renal colic

**Pathophysiology:** A stone (or, less often, clot, sloughed papilla, or tumor) obstructs urine flow, raising intraluminal pressure proximal to the blockage. Distension of the renal capsule/collecting system plus ureteral peristalsis/spasm and prostaglandin release (increasing renal blood flow and diuresis) cause intense visceral pain. Pain is classically "colicky" (waves), with severity often described as worse than childbirth.

**Stone sites of narrowing (high-yield):** (1) ureteropelvic junction, (2) where the ureter crosses the pelvic brim/iliac vessels, (3) ureterovesical junction (narrowest). Stones <5 mm usually pass spontaneously; >10 mm rarely do.

**Clinical presentation:** Sudden severe unilateral flank pain radiating anteriorly/inferiorly to the groin, testicle/labia (referred via T10-L1; ilioinguinal/genitofemoral nerves). Patient is restless and writhing (contrast with peritonitis, where patients lie still). Nausea/vomiting, hematuria (gross or microscopic; absent in some cases), dysuria/urgency if stone is near the bladder. Costovertebral angle tenderness may be present.

**Stone types:** - Calcium oxalate (~80%): radiopaque; associated with hypercalciuria, hyperoxaluria (Crohn disease, fat malabsorption), hypocitraturia; ethylene glycol ingestion yields envelope-shaped crystals. Treat with thiazides and citrate. - Calcium phosphate: alkaline urine, hyperparathyroidism, RTA type 1. - Struvite (magnesium ammonium phosphate): urease-producing bacteria (Proteus, Klebsiella); alkaline urine; staghorn calculi; coffin-lid crystals. - Uric acid: radiolucent on X-ray; acidic urine; gout, high cell turnover (leukemia/chemotherapy); treat with urinary alkalinization (potassium citrate), allopurinol. - Cystine: hexagonal crystals; cystinuria (AR defect of the COLA transporter: Cystine, Ornithine, Lysine, Arginine); positive urine cyanide-nitroprusside test; "ground-glass" radiopaque appearance.

**Diagnosis:** Non-contrast helical CT abdomen/pelvis is the gold standard (detects nearly all stones, including uric acid). Ultrasound is preferred in pregnancy and children (shows hydronephrosis; avoids radiation). Urinalysis for hematuria, infection, and crystals. BMP for creatinine. KUB X-ray is limited (radiolucent stones missed).

**Management:** - Analgesia: NSAIDs (ketorolac) first-line (reduce prostaglandin-mediated afferent arteriolar dilation and ureteral spasm); opioids if needed. Antiemetics and hydration. - Small stones (<5-10 mm, distal): conservative management with medical expulsive therapy (tamsulosin, an alpha-1 blocker) may be used, though its benefit is debated. - Urgent decompression (ureteral stent or percutaneous nephrostomy) plus antibiotics is required for an obstructing stone with infection (pyonephrosis, urosepsis), acute kidney injury, or obstruction of a solitary kidney. - Definitive: extracorporeal shock wave lithotripsy (ESWL) for smaller stones (<2 cm); ureteroscopy with laser lithotripsy for ureteral stones; percutaneous nephrolithotomy for large (>2 cm) or staghorn stones. - Prevention: high fluid intake (>2-2.5 L urine/day), reduce sodium and animal protein, normal dietary calcium (do not restrict; low calcium increases oxalate absorption), thiazides for hypercalciuria, potassium citrate for hypocitraturia/uric acid stones.

**Complications:** Obstructive uropathy/hydronephrosis, pyelonephritis, urosepsis, AKI, and chronic kidney damage.

**Differential diagnosis (must-not-miss):** Ruptured abdominal aortic aneurysm (older patient with hypotension, pulsatile mass), testicular torsion (in males with referred pain), ectopic pregnancy and ovarian torsion (in females), appendicitis, pyelonephritis, and aortic dissection.

**Boards pearls:** Hematuria + colicky flank pain + restlessness = stone until proven otherwise. Fever + obstructing stone = urologic emergency. Hydronephrosis with a stone and infection requires decompression, not just antibiotics. Hypocitraturia, hypercalciuria, and hyperuricosuria are key metabolic risk factors. Loop diuretics increase calcium excretion (stone risk), while thiazides decrease it.