renal calculi

Renal calculi form when urine becomes supersaturated with stone-forming substances, leading to crystal nucleation and aggregation. The four major types are: (1) Calcium oxalate/phosphate stones (~80%), associated with hypercalciuria, hyperoxaluria (e.g., Crohn disease, ethylene glycol ingestion), and hypocitraturia; radiopaque on X-ray. (2) Struvite (magnesium ammonium phosphate) stones, formed due to urease-producing organisms (Proteus, Klebsiella, Staphylococcus saprophyticus) that alkalinize urine; often form large 'staghorn' calculi filling the renal pelvis. (3) Uric acid stones, associated with low urine pH, gout, high-purine diets, and conditions with rapid cell turnover (tumor lysis syndrome); radiolucent on X-ray but visible on CT. (4) Cystine stones, due to a genetic defect (cystinuria) in renal tubular reabsorption of COLA amino acids (Cystine, Ornithine, Lysine, Arginine); form hexagonal crystals and classically occur in children/young adults.

Clinical presentation includes sudden-onset severe unilateral flank pain (renal colic) that may radiate to the groin as the stone moves down the ureter, hematuria (gross or microscopic), nausea/vomiting, and possible urinary urgency/frequency if the stone is near the bladder. Pain often waxes and wanes as the stone moves and obstructs at different points (ureteropelvic junction, pelvic brim where it crosses the iliac vessels, and ureterovesical junction—the three classic areas of narrowing).

Diagnosis: Noncontrast helical CT is the gold standard imaging modality. Urinalysis may show hematuria and crystals suggestive of specific stone types. Renal ultrasound is used in pregnancy to avoid radiation.

Management: Small stones (<5mm) often pass spontaneously with hydration and analgesia (NSAIDs, opioids); alpha-blockers like tamsulosin can facilitate passage. Larger stones may require extracorporeal shock wave lithotripsy (ESWL), ureteroscopy, or percutaneous nephrolithotomy. Struvite stones often require surgical removal due to infection risk. Prevention strategies include increased fluid intake, dietary modifications (reduced sodium, oxalate, animal protein), thiazide diuretics for hypercalciuria, and allopurinol for uric acid stones.

Complications include hydronephrosis, urinary tract infection/obstruction leading to pyonephrosis or sepsis, and chronic kidney disease with recurrent or bilateral obstruction.