This patient has a staghorn calculus. After the bedside US was performed, the patient underwent a CT scan which confirmed the diagnosis and noted dilated collecting system and thinning of the cortex on the affected kidney. It also demonstrated hypertrophy of the contralateral kidney with mild hydronephrosis. Urology was consulted and the patient was admitted to the Hospitalist service. She had a nuclear medicine renal scan which showed her renal function to be 79% on the left and only 21% on the right. She then underwent a right nephrectomy 8 days later. Her urine culture grew out Klebsiella.
Renal Calculi
Renal calculi, in general, affect around 10% of the population. Stones are usually comprised of calcium or uric acid. They are most commonly found in one of three areas: the UPJ (uretero-pelvic junction), the UVJ (uretero-vesicular junction) and the point at which the ureter crosses the pelvic brim. They appear on ultrasound as a hyperechoic crescent shape, usually associated with acoustic shadowing. You may also see the twinkle sign with renal stones. This sign is a color Doppler artifact that appears as a combination of red and blue Doppler signals distal to a stone. Small stones may not create an acoustic shadow, but by placing color Doppler on the kidney in a patient suspected to have renal colic, a stone may be identified by noting a twinkle sign.
Renal stones may cause complete or partial blockage of the ureter, resulting in hydroureter and hydronephrosis. A dilation of the ureter and renal collecting system may be present, evidenced by anechoic dilation of these structures. The ureters can be assessed by looking for ureteral jets in the bladder. Looking at the bladder in the transverse plane, scan through until the posterior wall appears flat horizontally. Using color Doppler, ureteral jets appear as red colored flame. If a jet is identified, it is reassuring that the ureter is at least partially patent.
A staghorn calculus is a stone that fills the renal pelvis branching into the calyxes, causing their dilation. They are most often composed of struvite (magnesium ammonium phosphate) and/or calcium carbonate apatite. These stones are often called 'infected stones' since they are usually associated with urinary tract infections with urea splitting organisms. These stones result in obstruction, either by obstructing the collecting system or the ureter. If left untreated, staghorn calculi can cause a kidney function to deteriorate causing end-stage kidney disease. As these stones are likely infected, they pose a risk of progression to sepsis. Thus, most patients require definitive surgical treatment.
On ultrasound, they may appear as various shapes depending on the scan plane, multiple curvilinear structures that outline the renal pelvis. They may or may not have distal shadowing. You may see hydronephrosis and a dilated kidney.