Stone Types and Radiopacity
Calcium oxalate (most common — 80%): radio-opaque (visible on KUB x-ray); caused by hypercalciuria, hyperoxaluria, hypocitraturia; associated with Crohn disease (fat malabsorption → oxalate absorption), primary hyperparathyroidism, vitamin D excess. Calcium phosphate: also radio-opaque; associated with renal tubular acidosis type 1 (RTA1) and primary hyperparathyroidism. Uric acid (5-10%): radiolucent (NOT visible on KUB); seen in gout, myeloproliferative disorders, tumor lysis syndrome, and low urine pH/acidic urine; dissolves with urine alkalization (sodium bicarbonate). Struvite (magnesium ammonium phosphate): radio-opaque; caused exclusively by urease-producing organisms (Proteus mirabilis most common, also Klebsiella, Pseudomonas); alkaline urine; forms staghorn calculi filling the renal pelvis; requires surgical removal. Cystine: faintly radio-opaque ('ground glass'); caused by cystinuria (AR defect in cystine transporter) — hexagonal crystals on urinalysis; treatment includes hydration + urine alkalization + D-penicillamine.
Clinical Presentation and Diagnosis
Classic presentation: sudden onset severe colicky flank pain ('writhing' — cannot find comfortable position) radiating to ipsilateral groin/labia/testis; nausea/vomiting; microscopic or gross hematuria. Pain occurs as stone passes through ureter, especially at three narrowing points: ureteropelvic junction (UPJ), crossing of the iliac vessels, and ureterovesical junction (UVJ — most common lodging point). Diagnosis: Non-contrast CT abdomen/pelvis = gold standard (sensitivity 95-98%, detects all stone types including radiolucent). Ultrasound preferred in pregnancy and children (no radiation). KUB x-ray detects calcium stones only. UA shows hematuria ± crystals. Basic metabolic panel: creatinine (obstruction?), calcium, uric acid.
Management
Stones ≤5mm: 95% pass spontaneously. Medical expulsive therapy: tamsulosin (alpha-1 blocker) relaxes ureteral smooth muscle, speeds passage of 5-10mm stones. Pain: NSAIDs (ketorolac) first-line; opioids for severe pain. Strainer to catch stone for analysis. Stones >10mm or persistent obstruction: urologic intervention. Ureteroscopy with laser lithotripsy: preferred for most ureteral stones. Shock wave lithotripsy (SWL): non-invasive, best for renal pelvis stones ≤2cm, avoid in pregnancy. Percutaneous nephrolithotomy (PCNL): for staghorn calculi, very large (>2cm) or hard stones. Urgent intervention for: infected obstructed kidney (sepsis), bilateral obstruction, single kidney with obstruction, or renal failure.
Prevention by Stone Type
All stone formers: increase fluid intake to >2.5L/day urine output. Calcium oxalate: reduce dietary sodium (reduces urinary calcium), maintain normal calcium intake (low-calcium diet paradoxically increases stone risk — dietary calcium binds gut oxalate), thiazide diuretics (decrease urinary calcium), potassium citrate (increases citrate — inhibits stone formation). Uric acid: potassium citrate or sodium bicarbonate to alkalinize urine (goal pH >6.5); allopurinol if hyperuricosuria. Cystine: high fluid intake + urine alkalization; D-penicillamine chelates cystine. Struvite: eliminate infection with appropriate antibiotics; acetohydroxamic acid (urease inhibitor) as adjunct.