Makindo Medical Notes.com

Due to inadequate reabsorption of dibasic acids causing nephrolithiasis which are cystine, ornithine, arginine, and lysine easily remembered as the mnemonic COAL

About

• Rare autosomal recessive cause of cystine stones
• Incidence of 1 in 2,500. Less than 3% of renal calculi

Genetics

• Cystinuria is caused by mutations in the SLC3A1 or SLC7A9 genes
• These are responsible for encoding proteins that help transport cystine/other amino acids
• Inability to properly reabsorb cystine in the renal tubules so excessive cystine in the urine.
• Impaired reabsorption of the dibasic amino acids

Pathophysiology

• Failure to reabsorb filtered COAL dibasic amino acids
  • Cystine
  • Ornithine
  • Arginine
  • Lysine
• Cystine is very insoluble and forms stones in urine.
• These compounds, however, are relatively soluble do not form stones
• Cystine stones can form a staghorn calculus or be the nidus for other stone formation

Clinical

• Renal colic and pain due to stones
• Obstruction and anuria, Infection
• Haematuria: Blood in the urine, which may be visible or microscopic.
• Recurrent urinary tract infections: Due to obstruction or irritation caused by stones.
• Frequent urination: Due to the irritation of the urinary tract.
• Obstruction of the urinary tract: Leading to AKI

Investigations

• Urine Tests: Microscopic examination to detect hexagonal cystine crystals and the cyanide-nitroprusside test for elevated cystine levels.
• 24-Hour Urine Collection: Measures cystine and other substances in the urine.
• Genetic Testing: Identifies mutations in the SLC3A1 or SLC7A9 genes.
• Imaging Studies: Ultrasound or CT scan to detect kidney stones.

Management

• Good fluid intake 3-4 L/day to dilute the urine and reduce cystine concentration, thereby reducing the risk of stone formation.
• Alkalinizing agents: Such as potassium citrate or sodium bicarbonate, to increase the pH of the urine and make cystine more soluble.
• Thiol-containing drugs: Such as tiopronin or penicillamine, which form a complex with cystine, making it more soluble and easier to excrete.
• Extracorporeal Shock Wave Lithotripsy (ESWL): Uses sound waves to break up stones into smaller pieces that can be passed in the urine.
• Ureteroscopy: A small scope is passed through the urethra and bladder to the ureter or kidney to remove stones.
• Percutaneous Nephrolithotomy: A surgical procedure to remove large stones directly from the kidney.