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Severe urinary tract obstruction, particularly bladder outlet obstruction, can lead to extravasation of urine and development of urinary ascites or a urinoma. These occur as complications of urinary tract obstruction either through rupture of the bladder or a calyceal fornix or tear in the renal parenchyma, which can be identified in the second or third trimester of labor. In one study, urinomas or urinary ascites were reported in 20% of cases of posterior urethral valves.
Some speculate that the kidney rupture that gives rise to a urinoma serves as a protective mechanism for the renal parenchyma in the setting of increased pressure due to obstruction.
On ultrasound, a urinoma appears as an ellipsoid or crescent-shaped anechoic cystic structure that is adjacent to the kidney (Figure). Depending on its location in the perinephric space, a large urinoma can displace the kidney anteriorly or posteriorly.
In addition to the presence of a fluid collection, an underlying urinary tract obstruction should also be observed, such as bladder outlet obstruction or bilateral hydroureteronephrosis. There may be evidence of a decompressed urinary tract obstruction because of rupture, with findings of a thickened bladder wall or renal dysplasia.
FigureUrinoma with bilateral renal pelvic dilation
Urinoma is strongly associated with underlying urinary tract obstruction, most commonly because of posterior urethral valves or ureteropelvic junction (UPJ) obstruction.
The differential diagnoses for a cystic structure adjacent to the kidney include lymphangioma, hemorrhagic neuroblastoma, mesenteric cyst, enteric duplication cyst, multicystic kidney disease, polycystic kidney disease, cystic renal tumor, and ureteric duplication. It is important to identify other intra-abdominal structures, such as the stomach and bladder, to determine the exact location of the fluid collection and accurately classify it. In most cases where the cystic structure is adjacent to the lumbar spine, the etiology is genitourinary.
Small urinomas may be mistaken for a dilated calyx in the presence of hydronephrosis; identifying a mass effect on the renal cortex can aid in distinguishing these two diagnoses.
In addition, intra-abdominal ascites can mimic a urinoma; a thorough evaluation of fluid in other compartments is warranted. There have also been cases of urinoma formation after trauma sustained during amniocentesis.
Relevant patient history and clinical setting should be reviewed in conjunction with ultrasound findings.
Genetic Evaluation
The suggested genetic evaluation is based on the recommended evaluation for the underlying urinary tract anomaly, which is usually diagnostic testing (amniocentesis or chorionic villus sampling) with chromosomal microarray analysis (CMA). If ultrasound findings or screening test results are suggestive of a common aneuploidy, it is reasonable to initially perform karyotype analysis or fluorescence in situ hybridization, with reflex to CMA if these test results are normal. If there are additional anomalies, consanguinity, or a family history of a specific condition, gene panel testing or exome sequencing is sometimes useful because CMA does not detect single-gene (Mendelian) disorders. If exome sequencing is pursued, appropriate pretest and posttest genetic counseling by a provider experienced in the complexities of genomic sequencing is recommended. After appropriate counseling, cell-free DNA screening is an option for patients who decline diagnostic evaluation particularly if a common aneuploidy is suspected.
Pregnancy and Delivery Management
A detailed ultrasound examination that focuses on identifying an underlying urinary tract obstruction should be performed when a urinoma is diagnosed. Serial ultrasound surveillance to monitor the size, evidence of progression of the underlying obstruction, and amount of amniotic fluid should be initiated. Magnetic resonance imaging may be helpful in cases where it is difficult to assess the relationship between the kidney and the mass and has been reported to aid in evaluating the renal parenchyma.
Prognosis depends on the cause of obstruction and the gestational age at presentation. A review of case reports suggests a worse prognosis with upper urinary tract obstruction compared with lower urinary tract obstruction. Preserved renal function was seen in 75% of cases associated with posterior urethral valves, but only in 7% of cases due to UPJ obstruction.
Associated anhydramnios portends an extremely poor prognosis, especially if seen in the second trimester of pregnancy.
Spontaneous resolution is a possible, although uncommon, outcome. In a series of 25 cases, 2 cases associated with UPJ obstruction spontaneously resolved.
The lack of improvement may be because the formation of a urinoma requires a functioning kidney, and thus, its resolution or lack of growth may indicate renal compromise. Other cases of spontaneous resolution in the setting of posterior urethral valves have been reported with normal postnatal kidney function.
Although a rare prenatal finding, the diagnosis of urinoma is an important one, as it indicates likely urinary tract obstruction with irreversible ipsilateral renal damage in most cases. This finding should prompt a thorough diagnostic evaluation, surveillance, and postnatal follow-up.
References
Lundar L.
Aksnes G.
Mørkrid L.
Emblem R.
Prenatal extravasation of urine seems to preserve renal function in boys with posterior urethral valves.
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