Staghorn calculi (also sometimes called coral calculi) obtain their characteristic shape by forming a cast of the renal pelvis and calices, thus resembling the horns of a stag.
Staghorn calculi are composed of struvite (magnesium ammonium phosphate) and are usually seen in the setting of infection with urease producing bacteria (e.g. Proteus, Klebsiella, Pseudomonas and Enterobacter). Urease hydrolyses urea to ammonium and increase in the urinary pH 1-3.
The struvite accounts for approximately 70% of these calculi, and is usually mixed with calcium phosphate thus rendering them opaque on both plain films and CT. Uric acid and cystine are the underlying component of a minority of these calculi .
The clinical presentation of patients with struvite stones can be variable. Consider struvite stones in patients with risk factors for developing urinary tract infections (eg, prior urinary diversion or urologic surgery, presence of indwelling catheters, neurogenic bladder, vesicoureteral reflux, other anatomic abnormalities).
Infections may result in pyelonephritis, pyonephrosis, or perinephric abscess. Symptoms may include flank pain classic for renal colic, fever, urinary symptoms (eg, frequency, dysuria), and hematuria (either gross or microscopic). However, struvite stones rarely manifest as a solitary ureteral stone with acute renal colic in the absence of prior intervention. Concomitant urinary obstruction and hydronephrosis may be present and can result in nausea or vomiting.
In institutionalized patients susceptible to infection stones, the ability to elicit symptoms may be limited; sepsis may be the only evidence of an underlying struvite staghorn calculus. Note that patients with struvite calculi can be asymptomatic, even when calculi occupy the entire renal collecting system. Even with progression to xanthogranulomatous pyelonephritis, 25% of patients may remain completely free of symptoms. Systemic manifestations of large struvite stones and associated chronic infection include generalized fatigue, malaise, and weight loss.
The vast majority of staghorn calculi are radiopaque and appear as branching calcific densities overlying the renal outline and may mimic an excretory phase IVP. Lamination within the stone is common.
The collecting system is filled with a densely calcific mass producing intense posterior acoustic shadowing.
Staghorn calculi are radiopaque and conform to the renal pelvis and calyces, which are often to some degree dilated. When viewed on bone windows they have a laminated appearance, due to alternating bands of magnesium ammonium phosphate and calcium phosphate 3.
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