Main Component of Urinary Stones After Terminal Ileum Resection
The main component of urinary stones complicating resection of terminal ileum is oxalate. 1
Pathophysiology of Oxalate Stone Formation After Ileal Resection
- Enteric (secondary) hyperoxaluria occurs following ileal resection due to fat malabsorption and consequent elevation of intestinal oxalate absorption 1
- The presence of an intact colon is a critical factor, as oxalate remains available for colonic absorption because of concomitant fat malabsorption and its binding of calcium 1
- Urinary oxalate excretion directly correlates with fat excretion, as demonstrated in studies of Crohn's disease patients undergoing intestinal resection 1
- The length of ileum resected is proportional to the degree of hyperoxaluria - longer resections lead to greater oxalate absorption and excretion 2, 3
Mechanism of Hyperoxaluria After Ileal Resection
- Decreased reabsorption of conjugated bile acids in the terminal ileum leads to excess transmission to the colon, where deconjugation by bacteria occurs 1
- Bile acids enhance oxalic acid absorption in both large and small intestine by increasing extracellular space 4
- Calcium, which normally binds to oxalate in the intestine preventing its absorption, instead binds to fatty acids due to fat malabsorption 1
- This leaves more free oxalate available for absorption, primarily in the colon 1, 4
Risk Factors for Stone Formation
- The risk of calcium oxalate stone formation is significantly higher in patients with ileal resection and intact colon compared to those with ileostomy 5
- Increasing dietary fat intake in these patients further increases urinary oxalate excretion 1, 2
- Patients who have undergone ileal resection show significantly higher intestinal oxalate absorption and 24-hour urinary oxalate excretion compared to healthy controls 3
- The prevalence of urinary stone disease in patients who have undergone bowel surgery can be as high as 9.4% 5
Prevention and Management
- A diet low in fat and oxalate and high in calcium is recommended for patients with hyperoxaluria following ileal resection 1, 2
- Restriction of dietary oxalate (teas and fruits mainly) is warranted particularly in those with recurring urinary tract stones 1
- Cholestyramine can help reduce oxalate absorption by binding bile acids, though it may worsen fat malabsorption in severe cases 1, 4
- Oral supplementation of calcium and magnesium, as well as alkali citrate therapy, should be considered as treatment options 3
- Adequate hydration to achieve urine volume of at least 2.5 liters daily is critical for stone prevention 1
Clinical Pearls and Pitfalls
- Calcium supplements, unlike dietary calcium, may actually increase the risk of stone formation and should be used cautiously 1
- Patients with a history of stones before intestinal surgery have a higher risk of developing additional stones postoperatively 6
- Pediatric Crohn's patients typically have lower urinary oxalate excretion than adults, likely due to shorter disease history and fewer bowel resections 1
- The beneficial therapeutic effect of cholestyramine in hyperoxaluria is mediated by its bile acid binding activity rather than direct binding of oxalic acid 4