urolithiasis

**Definition:** Urolithiasis (nephrolithiasis = kidney stones; ureterolithiasis = ureteral stones) is the presence of calculi in the urinary tract. Stones form when urine becomes supersaturated with stone-forming solutes, and when inhibitors (citrate, magnesium, Tamm-Horsfall protein) are deficient. Risk factors: dehydration, low urine volume, male sex, family history, obesity, and diets high in sodium, animal protein, or oxalate.

**Stone types (high-yield):** - **Calcium oxalate (about 80%):** Radiopaque. Envelope-shaped or dumbbell-shaped crystals. Associated with hypercalciuria (the most common metabolic abnormality), hyperoxaluria (Crohn disease/fat malabsorption, ileal resection, ethylene glycol ingestion), and hypocitraturia. Thiazides reduce urinary calcium and are used for prevention. Citrate (potassium citrate) alkalinizes the urine and chelates calcium. - **Calcium phosphate:** Radiopaque. Forms in alkaline urine (e.g., renal tubular acidosis type 1, hyperparathyroidism). - **Struvite (magnesium ammonium phosphate):** Radiopaque. Coffin-lid crystals. Caused by urease-producing organisms (Proteus, Klebsiella, Staphylococcus saprophyticus). Alkaline urine, staghorn calculi. Treat with antibiotics and surgical removal. - **Uric acid:** Radiolucent on plain film but visible on CT. Rhomboid or rosette crystals. Forms in acidic urine (pH < 5.5). Associated with gout, high cell turnover (leukemia, tumor lysis), and Lesch-Nyhan syndrome. Treat with urinary alkalinization (potassium citrate) and hydration. Allopurinol for recurrent stones with hyperuricosuria. - **Cystine:** Faintly radiopaque (ground-glass appearance, because of sulfur). Hexagonal crystals. Autosomal recessive defect of the proximal tubule transporter for cystine, ornithine, lysine, and arginine (COLA). Positive urine cyanide-nitroprusside test. Treat with alkalinization, hydration, and tiopronin or penicillamine.

**Clinical features:** Sudden, severe, colicky flank pain that radiates to the groin or testicle/labia. The patient writhes and cannot find a comfortable position (unlike peritonitis, where patients lie still). Nausea and vomiting, and hematuria (gross or microscopic). Stones lodge at three narrowing points: the ureteropelvic junction, the pelvic brim (where the ureter crosses the iliac vessels), and the ureterovesical junction (the narrowest point). Ureterovesical junction stones may cause urinary frequency and urgency.

**Diagnosis:** - **Noncontrast helical CT abdomen/pelvis** is the gold standard (detects all stone types, including uric acid). - Ultrasound is preferred in pregnancy and in children (hydronephrosis, no radiation). - UA: hematuria, with crystals and a pH that suggests the type (acidic for uric acid, alkaline for struvite). - BMP (creatinine), calcium, uric acid, and PTH if indicated. Check a 24-hour urine collection for recurrent stones.

**Management:** - **Small stones (< 5 mm) without complications:** Hydration, analgesia (NSAIDs such as ketorolac are first-line), and an alpha-blocker (tamsulosin) for medical expulsive therapy of distal ureteral stones. - **Larger stones or failure to pass:** Extracorporeal shock wave lithotripsy (ESWL) for stones < 2 cm in the kidney or proximal ureter. Ureteroscopy with laser lithotripsy for ureteral stones. Percutaneous nephrolithotomy for large (> 2 cm) or staghorn stones. - **Urologic emergency:** An obstructing stone with infection (fever, pyelonephritis/pyonephrosis), AKI, or a solitary kidney. Requires urgent decompression (ureteral stent or percutaneous nephrostomy) plus antibiotics. - **Prevention:** Increase fluid intake (urine output > 2-2.5 L/day), reduce sodium and animal protein, and keep normal dietary calcium (restricting calcium increases oxalate absorption and stone risk). Use thiazides for hypercalciuria and potassium citrate for hypocitraturia or uric acid stones.

**Complications:** Obstructive uropathy and hydronephrosis, urinary tract infection and urosepsis, AKI, and chronic kidney disease with recurrent stones.

**Key associations/pearls:** - Hypercalciuria is the most common metabolic cause of calcium stones. - Loop diuretics increase urinary calcium and can promote stones. Thiazides decrease it. - Acetazolamide and topiramate (carbonic anhydrase inhibitors) cause calcium phosphate stones (alkaline urine, low citrate). - Indinavir and triamterene can form drug-related stones. - Ethylene glycol poisoning causes calcium oxalate crystals ("envelope" shaped) in urine. - Renal tubular acidosis type 1 causes calcium phosphate stones and nephrocalcinosis. - Differential for flank pain: pyelonephritis, AAA rupture (older patients), ectopic pregnancy, and ovarian torsion.