cystine stones
Summary
Cystine stones are radiopaque (though less so than calcium stones), hexagonal-shaped kidney stones caused by cystinuria, an autosomal recessive disorder of defective renal tubular reabsorption of cystine, ornithine, lysine, and arginine (COLA amino acids) due to mutations in SLC3A1 or SLC7A9 transporters. They classically present with recurrent nephrolithiasis in children/young adults and can be diagnosed with the cyanide-nitroprusside test.
Detail
Pathophysiology: Cystinuria results from defective PCT and intestinal transport of dibasic amino acids (Cystine, Ornithine, Lysine, Arginine - 'COLA') due to mutations in SLC3A1 (type A) or SLC7A9 (type B) genes encoding the rBAT/b0,+AT amino acid transporter. Cystine is poorly soluble in urine, especially at acidic pH, leading to precipitation and stone formation. Clinical features: Recurrent staghorn or multiple bilateral stones, often presenting in childhood or early adulthood (unlike most other stones which peak later). Patients may have family history given autosomal recessive inheritance. Stones are hexagonal-shaped on urinalysis crystal exam - pathognomonic finding. Diagnosis: Urine microscopy showing hexagonal crystals; cyanide-nitroprusside test is a screening test (positive = pink-purple color) that detects cystine in urine; 24-hour urine cystine measurement confirms diagnosis. Imaging: stones are faintly radiopaque due to sulfur content (less radiopaque than calcium stones, more than uric acid stones). Management: Increase fluid intake (goal urine output >3L/day) - cornerstone of treatment. Urinary alkalinization (potassium citrate or sodium bicarbonate) to raise urine pH >7 to increase cystine solubility. Chelating agents (tiopronin, penicillamine) bind cystine and increase solubility for refractory cases. Dietary sodium restriction reduces cystine excretion. Avoid excessive protein/methionine intake. Surgical intervention (lithotripsy, percutaneous nephrolithotomy) for large/obstructing stones, though cystine stones are notoriously resistant to lithotripsy due to hardness. Complications: Recurrent UTIs, obstructive uropathy, chronic kidney disease from repeated stone episodes and interventions.
Sources
- First Aid for the USMLE Step 1
- Robbins and Cotran Pathologic Basis of Disease
- UpToDate: Cystinuria
- Harrison's Principles of Internal Medicine
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