Ectopic Kidneys

A kidney that is not typically located in the retroperitoneal renal fossa and is found elsewhere in the abdomen, pelvis or even rarely in the thorax is called ectopic (from the Greek word “ectopia” meaning “wrong position”). Instead, the contralateral kidney is in the normal position except for the case of horseshoe kidney¹. The most common types of renal ectopy are²:

• Pelvic kidney or simple congenital ectopy: one kidney located at a lower level in the pelvis
• Crossed renal ectopy: the ectopic kidney crosses the midline, with or without fusion with the orthotopic kidney (from the Greek words “orthos and topos” meaning “right position”).
• Horseshoe kidney: most common type of fusion anomaly. The lower poles of the two kidneys are fused, and the bridging tissue forms the isthmus, resulting in one large, fused kidney with two separate renal secretion units and ureters. The fusion itself prevents kidneys to ascend to their normal position and both kidneys are located at a lower level in the abdomen.

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• Simple renal ectopy is a challenging diagnosis antenatally, with an overall reported incidence of 1:1000 autopsies. There is no difference in the incidence among males and females, but there is a higher incidence of the left than the right kidney¹.
• Crossed fused ectopy has an estimated incidence of 1:2000 according to autopsy data; however, the prenatal incidence has not been described. An estimated 90% are fused to their ipsilateral mate¹.
• Horseshoe kidney can occur in 1:400 individuals, with a higher prevalence in males and in fetuses affected with genetic conditions, such as Turner Syndrome and Trisomy 18³. Horseshoe kidney is found in 30% of cases of Turner syndrome and in 20% of Trisomy 18. No prenatal incidence has been reported, as horseshoe kidney likely goes undetected during prenatal life.

The aetiology is generally unknown. Some genes have been described in association with CAKUT (Congenital Anomalies of Kidneys and Urinary Tract) and, more specifically, with ectopic and fused kidneys^4,5. Other causes could be ureteric bud maldevelopment, defective metanephric tissue that fails to induce ascent during embryonic life, maternal pathology, and other genetic or teratogenic causes. A vascular barrier that prevents upward migration of kidneys secondary to the persistence of fetal blood supply has also been postulated¹.

Metanephros failure to normally ascend during early embryonic life (5 to 9 weeks of gestation) from the pelvis to the level of the second lumbar vertebra. Fusion defects are thought to occur when metanephric masses fuse before normal tissue upward migration¹.

The ectopic kidney is typically smaller than the orthotopic kidney, losing its reniform shape, and it may be associated with dysplasia, hypoplasia, and/or reduced renal function. Horseshoe kidneys are displaced and malrotated, with lower poles medially oriented and symmetrically or asymmetrically fused^1-3.

A higher incidence of other genitourinary abnormalities may be associated with renal ectopy^1,2. These include:

• Vesicoureteral reflux with hydronephrosis (80% of cases)
• Contralateral renal dysplasia
• Müllerian agenesis and Mayer-Rokitansky-Küster-Hauser syndrome or unicornuate/bicornuate uterus in girls
• Cryptorchidism and hypospadias in boys

Horseshoe kidney and crossed fused ectopy can be associated with other non-renal anomalies (adrenal, cardiac, skeletal, and cloacal abnormalities) or as part of a syndrome^2,5 These include:

• Caudal regression syndrome (sacral agenesis or hypoplasia, hypoplastic vertebral bodies, anal atresia)
• CHARGE syndrome (coloboma of the eye, heart disease, atresia choanae, restricted growth and development, genital hypoplasia, and ear anomalies)
• VACTERL syndrome (vertebral, anal, cardiac, tracheal, esophageal, renal, limb anomalies)
• Congenital diaphragmatic hernia (in case of thoracic ectopic kidney)
• Turner syndrome and Trisomies 13, 18, and 21³.

Horseshoe and ectopic kidney cases have been reported in the same families and with other anomalies such as unilateral renal agenesis, suggesting a higher risk than in the general population. Therefore, screening first-degree relatives for renal disease is recommended⁶. In general ectopic kidney, when isolated, does not increase the risk of recurrence. In the specific case of horseshoe kidney, when the finding is isolated or part of Turner syndrome the risk of recurrence is not increased; when part of Trisomy 18 there is a recurrence risk of 1%⁶.

Antenatal diagnosis is challenging but feasible by ultrasound. When associated with normal amniotic fluid volume, one empty renal fossa and a normal size contralateral orthotopic kidney suggest an ectopic kidney as the most probable scenario.

The ectopy can be suspected during the fetal anatomy scan; however, sometimes, it is challenging to spot the ectopic renal parenchyma in the abdomen or pelvis, thus often it is identified during postnatal assessment. Renal arteries can be easily identified with colour Doppler. In ectopic kidneys, the vascular supply is seen branching from the abdominal aorta in a more acute angle, oblique and caudally oriented, or directly from iliac arteries. Fetal MRI may be helpful if a pelvic mass is suspected².

Postnatal ultrasound confirms the antenatal diagnosis of renal ectopy and defines the underlying anatomy, including the presence of hydronephrosis¹.

• Unilateral renal agenesis. In this case, renal artery is absent, and the contralateral kidney commonly presents hypertrophic as a compensatory mechanism. This is not typically seen in cases of ectopic or fused kidneys².
• Unilateral renal hypoplasia. The kidney can be visualized after intensive effort and the renal artery, though usually hypoplastic, can also be visualized.
• Pelvic mass (complex ovarian cyst, intra-abdominal sacrococcygeal teratoma)².

• Crossed fused ectopy is diagnosed by the sonographic finding of the normal kidney’s lower pole fused to the ectopic kidney’s upper pole. The sonographic clue is the empty renal fossa on one side and abnormal renal contour morphology of the contralateral kidney due to the fusion of poles with a “stacked kidney” appearance².
• Horseshoe kidneys can be easily missed on antenatal ultrasound, yet the medially rotated renal pelvises seen on transverse view along with sagging of the renal artery on coronal view are classic sonographic features⁷. The measurement of the renal pelvic angle on axial view has been described as a method to strongly support horseshoe kidney diagnosis. The isthmus may be visible on coronal view; however, it may be obscured if the bridging tissue is thin or by superposed bowel loops. The ureters are normally implanted in the bladder; however, there is always risk of late onset hydronephrosis.
• As pregnancy advances in the third trimester, renal parenchyma, vascularization, and shape are most easily recognized. The final contour of the fused kidney depends on the time, extent of fusion, and degree of renal rotation that has occurred. If the medial portion of the renal parenchyma is fused completely, the kidney loses its horseshoe appearance and appears as a flattened disc (“pancake”) or lump kidney, described as a round renal mass in the pelvis^1,9.

When an empty renal fossa is noted at the morphology scan, it should trigger screening for an ectopic kidney, most commonly located in the pelvis adjacent to the iliac artery or aortic bifurcation. Colour Doppler is of great value when searching for renal vascular supply. A vascular stalk of variable origin (aorta or iliac artery) should be seen feeding the bulk of the renal mass.

If renal ectopy or fusion is diagnosed, other anatomic anomalies should be ruled out by a targeted scan, especially in the case of horseshoe kidney^2,3.

Briefly imaging of maternal kidneys to check for gross renal anomalies is proposed if the sonographer feels confident to do so. The probe’s movement in this case is sliding up and laterally towards the maternal lumbar area.

Vesicoureteral reflux is the most common complication of ectopic or fused kidneys, and it can occur in different stages^5,9. Hydronephrosis is reported in approximately 70% of children with horseshoe kidney. Causes of hydronephrosis include: vesicoureteral reflux or obstruction of the collecting system by ureteropelvic junction obstruction, renal calculi, or external ureteric compression by an aberrant vessel^1,2. 

The majority of children with renal ectopy or horseshoe kidney are generally asymptomatic. In the absence of other anomalies, the prognosis of ectopic kidney is excellent¹.

Once other anomalies are ruled out, serial antenatal follow-up scans are not obligatory. However, monthly scans during the last trimester could be of value as they may detect late onset hydronephrosis. Pediatric urology or nephrology consultation and postnatal ultrasound to confirm the diagnosis are advised. Postnatal follow-up is also advised because of the risk of hydronephrosis, nephrolithiasis, and infections.

Folic acid supplementation has been shown to decrease folate-sensitive congenital anomalies like congenital urinary tract defects^10,12.

[1] Campbell, Walsh, and Wein, Urology, 12th ed. Elsevier, 2021.

[2] P. Woodward, A. Kennedy, R. Sohaey, J. Byrne, K. Oh, and M. Puchalski, Diagnostic Imaging: Obstetrics, 3rd ed. Elsevier, 2016.

[3] D. Bianchi, T. Crombleholme, M. D’Alton, and F. D. Malone, Fetology, 2nd ed. McGraw Hill Professional, Inc., 2010.

[4] S. Jain and F. Chen, “Developmental pathology of congenital kidney and urinary tract anomalies,” Clin. Kidney J., vol. 12, no. 3, pp. 382–399, 2019.

[5] M. Zajicek et al., “Crossed ectopic kidney: prenatal diagnosis and postnatal follow-up,” Prenat. Diagn., vol. 37, no. 7, pp. 712–715, 2017.

[6] E. McPherson, “Renal anomalies in families of individuals with congenital solitary kidney,” Genet. Med., vol. 9, no. 5, pp. 298–302, 2007.

[7] J. Y. Cho, Y.-H. Lee, A. Toi, and B. Macdonald, “Prenatal diagnosis of horseshoe kidney by measurement of the renal pelvic angle,” Ultrasound Obstet. Gynecol., vol. 25, no. 6, pp. 554–558, Jun. 2005.

[8] S. Perlman et al., “OP06.04: Fetal pancake kidney: prenatal diagnosis and postnatal follow‐up,” Ultrasound Obstet. Gynecol., vol. 54, no. S1, pp. 102–102, Oct. 2019.

[9] G. Krzemień, M. Roszkowska-Blaim, I. Kostro, J. Wojnar, M. Karpińska, and R. Sękowska, “Urological anomalies in children with renal agenesis or multicystic dysplastic kidney,” J. Appl. Genet., vol. 47, no. 2, pp. 171–176, Jun. 2006.

[10] R. D. Wilson, “SOGC No. 324 -Pre-conception Folic Acid and Multivitamin Supplementation for the Primary and Secondary Prevention of Neural Tube Defects and Other Folic Acid-Sensitive Congenital Anomalies.,” J Obs. Gynaecol Can, vol. 37, no. 6, pp. 534–549, 2015.

[11] Y. Goh, E. Bollano, T. Einarson, and G. Koren, “Prenatal multivitamin supplementation and rates of congenital anomalies: a meta-analysis.,” J Obs. Gynaecol Can, vol. 28, pp. 680–9, 2006.

[12] A. Czeizel, “Reduction of urinary tract and cardiovascular defects by periconceptional multivitamin supplementation.,” Am J Med Genet, vol. 62, pp. 179–83, 1996.

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