staghorn calculus

**Definition:** A staghorn calculus is a branched stone that occupies the renal pelvis and extends into at least 2 calyces. It is a cast of the collecting system.

**Composition:** - Most are struvite (magnesium ammonium phosphate), often mixed with carbonate apatite (calcium phosphate). This is why they are called "infection stones." - Less common types include cystine, uric acid, and calcium oxalate. Cystine and uric acid stones can also form staghorns, but struvite is by far the most common.

**Pathophysiology:** 1. Urease-producing bacteria infect the urinary tract. Examples are *Proteus mirabilis* (the classic one), *Klebsiella*, *Pseudomonas*, *Providencia*, *Serratia*, *Morganella*, *Staphylococcus saprophyticus*, and *Ureaplasma urealyticum*. *E. coli* is NOT a urease producer. 2. Urease hydrolyzes urea to ammonia and CO2. Ammonia raises urine pH (>7), and ammonium and phosphate precipitate with magnesium. 3. Alkaline urine favors precipitation of struvite and apatite. The stone keeps growing, and bacteria trapped inside the stone are protected from antibiotics. This causes recurrent UTIs.

**Risk factors:** - Female sex (more UTIs) - Recurrent UTIs - Urinary stasis or obstruction, such as neurogenic bladder, spinal cord injury, ileal conduit, or anatomic abnormalities - Chronic indwelling catheters

**Clinical presentation:** - Often has a silent or indolent course. Because the stone is large and not passing, it often does NOT cause classic renal colic. - Recurrent UTIs, flank pain, hematuria, and foul-smelling alkaline urine. - Complications include obstruction with hydronephrosis, pyelonephritis, renal abscess, and xanthogranulomatous pyelonephritis. Chronic infection and obstruction can lead to loss of renal function. Urosepsis can occur. - There is a possible association with squamous cell carcinoma of the renal pelvis (chronic irritation and infection).

**Diagnosis:** - Urinalysis shows alkaline urine (pH >7), with possible bacteriuria and hematuria. Urine microscopy shows **coffin-lid** shaped crystals (struvite). - Imaging: non-contrast CT is the most sensitive test. A plain abdominal film (KUB) shows a radiopaque, branching stone, because struvite is radiopaque. Ultrasound can also help. Pure cystine stones are faintly radiopaque, and uric acid stones are radiolucent.

**Management:** - Treatment is complete stone removal, because any leftover fragments harbor bacteria and cause recurrence. - **Percutaneous nephrolithotomy (PCNL)** is first-line. - Extracorporeal shock wave lithotripsy (ESWL) alone is not effective for most staghorn stones, and ureteroscopy is generally not used for large stones. Open surgery is rarely needed. Nephrectomy is an option if the kidney is nonfunctional. - Give antibiotics guided by culture, but antibiotics alone do not cure the stone. - Acetohydroxamic acid, a urease inhibitor, is an adjunct in select patients when surgery isn't possible. It has significant side effects. - Prevent recurrence by treating UTIs and relieving obstruction.

**Board pearls:** - Alkaline urine + urease-positive organism (Proteus) + coffin-lid crystals + branched radiopaque stone = struvite staghorn calculus. - Comparison of stone types: calcium oxalate is the most common stone overall, uric acid stones form in acidic urine and are radiolucent, and struvite stones form in alkaline urine. - Treat with PCNL, not thiazides or hydration alone.