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ectopic kidney

Ectopic kidney

Ectopic kidney also called “renal ectopia”, is a developmental renal anomaly characterized by one or both of the kidneys that isn’t located in its proper location 1. Ectopic kidneys don’t move up to the usual position. They can be located anywhere along the path they usually take to get to their normal place in the upper abdomen.

One may also cross over so that both kidneys are on the same side of the body. When a kidney crosses over, the two kidneys on the same side often grow together and become fused.

  • Simple ectopic kidney refers to a kidney that’s located on the proper side but in an abnormal position.
  • Crossed ectopic kidney refers to a kidney that has crossed from its side, to the other side. Both kidneys are located on the same side of the body. Crossed ectopic kidney is characterized by the ureter of the ectopic kidney crossing the midline before entering the bladder on the expected side 2. These kidneys may or may not be connected. Crossed renal ectopia is more common for the left kidney to be ectopically located on the right side. More than 85% of these get fused resulting in crossed fused renal ectopia. Less than 15% cases are non-fused. Most crossed renal ectopias are associated with fusion and left to right cross-over 3. Crossed renal ectopia is usually asymptomatic and incidentally found on imaging workup. It is usually seen fixed to iliac crest, but below the level of L2.
    • Crossed fused renal ectopia refers to an anomaly where the kidneys are fused and located on the same side of the midline. Cross fused renal ectopia is a rare anomaly with the estimated incidence is around 1 out of 1000 births 4. There is a recognized male predominance with a 2:1 male to female ratio. More than 90% of crossed renal ectopia results in fusion.

McDonald and McClellan’s classification of crossed renal ectopias 5:

  1. bilateral crossed renal ectopia without fusion
  2. unilateral crossed renal ectopia
  3. bilaterally crossed renal ectopia: represents 90% of all crossed ectopias and includes crossed fused renal ectopia
  4. crossed unfused renal ectopia

McDonald and McClellan also described six forms of crossed fused renal ectopia in decreasing order of frequency 5:

  • type a: inferior crossed fusion, the upper pole of the ectopic kidney fuses with the lower pole of the normal kidney
  • type b: sigmoid or S-shaped kidney, the hilum of the ectopic kidney faces laterally and that of the normal kidney faces medially
  • type c: lump kidney, both kidneys fuse over a wide margin with the ureter of the ectopic kidney crossing midline
  • type d: L-shaped kidney, a.k.a. tandem kidney, in which the ectopic kidney is horizontally oriented and fused with the lower pole of the normal kidney
  • type e: disc/pancake kidney, with extensive fusion of both kidneys forming a disc-shaped mass
  • type f: superiorly crossed fused, the ectopic kidney is placed above the normal kidney and fused with its upper pole

Ectopic kidney is often linked to birth defects in other organ systems. Embryologically, renal ectopia occurs in the first trimester as a result of an abnormal development of the ureteric bud and metanephric blastema between 4 and 8 weeks’ gestation 1. Mechanism involves arrest during kidney’s migration, migration beyond normal limit, metanephric ectopia, contralateral metanephros induced by wandering ureteric bud or duplex Wolffian ducts 1.

Ectopic kidney may be associated with the following abnormalities:

  • Contralateral renal agenesis
  • Bilateral ectopia
  • Genital anomalies such as bicornuate or unicornuate uterus
  • Absent uterus
  • Duplicate or rudimentary vagina
  • Undescended testes
  • Hypospadias
  • Duplicate urethra
  • Rarely adrenal anomalies.
  • Cardiac and skeletal anomalies are more common

Ectopic kidneys are thought to occur in about 1 out of 900 births. But only about 1 out of 10 of these are ever diagnosed 6. They may be found while treating other conditions. While most patients are asymptomatic, ectopic kidneys carry increased risk of urinary calculus formation and hydronephrosis 7.

Figure 1. Ectopic kidney ultrasound (crossed fused ectopic kidney)

crossed fused ectopic kidney ultrasound

Footnote: 34 year old male with abdominal pain. Both kidneys are located on the right side of the midline and show fusion at their poles with renal sinuses directed away from each other. The left renal fossa is empty. Features suggest a sigmoid type of Crossed fused renal ectopia.

[Source 8 ]

Ectopic kidney causes

Ectopic kidney occurs during fetal development. Kidneys, in general, develop as outgrowths or buds within the pelvis before they move upwards to their position at the rear end of the rib cage. The upward movement of the kidney occurs between the 4th and 8th weeks of embryo development. ‘Ectopic’ usually refers to an organ that is out of place. When one of the kidneys remains in the pelvis, or moves upward but fuses with the second kidney, or moves higher than its normal position, an ectopic kidney is formed. Some of the causes for this defect in movement are:

  • Genetic defects
  • Defects in the kidney tissue that provides the signal to move upward from the pelvis
  • Exposure of the pregnant mother to a drug, or a chemical, or an illness that results in defects in kidney development in the baby
  • Under-developed kidney bud

Ectopic kidney results as a consequence of abnormal renal ascent in embryogenesis with the fusion of the kidneys within the pelvis. It is thought to occur in the first trimester, at around 4th-8th week of fetal life (in a normal situation the kidney reaches its appropriate position at the L2 level at the end of the 2nd month).

Some evidence supports that an abnormally situated umbilical artery prevents normal cephalic migration. Another theory is that the ureteric bud crosses to the opposite side and induces nephron formation in the contralateral metanephric blastema. The result is a single renal mass with two collecting systems being located on one side of the abdomen.

Normal ascent of the kidneys is required for formation of the extraperitoneal perirenal fascial planes and therefore ectopia (or renal agenesis) results in failure of development of fascial layers in the flanks on the side not occupied by renal tissue. The lack of restraining fascia leads to possible malposition of bowel into the extraperitoneal fat of the empty renal fossa and relaxation of mesenteric supports for bowel loops in this region.

The precise mechanism of development of renal fusion anomalies is not fully understood and several theories have been put forward to explain the anomaly :

  • The Mechanical Theory proposes that during cephalad migration, the kidneys pass through the fork between the two umbilical arteries and any positional change in these arteries squeeze the kidneys close together allowing their fusion. Fusion of both nephrogenic blastemas with early arrested migration result in completely fused pelvic kidney. Abnormal position of an umbilical artery can result in abnormal migration of a renal unit to the contralateral side following the path of least resistance (crossed renal ectopia).
  • The Theory of Abnormal Caudal Rotation proposes that fusion occurs due to lateral flexion and rotation of the caudal end of the embryo disturbing the relative position of the nephrogenic blastema and ureteric bud. The distal curled end of the vertebral column permit one ureter to cross the midline and enter the opposite nephrogenic blastema or transplant the kidney and ureter to the opposite side during ascent. Association of scoliosis with crossed renal ectopia supports this theory.
  • The Ureteral Theory states that cross over is strictly a ureteral phenomenon with the developing ureteral bud wandering to the opposite side and inducing the differentiation of the contralateral metanephric blastema and it is assumed that the metanephric tissue that does not receive a ureteric bud regresses.
  • The Teratogenic Theory suggest that renal ectopia results from abnormal migration of posterior nephrogenic cells due to teratogenic insult forming a parenchymal isthmus. The increased incidence of malignancies and other organ system anomalies associated with renal ectopia possibly supports this theory.
  • The Genetic Theory suggests that genetic influence may play a role because some renal fusion anomalies have been reported to occur in identical twins and siblings within the same family. It is suggested that the sonic hedgehog gene signal is critical for kidney positioning along the mediolateral axis and its disruption will result in renal fusion.

Ectopic kidney symptoms

The most common symptoms linked to the ectopic kidney are:

  • Urinary Tract Infections (UTI)
  • Belly pain
  • A lump in the abdomen

A kidney in an abnormal spot may still work properly. But because of the change, it may have problems draining. Up to 1 out of 2 ectopic kidneys are at least partly blocked. Over time, these blockages can lead to serious problems, such as:

  • Urinary Tract Infections (UTIs)
  • Kidney Stones
  • Kidney Failure

Ectopic kidneys are also linked to vesicoureteral reflux (VUR ). Vesicoureteral reflux is a condition where urine backs up from the bladder through the ureters into the kidneys. Over time, vesicoureteral reflux can lead to infections. Infections can cause damage to the kidney that can’t be fixed. The non-ectopic kidney can also have problems like blockages or vesicoureteral reflux.

Ectopic kidney complications

In a crossed fused fused ectopic kidney, complications such as nephrolithiasis, infection, and hydronephrosis approaches ~50%.

Ectopic kidney diagnosis

Only about 1 out of 10 ectopic kidneys are ever diagnosed. Often, health care providers find ectopic kidneys while treating other conditions. A health care provider may also find them when looking for the cause of symptoms mentioned earlier. These imaging tests could help your health care provider find an ectopic kidney:

  • Ultrasound
  • Intravenous Pyelogram (IVP)
  • Voiding Cystourethrogram (VCUG)
  • Radionuclide Scan
  • Magnetic Resonance Imaging (MRI)

Your health care provider may also order blood tests to see how well your kidneys are working.

Ectopic kidney treatment

An ectopic kidney only needs to be treated if there’s a blockage or urine backing up into the kidney (VUR).

If the kidney isn’t badly damaged by the time the abnormality is discovered, surgery can remove the blockage or fix VUR. But if the kidney is badly scarred and not working well, it may need to be removed.

It’s possible to live a normal life after a kidney is removed if the remaining kidney is working well.

References
  1. Alfaseh A, Ilaiwy A. RCC in cross ectopic kidney: a challenging diagnosis and management. Case Reports 2018;2018:bcr-2018-226879. http://dx.doi.org/10.1136/bcr-2018-226879
  2. Shapiro E, Bauer SB, Chow JS. Anomalies of the upper urinary tract. Campbell-Walsh Urology 2012;4:3145.
  3. Abeshouse BS, Bhisitkul I. Crossed renal ectopia with and without fusion. Urol Int 1959;9:63–91.doi:10.1159/000277442
  4. Dunnick NR. Textbook of uroradiology. Lippincott Williams & Wilkins. (2001) ISBN:0781723892
  5. MCDONALD JH, MCCLELLAN DS. Crossed renal ectopia. Am J Surg. 1957 Jun;93(6):995-1002 https://doi.org/10.1016/0002-9610(57)90680-3
  6. Ectopic kidney. https://www.urologyhealth.org/urologic-conditions/ectopic-kidney
  7. Boyan N, Kubat H, Uzum A. Crossed renal ectopia with fusion: report of two patients. Clin Anat 2007;20:699–702.doi:10.1002/ca.20464
  8. Crossed fused renal ectopia. https://radiopaedia.org/cases/crossed-fused-renal-ectopia-6
Health Jade Team

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