calcium oxalate stones

Pathophysiology: Calcium oxalate stones form when urine becomes supersaturated with calcium and oxalate, promoting crystal nucleation and aggregation. Risk factors include hypercalciuria (idiopathic, hyperparathyroidism, distal RTA type 1), hyperoxaluria (excessive dietary oxalate intake, enteric hyperoxaluria from fat malabsorption in Crohn's disease/bariatric surgery leading to increased oxalate absorption, ethylene glycol poisoning, vitamin C excess), hypocitraturia (citrate normally inhibits stone formation by chelating calcium), and low urine volume/dehydration. Fat malabsorption syndromes are classically tested: unabsorbed fatty acids bind calcium in the gut, leaving oxalate free to be absorbed and excreted renally.

Clinical Presentation: Sudden onset severe flank pain radiating to the groin (renal colic), gross or microscopic hematuria, nausea/vomiting, and possible urinary urgency/frequency if stone is near the bladder. Costovertebral angle tenderness on exam.

Diagnosis: Noncontrast CT abdomen/pelvis is gold standard (most stones are radiopaque, though less radiopaque than calcium phosphate stones). Urinalysis may show hematuria and envelope-shaped (bipyramidal) or dumbbell-shaped crystals under microscopy. 24-hour urine collection helps identify underlying metabolic abnormalities (calcium, oxalate, citrate, uric acid levels).

Management: Acute management includes analgesia (NSAIDs preferred), hydration, and alpha-blockers (tamsulosin) to facilitate stone passage for stones <10mm. Larger stones or those causing obstruction may require lithotripsy, ureteroscopy, or percutaneous nephrolithotomy.

Prevention: Increased fluid intake (>2-3L/day) is first-line for all stone types. Thiazide diuretics reduce urinary calcium excretion (useful for hypercalciuria). Dietary sodium restriction, potassium citrate supplementation (increases urinary citrate, an inhibitor of stone formation), and normal (not restricted) dietary calcium intake are recommended, as low dietary calcium paradoxically increases oxalate absorption and stone risk. Avoid excessive vitamin C and oxalate-rich foods (spinach, nuts, chocolate) in susceptible individuals.

Associations: Crohn's disease, short bowel syndrome, primary hyperparathyroidism, distal (type 1) renal tubular acidosis, and hyperoxaluria disorders (primary hyperoxaluria - autosomal recessive enzyme deficiency).