Teaching Case

A 52-year-old woman visited the ER with acute left flank pain persisting for a day. She had experienced occasional, mild, colicky pain for 3 years, but no lower urinary tract symptoms or fever. Urinalysis revealed a pH of 7.5 and other normal parameters. Ultrasound detected large echogenic foci in the left kidney without hydronephrosis.

The images were obtained from Toshiba Aquilion 128 slice, a third-generation CT scanner with a slice thickness of 0.625mm. Subsequently, multi-planar images were reconstructed.

Figure 1 displays a staghorn calculus measuring approximately 1.9 x 4.2 x 5.9 cm (AP x TR x CC) with an average HU of 1320. It is observed in the left kidney, extending from the renal pelvis into the entire calyceal system.

Furthermore, Figure 2 shows the axial section of the calculi. No features of hydronephrosis or inflammatory changes are noted in both sections. There are no visible underlying predisposing anatomical factors.

In the 3D reconstruction image (Figure 3), the calculus takes on a distinctive shape, resembling stag horns, as it forms a cast of the renal pelvis and calyces.

Staghorn calculi, also known as coral calculi, struvite calculi, or triple phosphate, are renal calculi that obtain the shape of stag horns by occupying multiple portions of the renal pelvis and calyces [1–4]. Struvite stones are crystalline compounds made up of three cations (calcium, magnesium, ammonium) and one anion (phosphate) [5]. 

Patients with staghorn calculus can be asymptomatic even when the stones have grown large and involve the entire calculus. This lack of symptoms is because large staghorn calculi may not lead to acute urinary tract obstruction and hydronephrosis.  However, chronic flank pain and recurrent urinary tract infections with the same organism are key clinical indicators of staghorn calculus [4,5]. Furthermore, urinalysis typically reveals an alkaline pH (>7.2) and crystals with a “coffin-lid” morphology [4,5].

The gold standard for diagnosing any renal stone, particularly staghorn stones, is a non-contrast Computed Tomography (CT) scan. If a CT scan is unavailable, ultrasonography combined with a plain radiograph can be employed. Struvite stones are usually radio-opaque due to their calcium content, and a typical struvite stone will demonstrate a density reading on CT of 900 Hounsfield units or less [6,7]. 

On a plain radiograph, the struvite calculi appear as branching calcific densities overlying the renal outline, resembling an excretory phase intravenous pyelogram. Lamination within the stone is common. In ultrasound, the collecting system appears filled with an echogenic focus, resulting in posterior acoustic shadowing [2,8].

Struvite calculi can lead to significant morbidity and mortality. The overall rate of renal deterioration, with or without definitive treatment, has been reported as high as 28%. A 10-year follow-up study historically found a mortality rate of 7.2%  in patients who underwent stone-removal procedures compared to 28% in those managed conservatively [9,10].  The presence of struvite calculi is associated with xanthogranulomatous pyelonephritis, which can result in chronic parenchymal destruction and kidney function loss [11,12]. However, the recent studies have reported a very low overall mortality rate of less than 1% for percutaneous nephrolithotomy (PCNL) and shock wave lithotripsy treatment of calculi [5]. A morphometry-based classification of staghorn calculus offers a meaningful clinical assessment of these staghorn stones, predicting the outcome of PCNL for staghorn stones [13]. The long-term renal preservation in the staghorn calculi patient may depend on factors such as normal blood pressure, staghorn size, absence of voiding dysfunction, and complete stone eradication  [9].