by Yongen Chang, MD, PhD, Assistant Clinical Professor, Division of Nephrology and Hypertension, University of California-Irvine.
5 Borghi L, Schianchi T, Meschi T,et al. Comparison of two diets for the prevention of recurrent stones in idiopathic hypercalciuria. N Engl J Med 2002;346:77-84. doi:10.1056/NEJMoa010369.
Nephrolithiasis affects 13% of men and 7% of women in the United States Worldwide prevalence is increasing1. It often affects the young and middle-aged population. Stone recurrence rate is high: 30% to 50% of patients will have another stone within 10 years1.
Calcium stones (calcium oxalate and/or calcium phosphate) are the most common stone type. They affect men more than woman. Calcium stones are associated with a number of risk factors, including low urine volume, hypercalciuria, hypocitriuria, hyperoxaluria, and hyperuricosuria.2 Based on his urine profile, this patient has relatively low urine volume, hypercalciuria, and hypocitriuria, all of which promotes calcium salt crystallization and increase his risk of stone formation. The underlying etiology is most likely renal tubular acidosis type I (distal RTA) due to renal tubulointerstitial disease associated with chronic urinary obstruction. Distal RTA classically presents non-gap metabolic acidosis, hypokalemia, and inappropriately high urine pH above 5.5. It is often associated with hypocitriuria due to increased absorption of citric acid in the proximal renal tubules that, in turn, increases the risk of calcium stones in these patients. The treatment for distal RTA and calcium stones is potassium citrate to replete serum potassium and correct acidosis. It also serves to raise urinary citrate concentration, thereby increasing the solubility of calcium crystals.
For all patients with nephrolithiasis regardless of stone type, high fluid intake is recommended to target urine output 2.5 L per day3. For patients with calcium stones and/or hypercalciuria, limiting salt intake to less than 2.5 g per day is beneficial.2 This is because reabsorption of sodium and calcium in the renal tubules is closely connected. High salt diet leads to increased sodium excretion and inhibits calcium reabsorption. For those with calcium stones, a diet low in oxalate is recommended to decrease urinary calcium oxalate crystal formation.4 In contrast, a randomized control showed that increasing, not decreasing, dietary calcium intake (in conjunction with a diet low in salt and animal protein) reduced stone recurrence in patients with calcium stones5. Impact of dietary calcium differs from supplemental calcium. The later has been linked with increased calcium stone risk.4 This may be because dietary calcium binds to oxalate in the gut decreasing gut absorption of oxalate. Supplemental calcium taken between meals does not have the same effect.