As the kidneys begin their development in the pelvis and migrate cranially during embryogenesis, errors may occur in rotation and the final position of the kidneys. Normally, the renal pelvices will be oriented anteriorly in the pelvis and will rotate 90° ventromedially as they rise. Lack of any rotation whatsoever, known as ventral position, is the most common form of malrotation. The rarest form is dorsal position, where there is an overrotation causing the pelvices to orient dorsal to the parenchyma. This causes the vessels to pass posterior to the kidney to enter into the hilum. Finally, when the pelvices are oriented laterally it may be due to rotation; either up to 180° in the reverse direction or a full 360°. The vasculature will indicate the direction of the rotation, with vessels entering anterior to the kidney indicating reverse rotation and those posterior to the kidney indicating excessive rotation.

Renal malrotation itself is asymptomatic but may be associated with ureteral or vascular compression depending on the degree of rotation. Consideration of the kidney’s position is important when planning surgery. This is particularly true with horseshoe kidneys, where the calyces will be oriented more caudal and dorsal than in a normal kidney.

In contrast, renal ectopia is a condition where the kidney is located anywhere from the pelvis to the diaphragm. The incidence is 1/1000 live births, and it is seen more commonly on the left side. ^, The most common ectopia is a pelvic kidney. The ectopic kidney is often malrotated, and in cases of pelvic kidneys the renal pelvis will be oriented anteriorly. The right kidney can migrate more cranially than its orthotopic position in cases of omphalocele when the liver herniates into the omphalocele sac, as there is no structure to prevent this migration. However, it will remain subdiaphragmatic. There are also rare reports of “thoracic kidney,” where the kidney is located supradiaphragmatic in the thoracic cavity. This results from either delayed development or closure of the diaphragm. The adrenal glands will be located in their orthotopic location in cases of renal ectopia as they have independent embryogenesis from the kidneys.

Renal vasculature is typically aberrant in ectopic kidneys and close attention must be given to this when planning surgical procedures. Renal ectopia is mostly asymptomatic, but may be associated with other urologic anomalies that cause symptoms and require treatment, such as hydronephrosis, ureterovesical junction obstruction (UPJO), vesicoureteric reflux (VUR, 30%–50%), contralateral renal dysplasia, cryptorchidism, hypospadias, vaginal or uterine agenesis, and others. ^,

Horseshoe kidney is the most common renal fusion anomaly, with an incidence of 1/500. Horseshoe kidneys are marked by two renal units fused together by an isthmus of functional renal parenchyma or fibrous tissue that may be oriented in the middle, or to one side or the other, resulting in an asymmetric kidney. This isthmus typically arrests the ascent of the kidney upon contact with the inferior mesenteric artery (IMA) at the level of L3 during embryogenesis; however, some horseshoe kidneys are found in the pelvis, well below the level of the IMA. Due to the failure of appropriate ascent, these kidneys are also malrotated, with the upper poles oriented more medial and posterior than the lower poles and the pelvis oriented ventromedial to normal. Each moiety maintains its own ureter, which courses over the isthmus. Fig. 52.2 demonstrates variations of horseshoe kidney with regard to location of vasculature.

Variations of horseshoe kidney.

From F.T. Graves. The arterial anatomy of the congenitally abnormal kidney. Br J Surg, 56 (1969), pp. 533-541

Horseshoe kidneys typically function normally. However, given their abnormal orientation, these kidneys are predisposed to issues such as UPJO (25%, typically with a high ureteric insertion), impaired drainage, hydronephrosis, ureteral duplication (10%), VUR (50%), nephrolithiasis, and malignancy. Treatment of nephrolithiasis in a horseshoe kidney may be more challenging. Increased skin-to-stone distance and presence of obstructing vertebral bodies may make shockwave lithotripsy less effective and there may be impaired drainage of the fragments. If attempting percutaneous nephrolithotomy (PCNL), care should be taken to target a superior calyx for access in order to maximize maneuverability of the nephroscope. Complication and stone-free rates are similar to normal kidney PCNLs. Ureteroscopy and laparoscopic/robotic pyelolithotomy also remain good options for stone clearance in horseshoe kidneys as well. The risk of malignancy is increased in horseshoe kidneys with incidence of renal cell carcinoma of 3–4 times baseline, Wilms tumor two-fold, and carcinoid tumor 62–82 times that of normal kidneys. Malignancies should be managed in a similar fashion to normal kidneys, with extra care taken to ensure location of aberrant renal vascular anatomy prior to surgical intervention.

Crossed fused renal ectopia is a rare anomaly that occurs when an ectopic kidney crosses the midline and fuses with a normal orthotopic kidney. The ectopic kidney’s ureter will typically be found in its normal orthotopic location of its original side, indicating a midline migration occurred (left-to-right migration is more common). These kidneys can also be associated with UPJO, VUR, ureteral abnormalities such as stricture, and renal dysplasia.

Renal dysplasia is the abnormal development of the kidney affecting size, shape, structure, and often function. The cause of renal dysplasia may be either a primary failure in development of the nephrons and glomeruli leading to primitive tubules, interstitial fibrosis, renal cysts, or cartilage in the parenchyma, or secondary to congenital urinary tract obstruction (e.g., posterior urethral valves [PUV], ureteropelvic junction obstruction, or vesicoureteral reflux). ^, Dysplastic kidneys are small, have loss of corticomedullary differentiation, and appear hyperechogenic compared to the liver on ultrasound. Renal hypoplasia refers to kidney size that is less than two standard deviations below the mean for age. These kidneys typically have normal morphology but a fewer number of nephrons. Two types of renal hypoplasia are clinically relevant: oligomeganephronic type and Ask-Upmark kidney.

Oligomeganephronic-type kidneys have a reduced number of nephrons and those present demonstrate hypertrophy. This condition results from a failure of metanephric blastema to complete development. Boys appear to be affected more than girls, and it is associated with low birth weight <2500 g. These patients present with polyuria and hyposthenuria (secretion of dilute urine/inability to concentrate urine normally) and are typically normotensive. Kidneys appear small on ultrasound, and the diagnosis is made by renal biopsy. These patients are treated medically with protein restriction and increased fluid and salt intake. Angiotensin-converting enzyme inhibitors (ACEi) can potentially slow renal failure. This condition is progressive, and these patients eventually require dialysis and renal transplantation.

Ask-Upmark kidney is also known as segmental renal hypoplasia. This is thought to result from chronic atrophic pyelonephritis caused by severe acquired reflux nephropathy. These kidneys are also small and biopsy demonstrates absent glomeruli, atrophic tubules, and hyperplastic vasculature. Girls are affected more than boys. These patients typically present in adolescence or early adulthood with severe hypertension (up to half of patients demonstrate retinopathy) and VUR. Renal atrophy can continue after surgical repair of VUR even if there are no further episodes of pyelonephritis. Partial or total nephrectomy has been shown to control hypertension in unilateral disease. Bilateral disease is managed medically with dialysis and subsequent transplantation.

Renal hypodysplasia is secondary renal architectural change leading to renal insufficiency due to a variety of other congenital urologic diseases, such as primary obstructive megaureter, ureterocele, PUV or other urethral obstruction, ureteral ectopia, VUR, prune-belly syndrome, or others. These will be discussed elsewhere.

Complete absence of one or both renal units is termed renal agenesis. This can be due to failure of the ureteric bud to form the ureter and collecting system structures or by involution of a multicystic dysplastic kidney (MCDK). The presence or absence of a ureter can help distinguish etiology of the missing renal unit, with absence of a hemitrigone implying failure of appropriate ureteric bud formation. A normal trigone, often with a ureteric nubbin, implies involution of an MCDK.

Unilateral renal agenesis occurs in 1 per 2000 live births with a 2:1 male predominance and occurs more frequently on the left side. Compensatory hypertrophy of the contralateral kidney is often identified in utero. Unilateral renal agenesis is often an isolated, clinically silent congenital anomaly; however, it can be associated with other conditions affecting the contralateral kidney, and even extrarenal anomalies. UPJO and VUR are often seen in the contralateral kidney. Cardiac, gastrointestinal, musculoskeletal, dermatologic, endocrine, gynecologic, and ear, nose, and throat anomalies have also been associated. Care should be taken to assess for signs of VACTERL, a congenital syndrome with Vertebral anomalies, Anal atresia, Cardiac malformations, Tracheo-Esophageal fistula with esophageal atresia, Renal anomalies, and Limb abnormalities. Up to 24% of children diagnosed with VACTERL demonstrate unilateral renal agenesis.

Males with unilateral renal agenesis are at risk for abnormal Wolffian structures, given the embryologic origin of the ureteric bud from the mesonephric/Wolffian duct. The vas and seminal vesicles may be absent, or the seminal vesicle may be present as a cyst, but this will only occur on the ipsilateral side of the absent kidney. The contralateral testis is typically normal. In cases of involution of an MCDK, the seminal vesicle/vas deferens will be present. Studies have demonstrated that in patients with congenital unilateral absence of the vas deferens (CUAVD), up to 43% have a cystic fibrosis transmembrane conductance regulator (CFTR) variant, which is the gene responsible for cystic fibrosis. If a vas is found to be abnormal or absent during a hernia repair or orchiopexy, the surgeon should evaluate the kidneys with a renal ultrasound and consider CFTR testing. Zinner syndrome is a condition with seminal vesicle cysts and ejaculatory duct obstruction associated with ipsilateral renal agenesis. ^, These cysts are typically asymptomatic, but may cause dysuria, perineal pain, epididymitis, or pain with ejaculation as they enlarge. Infertility occurs in up to 45% of cases as a result of obstructive azoospermia, although the mechanism is unclear. In some cases in adults, transurethral resection of the ejaculatory duct can improve the quality of the semen in an effort to treat infertility. Vesiculectomy may be performed in those that have bothersome symptoms or if the cysts are felt to contribute to infertility. This may be done using a transvesical, retropubic, perineal, or transrectal approach.

The female equivalent of Zinner syndrome is Mayer-Rokitansky-Kuster-Hauser (MRKH) syndrome. This is a congenital disorder where the uterus and vagina fail to develop properly in 46XX females. These females will develop secondary sexual characteristics and have normal female external genitalia and ovaries, but demonstrate abnormalities of Mullerian duct structures. This typically includes aplasia or hypoplasia of the uterus and the upper two-thirds of the vagina. Up to 40% of females with MRKH will have upper urinary tract malformations, including unilateral renal agenesis (23%–28%). These patients typically present with primary amenorrhea. Magnetic resonance imaging (MRI) can establish the diagnosis. Box 52.1 demonstrates the differences in male and female patients with unilateral renal agenesis.

Bilateral renal agenesis occurs in 1 in 3000–4000 pregnancies and is usually considered a fatal condition. Prenatal ultrasound shows oligohydramnios, pulmonary hypoplasia, and Potter’s faces (low-set ears, a broad flat nose, prominent skin-fold beginning over the eye and extending to the cheek). Given their embryologically unique development, the adrenal glands are present but will appear large and round due to the lack of compression by the kidneys. The renal arteries and ureters will be absent and there is an absent or severely underdeveloped bladder. Almost all these fetuses die soon after birth secondary to severe pulmonary hypoplasia. Pregnancy termination may be offered upon confirmation of the diagnosis as the mortality rate is nearly 100% despite intervention. ^,

Supernumerary kidney is a rare congenital anomaly where a third kidney forms in addition to the other two. There are only a few cases reported in the literature. These kidneys may be completely separate or fused to one of the other two kidneys. The additional kidney has its own blood supply and parenchyma, which is usually situated caudal to a normal kidney. It is typically smaller than a normal kidney. The additional kidney forms due to aberrant induction of the metanephric blastema by an abnormally directed ureteric bud, either as a separate ureteral bud from the mesonephric duct or as part of a “Y” duplication. Supernumerary kidneys have their own blood supply, independent collecting systems/ureters, and renal capsules, which differentiate a fused supernumerary kidney from a duplex kidney. Supernumerary kidneys are often associated with other congenital anomalies such as ectopic ureters, urethral duplication (both male and female), vaginal atresia, horseshoe kidneys, imperforate anus, ventricular septal defects, coarctation of the aorta, and myelomeningocele. ^,

Supernumerary kidneys are typically asymptomatic and do not require treatment. They may be associated with other pathologies such as nephrolithiasis, hydronephrosis, pyelonephritis, and malignancies. Fig. 52.3 shows the renal and excretory system of a supernumerary kidney with a fusion anomaly.

3D rendering of renal arteries (A) and excretory system (B) of supranumary kidney.

From Mebis W, Peters B, van der Zijden T. S-shaped kidney: A rare occurrence of supernumerary kidney with fusion-malrotation. J of Belgian Society of Radiology. 2018; 102 (1): 73, 1–2.