Cystinuria
Revision as of 02:36, 18 March 2026 by Ostermayer (talk | contribs) (Created page with "Cystinuria is an autosomal recessive disorder of renal tubular amino acid transport causing excessive urinary excretion of '''cystine''', which precipitates into '''recurrent kidney stones'''.<ref name="StatPearls">Cystinuria. ''StatPearls''. NCBI. 2023.</ref> It accounts for ~1-2% of adult and '''6-8% of pediatric''' kidney stones. Patients present with '''renal colic indistinguishable from other stone types''' but tend to form '''larger stones, more frequently, startin...")
Cystinuria is an autosomal recessive disorder of renal tubular amino acid transport causing excessive urinary excretion of cystine, which precipitates into recurrent kidney stones.[1] It accounts for ~1-2% of adult and 6-8% of pediatric kidney stones. Patients present with renal colic indistinguishable from other stone types but tend to form larger stones, more frequently, starting at a younger age. The key EM considerations are recognizing the diagnosis (young/recurrent stone former), knowing that cystine stones are resistant to ESWL, and ensuring aggressive IV hydration and appropriate urologic follow-up.
Background
- 80% form their first stone before age 20[1]
- Cystine stones are faintly radiopaque on plain film (often missed on KUB) but visible on CT
- Pathognomonic finding: hexagonal crystals on urine microscopy
- Up to 40% form mixed stones (cystine + calcium oxalate/phosphate)
- NOT Cystinosis — cystinuria causes stones only; cystinosis is a lysosomal storage disease causing systemic cystine accumulation, Fanconi syndrome, and multiorgan disease
Clinical Features
- Presents identically to other renal colic: acute flank pain radiating to groin, hematuria, nausea/vomiting
- Suspect cystinuria when:
- Young patient (child, adolescent, young adult) with kidney stones
- Recurrent stone former with multiple procedures
- Bilateral or staghorn calculi in a young person
- Family history of early-onset stones
- Complications: ureteral obstruction, infected stone, chronic kidney disease (higher CKD rate than typical stone formers)
Differential Diagnosis
- Other causes of renal colic (stone type cannot be determined clinically):
- Calcium oxalate/phosphate (most common), uric acid (radiolucent; dissolves with alkalinization), struvite (infection stones)
- Other causes of recurrent pediatric stones: primary hyperoxaluria, distal RTA, hyperparathyroidism
- Cystinosis — completely different disease despite similar name
Evaluation
Workup
- CT abdomen/pelvis without contrast: identifies stone; cystine density typically 600-1200 HU (lower than calcium stones)
- Urinalysis: hematuria; look for hexagonal crystals (pathognomonic but not always present)
- BMP: creatinine, electrolytes
- Urine culture if fever or pyuria
- KUB is unreliable (cystine stones are faintly radiopaque and easily missed)
Diagnosis
- Stone analysis after passage or retrieval is the most reliable confirmation
- Hexagonal crystals on urine microscopy — pathognomonic
- Cyanide-nitroprusside test: positive (turns purple) when urine cystine >75 mg/L
- 24-hour urine cystine (>300 mg/day confirms; normal <30 mg/day) — not an ED test
Management
- Acute episode: manage per standard renal colic protocols
- Aggressive IV hydration (both therapeutic and helps dilute cystine)
- Analgesia: ketorolac, opioids as needed
- Antiemetics: ondansetron
- Tamsulosin for medical expulsive therapy if stone <10 mm
- Obstructing stone + infection: urology emergency — urgent decompression (stent or nephrostomy) + IV antibiotics
- Continue home medications: potassium citrate, tiopronin if already prescribed
- Cystine stones are resistant to ESWL — ureteroscopy with laser lithotripsy or percutaneous nephrolithotomy is preferred for stones requiring surgical intervention[1]
Disposition
- Admit: obstructing stone with infection/sepsis, intractable pain/vomiting, AKI, solitary kidney with obstruction
- Discharge: small (<10 mm) non-obstructing stone, controlled pain, tolerating PO, normal renal function, no infection
- Urology follow-up within 1-2 weeks
- Strain urine and send any passed stone for analysis
- If cystinuria newly suspected (young/recurrent), arrange nephrology or urology referral for 24-hour urine and long-term prevention planning
- Key patient instruction: drink enough to produce >3 L urine daily, including a large glass at bedtime and overnight — the single most important preventive measure
See Also
External Links
- StatPearls — Cystinuria
- Urol Ann — Cystinuria: Overview of Diagnosis and Medical Management (2022)
- GeneReviews — Cystinuria