Department of Ob/Gyn, Division of Maternal-Fetal Medicine, Vanderbilt University School of Medicine, B-1100 Medical Center North, Nashville, TN 37232-2519 Ph: 615-322-0122, Fax: 615-343-8881

Synonyms: Ectopic kidney.

Definition: A kidney located outside the renal fossa and within the pelvis.

Prevalence: 8.3-11:10,000, M1:F1¹

Etiology: Failed migration of the ureteral bud and developing metanephric blastema to the renal fossa.

Pathogenesis: Unknown.

Associated anomalies: Contralateral renal agenesis, bilateral ectopia, genital anomalies such as bicornuate or unicornuate uterus, absent uterus, duplicate or rudimentary vagina, undescended testes, hypospadias, duplicate urethra, and, rarely, adrenal anomalies. Cardiac and skeletal anomalies are more common¹.

Differential diagnosis: Hydro­nephrosis, ureteropelvic junction obstruction, duplicate collecting systems, ovarian cysts, mesenteric cysts, or sacrococcygeal teratoma.

Prognosis: Good, in the absence of other serious anomalies. However, hydronephrotic complications and calculi are more common with renal ectopia.

Recurrence risk: Unknown, probably small.

Management: Search for coexisting anomalies, otherwise normal obstetrical care. Perform urologic evaluation following delivery.

Introduction

Ectopic kidneys have been found  in 1 out of 900 autopsies¹. Abnormal positions are described as pelvic, iliac, abdominal, thoracic, and crossed. Despite the fact that fetal kidneys are easily visualized on ultrasound, few case reports on the antenatal diagnosis of ectopic kidneys exist^2-4. The recognition of an ectopic kidney is important because of the association with other more serious anomalies.

Case report

A 22-year-old white female G₃P₁₀₁₁ was referred to the obstetrical ultrasound clinic at Vanderbilt University for evaluation of possible fetal renal anomalies. The patient had a 4-year-old child at home reported to be normal.

An ultrasound examination was performed which revealed a 25 week fetus, consistent with her stated gestational age. The kidneys were not seen within the renal fossae. A mass seen between the bladder and the sacrum was felt to represent a pelvic kidney (fig. 1, 2). The amniotic fluid volume was normal. Subsequent sonograms at 29 and 33 weeks confirmed this finding. The patient underwent a normal spontaneous delivery at term without complication. The infant was discharged  without complications.

Figure 1: The pelvic kidney is seen between the iliac vessels.

Figure 2: The pelvic kidney is seen between the iliac vessels in the upper portion of the image. The color Doppler demonstrates a renal

An ultrasound performed in the second week of newborn life confirmed the presence of a pelvic kidney as well as the absence of any normally located kidneys. The infant was doing well three months postpartum.

Discussion

Definition

Pelvic kidney is the most common example of renal ectopia. Other sites of ectopic kidneys include the iliac region, the abdomen, the chest, and, in some cases, the contralateral side, referred to as "crossed†(fig. 3).

Figure 3: Ectopic kidneys may be thoracic, crossed, iliac or pelvic.

Pelvic kidneys are located within the bony pelvis. Of great clinical importance is the fact that pelvic kidneys may represent the patient"s sole kidney, and the diagnosis must be considered in the evaluation of any unusual pelvic masses to prevent an unintended nephrectomy. Pelvic kidneys may also, on occasion, occur as fused midline horseshoe kidneys.

Incidence

The incidence of renal ectopia (1:900)¹ is similar in both sexes and may be bilateral in 10% of cases1. When unilateral, there is a slight predilection for the left side¹.

Prenatal diagnosis

Given the reliability of sonographic visualization of the fetal kidneys in the second trimester, it is surprising that out of four cases reported on the antenatal diagnosis of pelvic kidney, the gestational ages were all 28 weeks or greater2-4.The present case illustrates that visualization of an empty renal fossa as well as an ectopic kidney can be accurately diagnosed earlier in pregnancy.

Embryology

Begining in the fourth week of pregnancy, the early signs of kidney development can be seen. A ureteral bud separates from the wolffian duct and ascends toward the urogenital ridge. In the fifth week the metanephric blastoma develops, appearing above the migrating bud. A period of rapid caudal growth in the embryo appears to assist in migration of this structure out of the pelvis and into its eventual retroperitoneal location in the renal fossa. With ascension comes a 90^o rotation from a horizontal  to a vertical position with the renal hilum finally directed medially.

By the eighth week, migration and rotation appear to be complete. Factors which interfere with development such as teratogens, genetic factors, ureteral bud or metanephric maternal disease may result in abnormal migration of the kidney and renal ectopia. The vascular supply to the organ changes several times during the migratory event, and abnormal origins of renal arteries are related to when migration was arrested^1,5.

Associated anomalies

The documentation of fetal ectopic kidneys is important because it signals the need to search for associated anomalies which frequently involve the genitourinary, cardiac, and skeletal systems (Table 1).

Table 1: Associated anomalies.

Prognosis

The prognosis for children born with a pelvic kidney is excellent in the absence of coexisting anomalies or chromosomal disorders. Because the renal fossae can be visualized early in pregnancy, renal ectopia can potentially serve as a useful indicator of other serious anomalies. In the otherwise healthy newborn, pelvic kidneys are associated with an increased risk of hydronephrosis secondary to alterations in the course of the ureter. Renal injury from progressive hydronephrosis can be averted by timely urological evaluation.

Management

Prenatal: A thorough anatomic survey should be undertaken to look for coexisting anomalies. Particular attention should be paid to defining the fetal urinary system in its entirety since the contralateral kidney may be abnormal or absent. The genital system should be visualized, though its evaluation may be more difficult. Fetal echocardiography is essential to rule out serious, potentially life-threatening anomalies.

Postnatal: These children should be evaluated after delivery because of the increased incidence of hydronephrosis and parenchymal injury⁴.

References

1. Walsh P, Gittes R, Perlmutter A, et al; Campbell"s Urology, Second Volume, Fifth edition. Philadelphia, WB Saunders, 1986, p 1674-5.

2. King KL, Kofinas AD, Simon NV, Deardorff J. Prenatal diagnosis of fetal pelvic kidney: A case report. J Reprod Med 38:225, 1993.

3. Colley N, Hooker JG. Prenatal diagnosis of pelvic kidney. Prenat Diagn 9:361, 1989.

4. Hill LM, Peterson CS. Antenatal diagnosis of fetal pelvic kidneys. J Ultrasound Med 6:393, 1987.

5. Moore KL. The Developing Human: Clinically Oriented Embryology, 3rd ed. Philadelphia, WB Saunders, 1982, pp 255-271.