Teaching Case

Male patient with vague, long-standing history of urolithiasis and benign prostatic hypertrophy, admitted to Emergency Department with acute right flank pain, without associated macroscopic haematuria nor significantly abnormal laboratory tests.
Sufficiently preserved renal function (estimated Glomerular Filtration rate 65 ml/min using MDRD equation).

With the patient still in Emergency, urgent unenhanced CT was requested to investigate suspected renal colic due to urolithiasis and exclude possible alternative diagnoses. After catheterisation, the bladder appeared contracted with severe circumferential mural thickening and multiple calculi, consistent with chronic urinary retention due to prostatic enlargement. Moderate right-sided hydronephrosis was observed, associated with fluid-like effusion surrounding the renal pelvis and proximal ureter.
Further investigation with enhanced CT was performed: after contrast administration, fair nephrographic phase and urinary opacification were observed bilaterally. The right-sided peri- and posterior pararenal fluid extravasation became strongly hyperdense in the excretory phase corresponding to extravasated urine, allowing diagnosis of retroperitoneal urinoma.
Completion with 3D volume rendering images allowed visualisation of size and shape of the urinoma, with its spatial relationship to the kidney, pelvis and proximal ureter.
After 10 days of conservative treatment including antibiotics, perinephric fluid collection was no more identifiable at follow-up ultrasound.

Perinephric urine extravasation caused by spontaneous pelvicalyceal or ureteral injury represents a serious but rarely reported complication of obstructive uropathy. Clinical manifestations include flank pain, sudden oliguria or worsening renal function [1-3].
Pathogenesis of urinary cavities’ rupture involves increased intraluminal pressure secondary to acute obstruction during renal colic, or conversely to chronic urine retention from retroperitoneal fibrosis, pelvic tumours, ureteral stones, pregnancy or bladder outlet obstruction [1-5]. As a result, urinomas (defined as abnormal extraluminal urine collections) develop in the perirenal space and sometimes extend into the retroperitoneum [5, 6].
Most often secondary to iatrogenic injury, blunt or penetrating trauma, urinomas may remain clinically occult or conversely lead to further complications such as abscessualisation, hydronephrosis and paralytic ileus. In most cases, conservative treatment with nephrostomy and/or ureteral stent placement is sufficient, whereas drainage may be required when the patient develops sepsis [1, 4, 6].
Unenhanced CT, currently performed as volumetric acquisition on multidetector scanners using low-kilovoltage protocols to limit radiation dose and completed with coronal reformations, is the modality of choice to investigate urolithiasis and acute renal colic. Very fast and widely available, MDCT allows detection of even tiny calculi without administration of intravenous contrast, and identification of associated secondary signs of acute renal colic (including unilateral renal enlargement with absent white pyramids, hydronephrosis, perinephric – periureteral oedema, perirenal fluid and thickened lateroconal fascia). Furthermore, MDCT provides confident differentiation from other urinary and extra-urinary conditions clinically mimicking renal colic, such as acute gynaecological diseases, pyelonephritis, renal masses, appendicitis, diverticulitis, vascular and musculoskeletal abnormalities [7].
As this uncommon case demonstrates, contrast-enhanced CT including excretory phase acquisition represents the modality of choice to identify and differentiate urinomas from other fluid-like collections such as confined ascites, abscesses, hematomas or cystic masses. Urinomas appear as fluid-density (0-15 HU) collections usually located nearby the renal pelvis, showing increased density (80-200 HU) in excretory phase corresponding to extravasated opacified urine. Sometimes associated a thin capsule or free peritoneal effusion are observed. Diagnostic aspiration may be rarely required if the diagnosis remains questionable or when superinfection is suspected. Furthermore, multiplanar and three-dimensional reformations may be helpful to visualise urinoma extent and relationship to the kidney and urinary tract [5, 6].