Urinoma

A urinoma is a fluid collection of urine which has extravasated into the perirenal space or retroperitoneum^1,2. Typically, this collection is encapsulated and localized to the peri-renal area. It is also known as pseudohydronephrosis or pararenal pseudocyst in the literature.

N36.8- Other specified disorders of urethra

Q62.39- Other obstructive defects of renal pelvis and ureter

The exact incidence of fetal urinoma is not known. It is a rare finding, with about 40 cases reported in the literature¹. It is more common among males than females².

A urinoma is thought to occur as a result of rupture of the collecting system of a functional kidney due to an underlying obstruction in the majority of cases³. The mechanism involves microperforation of the renal pelvic walls with extravasation of urine into the renal fascia (Gerota’s fascia) or the retroperitoneal space⁴.

The process of urine extravasation is not completely understood. There are two major hypotheses. An older theory suggests that urinomas are a result of a “pop-off” mechanism which acts as a “valve” to relieve increased pressure in the kidney in order to protect kidney function. A more recent theory is that urinomas occur as a result of exposure of the kidney to a high-pressure system which causes renal damage and ultimately rupture⁵. Based on renal function outcomes which are commonly poor in the affected kidney, the latter hypothesis appears to be more accurate.

In adults, urinomas are rare complications of trauma, surgery and obstructive uropathies¹.

Overall, urinomas are more common in males. In female fetuses, urinomas are more commonly related to upper urinary tract obstruction¹. In the prenatal period, urinomas are usually associated with obstructive uropathy, usually secondary to posterior urethral valves or ureteropelvic junction obstruction. These conditions can generate a high-pressure system in the upper urinary tract during fetal life which can cause rupture of the renal parenchyma. The proposed relationship between high pressure in the urinary tract and renal rupture is also supported by other reported obstructive aetiologies^4,6. Therefore, increased pressure within the urinary tract seems to be a prerequisite for rupture and urine extravasation².

There are some reported cases of fetal urinomas without an identifiable obstructive uropathy. These typically are non-obstructed urinomas, and can occasionally be caused by in utero trauma, due to traumatic amniocentesis (2, 5) or trauma from delivery (7). This suggests that urinomas can also occur in the absence of urinary tract obstruction, and in low-pressure systems. Non-obstructed urinomas seem to be reported exclusively in female fetuses (2).

Often, the underlying cause of a fetal urinomas is unknown.

Fetal urinomas are strongly associated with obstructive uropathies, including obstruction of the upper-pole, multicystic dysplastic kidney, ureterocele, ureteral atresia or obstruction, posterior urethral valves and cloacal malformation^4,6.

Urinoma is a sporadic anomaly, and the recurrence risk is no greater than the baseline risk.

Urinomas may be diagnosed during prenatal period by fetal ultrasound.

The best diagnostic clue is the presence of a unilocular flank cyst that displaces the kidney⁸.

They are typically elliptical or crescent-shaped, thin-walled, cystic masses adjacent to the renal parenchyma and adjacent to the fetal spine. The anechoic fluid around or abutting the kidney is more likely unilocular than septated⁸. This mass pushes the renal fascia out from the kidney due to mass effect⁵. The mass is adjacent to but not contiguous with the renal parenchyma and pyelocalyceal systems¹.

Urinomas are usually unilateral (60%)⁸.

Fetuses with large urinomas can be difficult to evaluate as the mass effect can distort and displace the affected kidney from its normal location in the renal fossa, rendering its identification difficult^2,5. Small urinomas can mimic a dilated calyx and can be overlooked⁵.

Fetal magnetic resonance imaging, if available, may help visualize the ipsilateral kidney and differentiate the mass from other conditions² but may not change the management of the pregnancy.

Differential diagnosis includes hydronephrosis, cystic renal diseases, duplication of the intrarenal collecting system, cystic lymphangioma, haemorrhagic neuroblastoma, mesenteric cyst, enteric duplication cyst and Wilm’s tumor^2,3,5.

Once a urinoma is identified, the other kidney should be evaluated to ensure that it is normal. The fetus should be carefully evaluated to ensure that there are no other fetal abnormalities, growth abnormalities or amniotic fluid abnormalities.

Reevaluation of the intra-abdominal findings, growth and evaluation of the amniotic fluid volume every three to four weeks is usually recommended.

Patients are usually advised that there may be damage to the affected kidney; the affected kidney may develop renal dysplasia and may not function well in the long run^5,6. The degree of impairment of kidney function can, unfortunately, not be determined in the prenatal period.

The finding of fetal urinoma is helpful to plan for timely postnatal follow up⁵. Unfortunately, urinoma formation may lead to impaired renal function of the ipsilateral kidney, which has been attributed to the pressure exerted by the urine confined within Gerota’s fascia on the developing kidney². The probability of a non-functioning kidney after a diagnosis of fetal urinoma remains to be fully established as data is limited, but it may be as high as 80%³.

Poor prognostic factors for postnatal kidney function include: female sex, association with upper urinary tract obstruction, diagnosis early in the fetal development (second trimester), ureteropelvic junction stenosis, intrauterine resolution and prenatal visualization of renal dysplasia^1,6.

Obstruction of the lower urinary tract is the most common cause of a fetal urinoma and is associated with better prognosis than those caused by upper urinary tract obstruction which are less common.

Postnatally, reduction in size of the affected kidney has been observed³.

Cases of a single kidney with a urinoma or bilateral urinomas typically have poor outcomes due to oligohydramnios and pulmonary hypoplasia. These findings may be suspected using serial ultrasound examinations.

Urinomas may spontaneously resolve in utero (decrease in size), remain stable or also increase in size. Unfortunately, reduction in size of the urinoma commonly reflects worsening renal function, which may predict poor postnatal renal function¹. Huge urinomas may cause abdominal dystocia at the time of delivery.

Fetal urinomas are most commonly conservatively managed².

Large urinomas interfering with the function of other organs may require specific treatment (drainage or diversion) or to allow vaginal delivery in the case of huge urinomas^2,6.

In-utero invasive procedures such as aspiration have no effect on renal function^2,6.

Vaginal delivery is suggested with caesarean delivery reserved for the usual obstetric indications.

Evaluation of the mass in the immediate newborn period is suggested with the aid of a multidisciplinary team and multimodality imaging. Scintigraphy is an ideal method for evaluation of renal function after birth.

1. Hernandez P, Sedano I, Iglesias S, Goñi A, Nasarre M. Urinoma prenatl y función renal posnatal. Progresos de Obstetricia y Ginecologia. 2013;56(5): 270-273

2. Zaccara A, Brizzi C, Mobili L, Nahom A, Carnevale E, Marciano A, et al. Fetal urinoma in females without obstructive uropathy. Fetal Diagn Ther. 2011;29(4):296-300.

3. Oktar T, Salabaş E, Kalelioğlu İ, Atar A, Ander H, Ziylan O, et al. Fetal urinoma and prenatal hydronephrosis: how is renal function affected? Turk J Urol. 2013;39(2):96-100.

4. Balcom AH, Pircon R, Worthington D, Carr M. Spontaneous resolution of an in utero perirenal urinoma associated with posterior urethral valves. Urology. 1999;54(2):366-7.

5. Gorincour G, Rypens F, Toiviainen-Salo S, Grignon A, Lambert R, Audibert F, et al. Fetal urinoma: two new cases and a review of the literature. Ultrasound Obstet Gynecol. 2006;28(6):848-52.

6. Cimador M, Castagnetti M, Rosone G, Lima M, De Grazia E. A giant fetal urinoma in a neonate without detectable obstructive uropathy. Eur J Pediatr Surg. 2003;13(5):355-9.

7. Ito S, Ikeda M, Asanuma H, Shishido S, Nakai H, Honda M. A giant urinoma in a neonate without obstructive uropathy. Pediatr Nephrol. 2000;14(8-9):831-2.

8. Woodward Paula J., ed.  Diagnostic Imaging: Obstetrics. Third edition, Salt Lake City, UT. Elsevier 2016. 

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