Normal development and migration disorders
Normally, the development of the urinary system begins far from its final location. Initially, the kidney buds form in the lesser pelvis. As the fetus grows, by the eighth week of gestation, they ascend to the proximal lumbar spine. This pathway is accompanied by an important process: the organs rotate 90 degrees around their axis. It is precisely due to this rotation that the renal hila (hila) normally face medially.
Any disruption in this delicate process leads to an anomaly of position—ectopy (renal ectopy). On average, it occurs in one in a thousand people.
Several displacement variants are distinguished:
- Pelvic kidney (pelvic kidney). The most typical scenario, where the organ does not leave the lesser pelvis and remains below the pelvic brim. Since the ascent stopped prematurely, malrotation occurs: the renal hila remain directed anteriorly rather than medially.
- Crossed renal ectopy (crossed renal ectopy). In this pathology, the displaced organ crosses the body's midline. In 85–90% of cases (with a frequency of 1 in 7,500), the ectopic kidney fuses with the contralateral kidney located in its normal position. The crossed form without fusion is extremely rare (1 in 75,000).
Fusion anomalies
Among all anomalies associated with organ fusion, the absolute leader is the horseshoe kidney (horseshoe kidney). The mechanism of its origin lies in the impaired migration of both kidneys, which fuse along the midline during development.
- In 90% of cases, the connection is formed by the lower poles, which simultaneously point medially.
- Once formed, the fused structure attempts to ascend, but its normal ascent is mechanically restricted by a major vessel—the inferior mesenteric artery (a. mesenterica inferior). It acts as the barrier.
- The site of fusion is called the isthmus (isthmus). In the vast majority of cases (85–95%), it consists of fully functioning parenchyma and only occasionally represents a simple fibrous cord.
Number and size abnormalities
Renal agenesis (renal agenesis) is the complete absence of the kidneys, which is divided into two forms with fundamentally different prognoses:
- Bilateral agenesis (13 per 100,000 births). Until recently, it was considered absolutely fatal, since the lack of renal function in utero leads to severe oligohydramnios (oligohydramnios) and the development of Potter sequence (Potter sequence).
- Unilateral agenesis (8 per 100,000 births). It is often asymptomatic, but for a physician, it is a clear marker of potential associated problems. It is associated with vesicoureteric reflux (vesicoureteric reflux), arterial hypertension, and VACTERL association. It can be accompanied by cardiac, respiratory, and gastrointestinal malformations, adrenal agenesis, skeletal anomalies (e.g., clubfoot), chromosomal pathologies (trisomies 21 and 22, Turner syndrome), as well as Müllerian duct anomalies in girls and seminal vesicle cysts in boys.
A less radical defect is hypoplasia (hypoplasia). It arises due to growth arrest in the first trimester of gestation. Small-sized kidneys are formed with a critically reduced nephron count. Imaging reveals a diminished organ, sometimes with signs of hyperechogenicity and cysts.
Pathologies of the pelvicalyceal system and ureters
Structural defects of the collecting system include:
- Calyceal diverticulum (calyceal diverticulum). This is a congenital outpouching from the renal calyx into the renal cortex. Often an incidental finding, this pouch predisposes to urinary stasis, which can lead to urolithiasis, obstruction, or recurrent infections. In such cases, the patient presents with flank pain and hematuria.
- Congenital megacalyces (congenital megacalyces). Enlargement of the calyces due to underdevelopment of the renal pyramids. The main differential criterion on ultrasound, CT, or MRI (helping to distinguish it from hydronephrosis) is the absolutely normal size of the renal pelvis. It is more common in males and is typically unilateral.
- Duplication of the ureter and collecting system (duplicated collecting system). This pathology is the most common congenital anomaly of the entire urinary tract. It can be complete or partial and begins to develop when two separate buds are formed.
Briefly: answers to topical questions
What specific malformations are part of the VACTERL association?
The VACTERL association includes the following concurrent congenital malformations:
- Vertebral anomalies (vertebrae) — spinal development defects.
- Anal atresia (imperforate anus/atresia) — anorectal malformations.
- Cardiac defects (cardiac anomalies) — congenital heart defects.
- Tracheoesophageal fistula (tracheoesophageal fistula) — frequently combined with esophageal atresia.
- Renal anomalies (renal anomalies) — various kidney malformations.
- Limb anomalies (limb anomalies) — limb development defects.
What is the Weigert-Meyer rule in complete ureteral duplication?
The Weigert-Meyer rule (Weigert-Meyer rule) describes the typical anatomical pattern of ureteral insertion in complete duplication of the renal collecting system. Sources do not provide the full formulation of the rule, but indicate that the ureter from the upper pole (upper pole ureteric moiety) may have an ectopic insertion (ectopic insertion) leading to the formation of a distal ureterocele. Meanwhile, the second ureter may insert in the normal anatomical location.
What anatomical variants of crossed fused renal ectopy are distinguished?
The source directly describes the most common L-shaped variant for crossed fused renal ectopy: the left dystrophic kidney fuses by its upper pole with the lower pole of the normally positioned right kidney. The source also lists renal fusion forms: S-, L-, and I-shaped, though they are not presented as an exhaustive list of variants specifically for crossed fused renal ectopy.
To easily remember what a horseshoe kidney 'trips over' during ascent, imagine a horseshoe hooked on a lower branch: this is the inferior mesenteric artery (a. mesenterica inferior).
Class and exam questions
How to differentiate congenital megacalyces from hydronephrosis on imaging?
In megacalyces, only the calyces are pathologically dilated, while the size of the renal pelvis remains normal. In hydronephrosis, the pelvis will also be dilated.
Which direction do the renal hilum face in pelvic ectopia?
Due to halted migration, the normal rotation process is not completed, so the renal hila of the ectopic pelvic kidney usually face anteriorly rather than medially.
Why is a thorough evaluation of other systems necessary when unilateral agenesis is detected?
Unilateral agenesis often acts as part of syndromic pathology. It may be accompanied by cardiac, gastrointestinal, and reproductive system anomalies, skeletal deformations, and be part of the VACTERL association.
What else to study on this topic
- Pathogenesis of complete and incomplete collecting system duplication
- Components of the VACTERL association and their imaging visualization
- Potter sequence: mechanisms of formation in oligohydramnios
- Clinical significance and imaging presentation of Müllerian duct anomalies
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