A 46-year-old woman is admitted to the hospital with dehydration secondary to excess output from an ileostomy. Five years ago, the patient had a total colectomy with a diverting ileostomy for colon cancer. For the last 6 months, she has had increased output from the ileostomy and has been admitted to the hospital twice with similar episodes of dehydration. On the second day of admission, she reports right flank pain. X-ray of the abdomen reveals a nonspecific bowel gas pattern with no evidence of renal calculi. Ultrasound of the abdomen shows a 4-mm stone in the distal right ureter. The patient is treated with analgesics and the stone passes spontaneously. Microscopic analysis of the stone reveals a pure uric acid stone. Which of the following is the most likely underlying mechanism leading to stone formation in this patient?

Question

A 46-year-old woman is admitted to the hospital with dehydration secondary to excess output from an ileostomy.  Five years ago, the patient had a total colectomy with a diverting ileostomy for colon cancer.  For the last 6 months, she has had increased output from the ileostomy and has been admitted to the hospital twice with similar episodes of dehydration.  On the second day of admission, she reports right flank pain.  X-ray of the abdomen reveals a nonspecific bowel gas pattern with no evidence of renal calculi.  Ultrasound of the abdomen shows a 4-mm stone in the distal right ureter.  The patient is treated with analgesics and the stone passes spontaneously.  Microscopic analysis of the stone reveals a pure uric acid stone.  Which of the following is the most likely underlying mechanism leading to stone formation in this patient?

Quizplus · Accepted Answer

11ec160b_3a49_f819_9249_33837d1c647f_MD0001_00 This patient with recurrent episodes of dehydration due to fluid loss from her ileostomy developed a uric acid stone. Pure uric acid stones are radiolucent and cannot be visualized on x-ray but appear as yellow/brown agglomerations of rhomboid-shaped crystals on gross/microscopic examination. Biochemical risk factors include low urine pH, low urine volume (eg, dehydration), and hyperuricemia. Patients with chronic diarrhea or those who have had a colectomy have reduced bicarbonate reabsorption from the gut, leading to a state of chronic metabolic acidosis. The kidneys compensate by increasing the excretion of hydrogen ions (H⁺) and reabsorption of bicarbonate in the collecting ducts. This lowers urine pH (acidic urine), increasing the conversion of soluble urate ion into insoluble uric acid. Conversely, alkalinization of the urine with potassium citrate favors formation of urate and can prevent, and in some cases dissolve, uric acid stones. Other commonly associated conditions include gout, high cell turnover states (eg, lymphoproliferative disorders), and metabolic syndrome. (Choice A) Dietary calcium and oxalate normally form insoluble complexes, which are eliminated in the feces. Disruption of the normal enterohepatic circulation of bile acids can cause malabsorption of dietary lipids, which then form soap complexes with calcium, allowing increased absorption of free oxalate. The excess oxalate is then excreted by the kidneys where it promotes formation of calcium oxalate stones. These stones are radio-opaque. (Choice C) Overproduction of uric acid (eg, gout, myeloproliferative disorders) can lead to uric acid stones. However, this patient has no features of these disorders, and her stone formation is likely due to abnormally concentrated and acidic urine. (Choice D) Hydrolysis of urea by urease-producing bacteria (eg, Proteus, Klebsiella) yields ammonia, which alkalinizes the urine and promotes formation of magnesium ammonium phosphate (struvite) stones. These stones are large and radio-opaque, and typically present with fever and moderate flank pain. (Choice E) Hyperparathyroidism leads to hypercalcemia and an increased filtered calcium load. The subsequent hypercalciuria increases the risk of radio-opaque calcium (oxalate, phosphate) stones. Educational objective: Formation of uric acid kidney stones is promoted by low urine pH, which favors formation of insoluble uric acid over soluble urate ion. Gastrointestinal bicarbonate loss due to chronic diarrhea leads to chronic metabolic acidosis and production of acidic urine, promoting formation of uric acid stones. __________ References: Uric acid nephrolithiasis: a systemic metabolic disorder. (http://www.ncbi.nlm.nih.gov/pubmed/25045326)

A 46-Year-Old Woman Is Admitted to the Hospital with Dehydration