What is the most likely finding during metabolic evaluation in a patient with Crohn’s disease (Crohn's disease) and persistent watery diarrhea, who has formed multiple urinary tract (UT) stones?

Metabolic Abnormalities in a Patient with Crohn's Disease and Kidney Stones

In a patient with Crohn's disease and persistent watery diarrhea who has formed multiple kidney stones, the most likely finding during metabolic evaluation is hyperoxaluria and hypocitraturia (answer D).

Pathophysiology of Stone Formation in Crohn's Disease

Patients with Crohn's disease have a significantly higher risk of developing kidney stones due to several metabolic abnormalities:

1. Hyperoxaluria: The primary metabolic abnormality in Crohn's disease patients with stones 1

   • Occurs due to fat malabsorption, especially with ileal involvement
   • Fat malabsorption leads to binding of calcium with fatty acids in the intestinal lumen
   • This reduces calcium available to bind with oxalate, resulting in increased free oxalate absorption

2. Hypocitraturia: The second most common metabolic abnormality 2

   • Caused by chronic diarrhea leading to metabolic acidosis
   • Acidosis increases renal citrate reabsorption, reducing urinary citrate excretion
   • Citrate is a critical inhibitor of stone formation, and its deficiency promotes crystallization

Evidence Supporting Hyperoxaluria and Hypocitraturia

The AUA guideline on medical management of kidney stones identifies that patients with Crohn's disease, particularly those with persistent diarrhea, commonly develop enteric hyperoxaluria 2. This occurs because:

• Urinary oxalate excretion directly correlates with fat excretion in patients with Crohn's disease 1
• Intestinal oxalate absorption and 24-hour urinary oxalate excretion are significantly higher in Crohn's patients with urolithiasis 3
• The length of ileal resection (common in Crohn's disease) correlates significantly with intestinal absorption and urinary excretion of oxalate 3

Regarding hypocitraturia:

• Patients with chronic diarrhea from Crohn's disease develop metabolic acidosis, leading to increased renal reabsorption of citrate and reduced urinary citrate excretion 4
• The Canadian Association of Gastroenterology guidelines recognize that bile acid diarrhea (common in Crohn's) contributes to metabolic abnormalities affecting stone formation 2

Why Other Options Are Less Likely

• Hypercalciuria (A): While this can occur in some patients with stones, it's less common in Crohn's disease where calcium is typically bound to fatty acids in the intestine, reducing urinary calcium excretion 3
• Hyperuricosuria (B): Not typically associated with Crohn's disease and diarrhea
• Hypermagnesuria (C): Crohn's patients actually tend to have lower urinary magnesium excretion compared to healthy controls 3
• Alkaline urine (E): Patients with chronic diarrhea typically develop metabolic acidosis with acidic urine, not alkaline urine 5

Clinical Implications

For patients with Crohn's disease and recurrent kidney stones:

1. Metabolic testing should include:

   • 24-hour urine collections analyzing volume, pH, calcium, oxalate, citrate, and other parameters 2
   • Serum electrolytes to assess acid-base status

2. Management recommendations:

   • Diet low in fat and oxalate, high in calcium 1
   • High fluid intake to achieve urine volume of at least 2.5 liters daily 2
   • Oral supplementation of calcium, magnesium, and alkali citrate therapy 3
   • Treatment of underlying Crohn's disease to reduce diarrhea and fat malabsorption 2

This case highlights the importance of recognizing the connection between intestinal disease and urinary stone formation, with hyperoxaluria and hypocitraturia being the key metabolic abnormalities in patients with Crohn's disease who form kidney stones.