Case of the Week #653

Denys Saitarly (1), Javier Cortejoso (2)
(1) Israel, Meir Medical Center; (2) Centro Médico Recoletas, Valladolid, Spain

Posting Dates: Jun 30, 2026 - Jul 14, 2026

19-year-old primigravid woman with non-contributive medical history presented at 24 weeks for routine second trimester anomaly scan.

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Answer

We present a case of Crossed Fused Renal Ectopia. The diagnosis was confirmed on postnatal ultrasound.

Our ultrasound images demonstrated the following findings:

Ultrasound image 1 and videos 1 and 2 demonstrate an empty left renal fossa and an inferiorly elongated left adrenal gland ("lying-down adrenal sign").
Image 2 and video 3 show left-to-right crossed fused renal ectopia, with the ectopic kidney located inferior to the orthotopic right kidney (inferior crossed ectopia).

Discussion

Congenital renal anomalies in position and fusion arise from disruption of the normal embryologic migration of the kidneys. Between the sixth and ninth gestational weeks, the metanephros normally ascends from the pelvis to the retroperitoneal renal fossa in the lumbar region, undergoing a 90‑degree medial rotation along its longitudinal axis during this process [1]. Failure of the mature kidney to reach its usual location in the renal fossa results in renal ectopia, which may occur in several positions: pelvic, iliac, abdominal, thoracic, or contralateral/crossed [2,3].

Simple renal ectopia refers to a kidney located on the correct side of the body but in an abnormal position, either unilaterally or bilaterally. The most common form is the pelvic kidney, situated opposite the sacrum and below the aortic bifurcation. Kidneys that cross the midline are termed crossed renal ectopia, and describes a kidney located on the side opposite to its ureteral insertion into the bladder. Crossed renal ectopia may occur with or without fusion to the contralateral kidney.

Beyond positional abnormalities, shape and fusion anomalies include not only crossed renal ectopia with fusion but also horseshoe kidney. Horseshoe kidney consists of two vertically oriented renal masses connected at their lower poles by a parenchymal or fibrous isthmus that crosses the midline. Crossed fused renal ectopia is the second most common renal fusion anomaly after horseshoe kidney, with a reported incidence of 1 in 1,000 to 1 in 7,500 live births and a male predominance [4,5]. The majority of cases (79%) are detected antenatally, typically during routine second-trimester anatomic surveys [6]. Its etiology remains uncertain. The most widely accepted explanation is the “mechanical theory”, which proposes that an altered course of the umbilical arteries creates a path of least resistance that directs one metanephric unit toward the opposite side, facilitating fusion. Other proposed mechanisms include abnormal rotation of the caudal embryo, as well as the “ureteral theory,” in which the ureteral bud crosses the midline and induces differentiation of the contralateral metanephric blastema while the ipsilateral metanephros regresses [5,7].

McDonald and McClellan [8] classified crossed renal ectopia into four major categories describing the presence or absence of fusion and whether the anomaly is solitary or bilateral, then six additional subtypes based on the specific fusion pattern. Left‑to‑right ectopia accounts for approximately two‑thirds of cases, although proportions vary across published series [6,7]. Renal fusion occurs in about 90% of cases, and in at least 80% of these the crossed ectopic kidney lies inferior to the orthotopic kidney, with its upper pole fused to the lower pole of the normally positioned kidney (inferior ectopia). Additional morphological variants include sigmoid or S‑shaped, lump, and L‑shaped kidneys, as well as rare forms such as disk kidney and superior ectopia [6]. The ectopic kidney is often smaller than the orthotopic one, and its ureter crosses the pelvis to insert normally into the bladder [6]. The orthotopic kidney frequently exhibits anomalous vascularization, with multiple renal arteries arising at different levels from the aorta. The crossed ectopic kidney is typically supplied by one or more branches originating from the aorta or the common iliac artery on the side where it is located [9].

Failure to visualize a fetal kidney in its usual location—an “empty renal fossa”—is often interpreted as renal agenesis; however, careful examination may reveal an ectopic kidney [10-12]. Normal amniotic fluid volume and a visible fetal bladder provide reassurance that at least one functioning kidney is present. A pelvic kidney may be difficult to distinguish from adjacent structures, particularly in the second trimester when perinephric fat is limited [13]. When ectopia is suspected on a transverse image, sagittal and coronal views of the lower fetal abdomen and pelvis can assist in identifying ectopic renal tissue. Associated hydronephrosis or other renal anomalies may facilitate detection [14]. Color Doppler is essential for confirming the diagnosis by demonstrating the aberrant vascular supply to the ectopic kidney [5,15].

The adrenal gland arises from the mesodermal ridge in the thoracolumbar region, with contributions from neural crest cells [16]. Consequently, its position is not affected by renal migration abnormalities. When a kidney is absent from the renal fossa, the adrenal gland, which is normally located superior to the kidney and characterized by a hypoechoic cortex and hyperechoic medulla, appears flattened and vertically oriented and occupies the empty fossa. This is known as the “lying‑down adrenal sign” [17,18].

The first prenatal diagnosis of crossed renal ectopia was reported by Greenblatt et al. [19] in 1985. Since then, it has been described only sporadically, within broader series of renal ectopia [10-12], in combined pre‑ and postnatal studies [6,7], or as isolated cases or small dedicated series [14,15,20-22]. In some fetuses with an empty renal fossa, meticulous sonographic evaluation reveals the typical appearance of crossed renal ectopia: one kidney, usually the right, occupies its normal position, whereas the contralateral kidney lies inferior to it and is malrotated [3]. A cross‑fused ectopic kidney may mimic an enlarged solitary kidney when the contralateral kidney is not visualized; therefore, assessment of renal morphology and contour, together with identification of two echogenic collecting systems, is essential for correct diagnosis [10]. Because renal ascent is completed by the eighth gestational week, a fetal pelvic kidney can be detected as early as the first trimester using high‑frequency transvaginal probes [22].

Urinary tract anomalies occur in 24-41% of cases and include hydronephrosis (38%), vesicoureteral reflux (41%), double collecting systems, and ureteropelvic junction obstruction [6,15,23]. Extraurinary anomalies are present in approximately 40-48% of cases and can include cardiovascular, skeletal, genital and gastrointestinal abnormalities [6,15, 24]. Cardiac anomalies include ventricular septal defect and persistent left superior vena cava [6,15]. Genital anomalies include hypospadias and cryptorchidism [6]. Gastrointestinal abnormalities include anorectal malformation, which may occur in isolation or as part of the VACTERL association (Vertebral anomalies, Anal atresia, Cardiac malformations, Tracheoesophageal fistula, Renal anomalies, Limb abnormalities) [7,24]. Single umbilical artery has also been associated with crossed fused renal ectopia [15].

The sonographic identification of any form of renal ectopia should not modify obstetric management. Once the diagnosis is established, a meticulous ultrasound examination is required to exclude associated anomalies, followed by periodic sonographic surveillance throughout pregnancy [15].

Cross‑fused ectopia should not be misinterpreted as unilateral renal agenesis with compensatory hypertrophy of the contralateral kidney. Beyond this scenario, the main differential diagnosis is a renal tumor [19]. Several sonographic features help distinguish renal ectopia from a neoplastic mass: a normally positioned kidney will be present in the contralateral renal fossa; the characteristic echogenic central collecting system is absent in tumors; and the echotexture of a mass typically differs from that of normal renal parenchyma.

Postnatal evaluation includes renal-bladder ultrasound, which is usually sufficient for confirmation of the diagnosis. Additional imaging with voiding cystourethrography (VCUG) and dimercaptosuccinic acid (DMSA) scan may be indicated to detect vesicoureteral reflux and assess differential renal function [6]. Postnatal nephrological follow-up is recommended for all cases [15]

Most individuals with an ectopic kidney remain asymptomatic throughout life, with the condition often identified incidentally. However, abnormal renal position and/or aberrant ureteral course may predispose to complications such as hydronephrosis, vesicoureteral reflux, ureteropelvic junction obstruction, urinary stone formation, and infection [25-30]. Impaired renal function in the ectopic kidney occurs in approximately 34% of cases on postnatal DMSA scanning, though chronic renal failure is uncommon in isolated cases [6,31]. Associated urinary anomalies require postnatal surgical intervention in approximately one-third of affected patients [6]. The presence of extraurinary malformations with crossed fused renal ectopia carries higher morbidity risk. In one series, 14% of patients with extraurinary anomalies developed chronic kidney disease, with mortality occurring in those with severe associated malformations [6].

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