Cystinuria
Overview
Plain-Language Overview
Cystinuria is a genetic condition that affects the kidneys and urinary system. It causes the body to have trouble processing certain amino acids, especially cystine, which can build up and form kidney stones. These stones can cause pain, block urine flow, and lead to infections. The condition mainly impacts the renal tubules, where cystine reabsorption is impaired. People with cystinuria often experience repeated episodes of kidney stones starting in childhood or early adulthood. Managing the condition involves monitoring kidney health and preventing stone formation.
Clinical Definition
Cystinuria is an autosomal recessive disorder characterized by defective renal tubular reabsorption of the dibasic amino acids cystine, ornithine, lysine, and arginine due to mutations in the SLC3A1 or SLC7A9 genes. The primary pathology is impaired transport of cystine in the proximal renal tubules and intestinal epithelium, leading to increased urinary cystine concentration. Because cystine is poorly soluble in urine, it precipitates and forms cystine kidney stones, which are often recurrent and can cause urinary obstruction and renal damage. The condition is clinically significant due to its chronic nature and potential for causing nephrolithiasis and associated complications. Diagnosis is important for guiding management to prevent stone formation and preserve renal function.
Inciting Event
Increased cystine concentration in urine due to defective reabsorption triggers stone formation.
Low fluid intake or conditions causing concentrated urine precipitate cystine crystallization.
Urinary tract infections may exacerbate stone formation and symptoms.
Latency Period
Variable latency from birth to symptom onset, often presenting in childhood or adolescence.
Stones may form months to years after initial cystine accumulation begins.
Symptoms typically arise after stone formation and urinary obstruction occur.
Diagnostic Delay
Misdiagnosis as other nephrolithiasis types delays recognition of cystinuria.
Lack of awareness of cystine crystals' characteristic hexagonal shape on urine microscopy.
Failure to perform urine cyanide-nitroprusside test or genetic testing early.
Intermittent symptoms and stone passage may lead to underestimation of disease severity.
Clinical Presentation
Signs & Symptoms
Recurrent flank pain due to nephrolithiasis is the hallmark symptom.
Hematuria is common during stone passage.
Patients may report urinary tract infections secondary to obstruction.
Renal colic with sudden onset severe pain radiating to the groin is typical.
Some patients have a family history of early-onset kidney stones.
History of Present Illness
Recurrent episodes of flank pain and hematuria due to kidney stones.
Colicky abdominal pain associated with urinary obstruction.
History of recurrent kidney stones from a young age is common.
Possible urinary tract infections secondary to obstruction.
Past Medical History
Previous nephrolithiasis or urolithiasis episodes increase suspicion for cystinuria.
History of urinary tract infections may be present.
No specific medication history typically influences cystinuria but poor hydration worsens symptoms.
Family History
Autosomal recessive inheritance pattern with affected siblings or parents.
Family members may have a history of early-onset kidney stones.
Known mutations in SLC3A1 or SLC7A9 genes in relatives.
Physical Exam Findings
Costovertebral angle tenderness may be present during acute nephrolithiasis episodes.
Signs of hydronephrosis such as flank fullness may be detected in severe obstruction.
Generally, physical exam is normal between stone episodes.
Diagnostic Workup
Diagnostic Criteria
Diagnosis of cystinuria is established by detecting elevated cystine levels in the urine, typically using a cyanide-nitroprusside test that produces a characteristic purple color. Confirmation involves quantitative measurement of urinary cystine and other dibasic amino acids by amino acid chromatography. Imaging studies such as non-contrast CT scan can identify characteristic cystine stones. Genetic testing for mutations in SLC3A1 and SLC7A9 may support diagnosis but is not required. The presence of recurrent kidney stones with positive urinary cystine tests confirms the diagnosis.
Pathophysiology
Key Mechanisms
Defective renal tubular reabsorption of cystine and dibasic amino acids due to mutations in the SLC3A1 or SLC7A9 genes.
Increased urinary cystine concentration leads to poor solubility and cystine crystal precipitation in the urinary tract.
Formation of cystine kidney stones causes urinary obstruction, pain, and potential renal damage.
| Involvement | Details |
|---|---|
| Organs | Kidneys are the primary organs affected by cystinuria, with stone formation causing obstruction and damage |
Bladder may be involved in stone passage and urinary symptoms | |
| Tissues | Renal proximal tubule tissue is the site of defective amino acid transport leading to cystinuria |
Urinary tract epithelium is affected by mechanical irritation and inflammation from cystine stones | |
| Cells | Renal tubular epithelial cells are responsible for defective reabsorption of cystine and dibasic amino acids in cystinuria |
Uroepithelial cells may be involved in local inflammation secondary to cystine stone irritation | |
| Chemical Mediators | Cystine is the poorly soluble amino acid that precipitates to form characteristic stones in cystinuria |
Sodium influences cystine solubility and excretion, with high sodium intake increasing cystine excretion |
Treatments
Pharmacological Treatments
Potassium citrate
- Mechanism:
Alkalinizes urine to increase cystine solubility and prevent stone formation
- Side effects:
Hyperkalemia
Gastrointestinal upset
Metabolic alkalosis
- Clinical role:
First-line
Tiopronin
- Mechanism:
Forms soluble disulfide bonds with cystine, reducing cystine stone formation
- Side effects:
Rash
Nephrotic syndrome
Proteinuria
- Clinical role:
Second-line
D-penicillamine
- Mechanism:
Binds cystine to form a more soluble complex, decreasing stone formation
- Side effects:
Nephrotoxicity
Bone marrow suppression
Rash
- Clinical role:
Second-line
Non-pharmacological Treatments
Increase fluid intake to maintain urine output above 3 liters per day to reduce cystine concentration
Adopt a low-sodium diet to decrease cystine excretion in urine
Limit animal protein intake to reduce cystine and acid load
Perform extracorporeal shock wave lithotripsy or surgical removal for large or obstructive cystine stones
Prevention
Pharmacological Prevention
Potassium citrate to alkalinize urine and increase cystine solubility.
Tiopronin or D-penicillamine to form soluble cystine complexes and reduce stone formation.
High fluid intake combined with pharmacotherapy to maintain urine dilution.
Non-pharmacological Prevention
Increased hydration to maintain urine output >3 liters/day and reduce cystine concentration.
Dietary sodium restriction to decrease cystine excretion.
Avoidance of excessive animal protein intake to reduce acid load and cystine precipitation.
Regular monitoring with urine cystine quantification and imaging to detect early stones.
Outcome & Complications
Complications
Obstructive uropathy leading to hydronephrosis and renal impairment.
Recurrent urinary tract infections including pyelonephritis.
Chronic kidney disease from repeated obstruction and infection.
Possible progression to end-stage renal disease in severe cases.
| Short-term Sequelae | Long-term Sequelae |
|---|---|
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Differential Diagnoses
Cystinuria versus Hyperuricosuria
Cystinuria | Hyperuricosuria |
|---|---|
Elevated urinary cystine with characteristic hexagonal crystals | Elevated urinary uric acid with normal cystine levels |
Cystine stones, radiopaque with characteristic appearance on imaging | Uric acid stones, radiolucent on X-ray |
Requires urinary alkalinization and cystine-binding thiol drugs for stone prevention | Responds well to urinary alkalinization and xanthine oxidase inhibitors |
Cystinuria versus Primary Hyperoxaluria
Cystinuria | Primary Hyperoxaluria |
|---|---|
Elevated urinary cystine excretion | Elevated urinary oxalate excretion |
Cystine stones | Calcium oxalate stones |
Autosomal recessive defect in renal cystine transporter | Autosomal recessive disorder affecting glyoxylate metabolism |
Cystinuria versus Dent Disease
Cystinuria | Dent Disease |
|---|---|
Autosomal recessive inheritance | X-linked recessive inheritance |
Elevated urinary cystine without proteinuria | Low molecular weight proteinuria with hypercalciuria |
Usually presents in childhood or adolescence with recurrent cystine stones | Childhood onset with progressive renal failure |
Cystinuria versus Renal Tubular Acidosis (Type 1)
Cystinuria | Renal Tubular Acidosis (Type 1) |
|---|---|
Normal acid-base status with cystinuria | Non-anion gap metabolic acidosis with hypokalemia and alkaline urine |
Cystine stones due to cystine precipitation | Calcium phosphate stones due to alkaline urine |
Requires urinary alkalinization and cystine-binding agents | Improves with alkali therapy correcting acidosis |