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Pelvic kidneys are usually smaller than expected for gestational age and have an aberrant blood supply. Most of these kidneys are malrotated on their vertical axis, with the hilum facing anteriorly instead of medially toward the spine.
A pelvic kidney is one located below the aortic bifurcation in the presacral area. The pelvic location results from failure of renal ascent in early embryonic development.
A pelvic kidney is usually discovered when an empty renal fossa prompts a thorough search for the absent kidney. Demonstration of a kidney within the pelvis can be challenging because of bowel and acoustic shadowing from the iliac wings.
Furthermore, the small kidney size, diagonal or horizontal orientation, and anteriorly facing the hilum make the visualization of the kidney more challenging. Frequently, dilation of the renal collecting system and cystic dysplasia further confound the ultrasonographic diagnosis because of distortion of normal anatomy. However, a reniform structure between the iliac vessels on the coronal view or adjacent to the bladder suggests a renal pelvic location, even if the appearance is somewhat atypical (Figure 1). Color Doppler tracing of the blood supply from the distal aorta, aortic bifurcation, or an aberrant pelvic vessel to the renal hilum may provide further supportive evidence for the presence of a pelvic kidney. Not infrequently, a follow-up examination is helpful in visualizing the ectopic kidney, as bowel echogenicity changes with gestational age.
Occasionally, the absence of the kidney from the renal fossa is overlooked because loops of bowel can be mistaken for a kidney. However, close inspection in the transverse and parasagittal views will reveal the absence of the slit-like renal pelvis and the lack of hypoechoic medullary pyramids converging toward the hilum. Other clues of an empty renal fossa are loss of the boomerang shape of the adrenal gland, which may appear flattened, elongated, and “lying down” in the parasagittal plane (Figure 2), and the inability to demonstrate a renal artery in the coronal plane using color Doppler.
Other pelvic masses, including teratoma, neuroblastoma, supernumerary pelvic kidney, and ovarian or mesenteric cysts, should be considered. An empty renal fossa is associated with renal agenesis in 47% of cases and an ectopic kidney in 42% of cases, usually in the pelvis.
Chromosomal anomalies are rare with an isolated pelvic kidney. If no further abnormalities are noted on ultrasound and the family history is unremarkable, no genetic evaluation beyond standard aneuploidy screening is typically recommended. If additional nongenitourinary anomalies are present, amniocentesis and chromosomal microarray analysis (CMA) should be offered. Syndromic conditions, such as VACTERL (vertebral defects, anal atresia, cardiac defects, tracheoesophageal fistula, renal anomalies, and limb abnormalities) and CHARGE (coloboma, heart defects, atresia choanae [choanal atresia], growth restriction, genital anomalies, and ear anomalies), should be considered in the presence of multiple anomalies.
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 anatomic survey focusing on the genitourinary and cardiovascular systems should be performed to exclude associated anomalies. A fetal echocardiogram should be considered if a cardiac anomaly is suspected. A follow-up ultrasound examination in the third trimester of pregnancy to assess for late development of hydronephrosis is warranted, and consultation with pediatric urology or nephrology specialists can be helpful to review childhood follow-up. The finding of a pelvic kidney in the absence of other anomalies should not affect timing and mode of delivery, which should be based on the usual obstetrical indications.
The prognosis of a pelvic kidney is favorable in most cases, although renal function in the affected kidney is usually reduced.
The diagnosis of pelvic kidney relies on a high index of suspicion on recognizing an empty renal fossa. Direct visualization of a kidney within the pelvis requires focused gray scale and color Doppler examination due to challenges posed by the fetal bowel and pelvic bones and, occasionally, atypical anatomic appearance. Amniocentesis and CMA are warranted only when additional, extragenitourinary malformations are present. Obstetrical management should not be altered, and the prognosis is generally good.
Ultrasound mass screening for congenital anomalies of the kidney and urinary tract.