- Definition: an inherited genetic disorder characterized by the accumulation of cystine in the kidneys and bladder due to a disruption of amino acid transporter function in the proximal convoluted tubule and intestine.
- Pathophysiology:
- Autosomal Recessive defect in renal proximal convoluted tubule (PCT) and intestinal amino acid transporter.
- Prevents reabsorption of dibasic amino acids (COLA):
- Cystine (only one insoluble in acidic urine)
- Ornithine
- Lysine
- Arginine
- Excess cystine in urine is poorly soluble precipitation in acidic urine.
Mnemonic
Cystine, Ornithine, Lysine, and Arginine (COLA) share a common transporter in kidney and intestine.
- Clinical features
- Recurrent nephrolithiasis (kidney stones).
- Onset typically in childhood or adolescence.
- Can form staghorn calculi.
- Diagnostics
- Urinalysis: Hexagonal crystals (pathognomonic).
- Urinary Cyanide-Nitroprusside test: Positive (turns purple/red).
- Treatment
- Hydration: Primary Tx to dilute urine.
- Urinary Alkalinization: Cystine is soluble at high pH.
- Agents: Potassium citrate, Acetazolamide.
- Chelating agents (refractory cases): Penicillamine, Tiopronin (forms soluble complexes).
- Diet: Low sodium (↓ Na+ reabsorption leads to ↓ cystine excretion), low methionine.