Renal fusion anomalies were first described by Wilmer in 1938, with the classification being expanded by McDonald and McClellan in 1957. Classification is based on characteristics such as crossed or uncrossed, and fused or unfused. They may be further subdivided, as described below.
The term pelvic kidney encompasses a range of anatomical abnormalities when the kidney fails to rise from the pelvis in its metanephros stage during embryogenesis. Most cases are asymptomatic, although they are generally associated with higher risks for traumatic injury, urinary tract infections, renal calculi, and other urological problems. They may also complicate other surgeries, such as for aortic aneurysms.
Congenital renal anomalies are among the most common birth deformities, exceeded only by cardiac and skeletal defects. Of all the different renal fusion anomalies, the horseshoe kidney is the most common, while a pancake or lump kidney is the rarest.
An ectopic kidneys' vascular supply is not consistent, and they may receive vascular access from a range of vessels as the fetal blood supply can be retained. Multiple vascular sources may supply the ectopic kidney. The iliac arteries, direct branches from the aorta, mid sacral vessels, or the hypogastric arteries have all been found supplying ectopic kidneys. Understanding this anatomy is essential for any surgeon operating on a patient with an ectopic kidney.
Ectopic kidneys are also associated with several other congenital abnormalities. This may be in the pelvis, such as Mullerian agenesis or unicornuate uterus in females. Ectopic kidneys can be a feature of multisystem congenital syndromes such as CHARGE syndrome (coloboma, heart disease, atresia choanae, retarded growth, genital hypoplasia, and ear abnormalities) or VACTERL malformations (vertebral, anal, cardiac, tracheal, esophageal, renal, and limb anomalies).
The kidney develops between weeks 6 and 8 after conception, and the embryologic kidney rises from the pelvis into the lumbar region in the 9th week. If the kidney fails to pass above the fork of the umbilical arteries, the blood supply degenerates, or there are other factors inhibiting renal migration, then the kidney fails to rise to its normal anatomical location and instead becomes ectopic.
The exact location can be varied, with most cases being in the contralateral pelvis, but in the cases of crossed renal ectopia, both kidneys can be on the same side of the spine or, more rarely, the kidney can be outside the pelvis or retroperitoneal space entirely and even become located within the thorax.
There are 6 anatomical subtypes of crossed fused ectopic kidneys that have been described.
Zhuo Yin et al. (2014) also report an additional subtype they termed "Y type" with a fusion of the ureters.
Ectopic renal units are usually a unilateral condition, but there are documented cases of bilateral ectopic kidneys. This differs from a horseshoe kidney, which has the inferior poles of both kidneys fused together and is usually within the retroperitoneal space.
As a rare subset of fused ectopic kidneys, a pancake kidney occurs when there is a fusion of the kidneys along their respective medial aspects leading to a round, discoid renal mass. There may be a central hole (donut kidney), or the medial aspect can fuse entirely. However, there are usually still two separate collecting systems with anterior, medial lying renal pelves that do not communicate with each other. Pancake kidneys are found more often in males (2.5:1) with the most common age for detection between 30 and 60 years.
The incidence quoted is variable worldwide but is often approximately 1 in 1000 births. A retrospective study of 13,701 antenatal scans in Turkey found an incidence of pelvic kidneys of 1 in 571, although this study only included scans with a normal amniotic fluid volume.
A Taiwanese study screening 132,000 school children found a lower incidence of only 1 in 5000, but it is thought this may have underestimated the actual incidence due to the screening method used. An ectopic, pelvic kidney is the most likely finding in a fetus where the prenatal ultrasound finds an absent or missing renal fossa, but the amniotic fluid is normal.
During normal renal development, the metanephros starts at 5 to 6 weeks after conception. The metanephros would be within the caudal pelvis and migrates from this position to the lumbar region by week 8. If the kidney does not enter the retroperitoneal fossa, it is termed ectopic, and if it remains within the pelvis, it is deemed a pelvic kidney. In very rare instances, the ectopic kidney may be in the thorax. This is usually associated with a diaphragmatic hernia.
Even in asymptomatic patients, the ectopic kidney often has reduced function relative to the contralateral kidney. Most patients with ectopic kidneys are asymptomatic, and if recognized at all, the diagnosis tends to be an incidental finding while investigating other pathology or on routine antenatal ultrasonography. However, urinary tract complications can develop, and patients may present with a range of pathologies, including increased incidence of urinary tract infections, ureteropelvic junction obstruction in the ectopic kidney, or increased risk of renal calculi. The most common associated abnormality is vesicoureteric reflux, which occurs in 30% of patients with simple renal ectopia.
Pancake kidneys are often associated with other congenital defects such as vaginal agenesis, undescended testes, tetralogy of Fallot, spina bifida, sacral agenesis, and strabismus. They are more likely to develop hydronephrosis, renal trauma reflux, and stones than normal kidneys.
The majority of cases of isolated renal ectopy are diagnosed as incidental findings in asymptomatic patients. For example, in a case series published by Guardino et al. (2004), 79 of the 99 patients included were asymptomatic at diagnosis. Of those that were symptomatic, 12 were found to have urinary tract infection. In the aforementioned case series, the physical findings were abdominal pain (4 patients), palpable abdominal mass (2 patients), hematuria, and urinary incontinence (1 patient each).
A small or even tiny ectopic pelvic kidney may rarely cause primary, persistent, continuous urinary incontinence in a female. It may sometimes be too small to be seen by ultrasound alone and may require a renal nuclear scan or magnetic resonance imaging with contrast to identify the associated hydronephrotic ureter and tiny ectopic renal tissue. Therefore, a high degree of suspicion should accompany any female patient with primary, continuous incontinence. There may even be compensatory hypertrophy of the contralateral renal unit. Treatment is by surgical removal of the ectopic pelvic kidney, usually laparoscopically or robotically.
On antenatal ultrasound, an empty renal fossa would be seen, and an ectopic kidney would be the most common cause of this finding, especially if the amniotic fluid is normal.
Investigation and evaluation of pelvic kidneys vary somewhat depending on the center. However, ectopic kidneys should always have initial, postnatal imaging to assess for hydronephrosis and to visualize the anatomy of the contralateral kidney. If there are no further positive radiological findings, such as hydronephrosis and normal renal biochemistry (creatinine), further action may not be needed. However, some advocate for serial ultrasounds to investigate renal growth as well as for the early detection of hydronephrosis and calculi.
In the event of severe hydronephrosis or urinary tract infections, a voiding cystourethrogram (VCUG) may be indicated. If this is normal, further evaluation with a mercaptoacetyltriglycine-3 (MAG-3) or diethylentriamine pentaacetic acid (DTPA) renal nuclear scan imaging should be considered, especially if the serum creatinine is elevated as an obstruction is likely. However, if there is only mild to moderate hydronephrosis, a follow-up study in 3 to 6 months to determine if there is a progression in the hydronephrosis and adequate renal tissue growth is reasonable and recommended. If abnormal progression is found, additional imaging and nuclear scans should be done to look for a ureteric obstruction.
If the contralateral kidney is anatomically abnormal or if there is impaired renal biochemistry, then the differential renal function should be assessed with a DMSA scan and specialist advice sought. Guardino et al. found that in 82 cases of a simple ectopic kidney, 74 exhibited reduced function in the ectopic kidney relative to the contralateral kidney on DMSA imaging. In that same case series published by Guardino, the incidence of concomitant pathology was found to be high with an ectopic kidney. Twenty percent of the cases with crossed renal ectopy exhibited vesicoureteral reflux and thirty percent of those with simple renal ectopy; in bilateral cases, it was seventy percent. Other abnormalities seen in smaller numbers included renal dysplasia in the anatomically normal kidney, one of which had renal agenesis. Cryptorchidism and hypospadias were seen in 5 cases each. Hydronephrosis was seen in 3 of the ectopic kidneys and 8 of the anatomically normal kidneys. In one of the ectopic kidneys, this was associated with ureteropelvic junction obstruction.
Given that pelvic kidneys are associated with additional pathology both inside and outside the urinary tract, such as VACTERL anomalies, it is important to assess patients for this and other congenital defects once the ectopic kidney diagnosis is made.
In simple pelvic kidneys without complications, no intervention is required. However, if there are complications such as renal calculi, the distorted anatomy may make the condition more difficult to manage, and it is, therefore, important to understand the anatomy and vasculature of pelvic kidneys.
In the event a patient develops renal calculi in the ectopic, pelvic kidney, operative intervention is much more complicated than in an anatomically normal renal unit due to the different location of the ureters. Laparoscopic-assisted percutaneous transperitoneal nephrolithotomy has been used to successfully treat stones too large for shockwave lithotripsy, which is more effective in renal stones under 2 cm in diameter. This procedure may require the mobilization of the colon, and there is a risk of urine leak into the abdomen, but it is an option if the stone is not contained within the renal pelvis and calyces. Mobilization of the bowel can be avoided with a transmesocolic pyelolithotomy or with robot-assisted surgery, which has the added benefit of being able to remain within the retroperitoneal space reducing the risk of abdominal urine leak and urinoma formation.
Specific problems with ectopic kidneys can be dealt with individually, if necessary. Separating a fused renal mass is not recommended due to the potential for vascular injury, renal infarction, tissue necrosis, and decreased renal function.
In general, significant surgery is discouraged in ectopic kidneys except for specific problems, and most cases can be managed conservatively. Percutaneous access to relieve an obstruction or for percutaneous nephrolithotomy may be more complicated or even impossible in some cases depending on the individual anatomy, and this needs to be taken into consideration. 24-hour urine testing for nephrolithiasis prophylaxis is recommended in patients that have had nephrolithiasis with ectopic kidneys due to the increased complexity of kidney stone surgery in these individuals and the risk of loss of renal function.
In the absence of other pathology, ectopic kidneys are rarely associated with disordered renal function or hypertension. Van den Bosch et al. (2010) published a series of 41 patients, all of whom underwent VCUG with 13 demonstrating vesicoureteric reflux. The relative function of the ectopic kidney was 38% on DMSA scanning, and 88% had an eGFR >90. These findings were consistent in both simple and crossed renal ectopia.
However, when there is malrotation and associated extrarenal calyces, these patients are at increased risk of recurrent urinary tract infection and hydronephrosis. Their incidence of vesicoureteric reflux and renal failure is also higher than the general population.
Pancake kidneys, like other ectopic kidneys, are usually asymptomatic but are also associated with an increased risk of recurrent urinary tract infection and stone formation. This increased risk of nephrolithiasis in renal ectopia is due to reduced urinary flow through a more tortuous ureter and the vascular anomalies that are associated with the condition.
A review of crossed fused renal ectopia published in 2019 found only 35 reported cases of stones and 30 cases of malignancy associated with the condition. Their function can be sufficient to use them as donor organs for renal transplant.
Therefore, while renal ectopia is not an entirely harmless condition, the overall prognosis is generally good.
As it has already been stated, most patients with pelvic kidneys have a good prognosis; however, the distorted anatomy of their ureters can impair urinary flow. This is associated with an increased incidence of vesicoureteric reflux and renal calculi formation. Vesicoureteric reflux, in turn, is associated with recurrent urinary tract infection and chronic kidney disease.
Ectopic kidneys are associated with aberrant ureteric anatomy. The course of the ureter can be more tortuous and the point of insertion higher in the kidney, impairing the flow of urine through the urinary tract. This can lead to the back-flow of urine, causing vesicoureteric reflux (VUR) or dilatation of the renal pelvis and calyces, resulting in hydronephrosis. The reported incidence of these complications is variable, with most studies showing they occur infrequently. Hydronephrosis can be diagnosed with ultrasound imaging; however, vesicoureteric reflux requires further investigation. The gold standard is voiding cystourethrography (VCUG). As vesicoureteric reflux is associated with recurrent urinary tract infections and renal impairment, a VCUG study should be considered in this patient cohort.
Calisti et al. (2008) published a series including 50 patients with a single ectopic kidney, none of whom had significant concomitant pathology. However, there are reports where the incidence of pathology is higher. Gleason et al. (1994) published a series of 77 patients with 82 ectopic kidneys from the Mayo Clinic. They found 56% had hydronephrosis, and 26% had vesicoureteric reflux.
The exact incidence of renal calculi in pelvic kidneys is unknown but is thought to be higher than the general population due to altered anatomy and impaired urinary flow rates. The altered anatomy does have significant implications in the operative management of renal calculi, and the risk of vascular injury is increased relative to the general population.
Malignancy in pelvic kidneys is rare, and the risk of malignancy is not generally thought to be significantly higher than anatomically normal kidneys. However, there are some reported cases, and it has been suggested by some experts that patients with ectopic fused renal anomalies are more likely to develop Wilms tumor or renal cell carcinoma than the general population. Patients with horseshoe kidneys, for example, are twice as likely to develop Wilms tumor than age-matched cohorts with normal anatomy. Like operative management of renal calculi, special consideration must be taken given the variable nature of the renal blood supply should such patients require surgery.
The nature of a consultation of a patient with renal ectopia will vary greatly depending on the nature of the presentation. In cases where it is an incidental finding, the consultation will likely be primarily focused on the initial pathology under investigation. In cases where renal ectopia is diagnosed as a result of a direct complication, such as vesicoureteric reflux or renal calculi, this will require more direct investigation. Particularly in cases of vesicoureteric reflux, these patients may need ongoing follow-up, given the risk of chronic kidney disease.
Given the implications for treatment options, it is important that patients with an ectopic kidney understand this and communicate with the clinicians delivering their care. This is particularly important in patients who develop long term complications such as vesicoureteric reflux as this may have long term health implications as well as an increased risk of developing stones, urinary tract infections, and chronic kidney disease.
Ectopic kidneys are often incidental findings, so they will often be found by non-urologists. Given the relatively uncommon nature of the condition with even fewer cases presenting clinically, most of the studies are small case series without adequate controls. [Level 5] It is therefore important that all clinicians are aware of the potential complications and associated conditions so that patients with an ectopic kidney can be assessed and referred appropriately.
An interprofessional team approach is needed to do this effectively with good communication between the diagnosing clinician and, if needed, pediatric urologists in antenatally diagnosed cases who would continue care after delivery. In the case of a postnatal diagnosis, the clinician would also need to arrange appropriate follow up. Urology should be involved early to help with diagnosing specific problems associated with ectopic pelvic kidneys and to correct them if necessary. This is of particular importance if the patient develops long term complications such as chronic kidney disease or stones. Patients need to be empowered to take ownership of their care and fully understand their condition and treatment options.
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