Imaging anatomy and diagnostics
On imaging, particularly during contrast-enhanced computed tomography (CECT), a horseshoe kidney can be identified by two main features:
- Medial deviation of the lower poles. Unlike normal anatomy, where the renal axes diverge inferiorly, here the lower regions converge toward the midline.
- Presence of an isthmus. The lower poles are joined by a parenchymal band. This structure is well visualized with contrast enhancement, confirming the fusion of the two kidneys into a single functional unit.
Associated urinary tract anomalies
A horseshoe kidney is part of a broader spectrum of congenital urinary system anomalies. Although it may be asymptomatic on its own, it is important for students to know other common developmental disorders that physicians encounter in diagnostic imaging:
- Duplication of the urinary tract. The ureters from the upper and lower renal moieties develop from a single Wolffian duct. Their insertion anatomy is described by the Weigert-Meyer law: the ureter from the upper moiety always inserts medially and inferiorly (distally), is frequently ectopic (into the urethra or vagina), and is associated with a ureterocele. The ureter from the lower moiety inserts normally, but is often accompanied by vesicoureteral reflux.
- Ureteropelvic junction obstruction. Most commonly associated with a congenital defect of the muscular layer in the proximal ureter. In 10% of cases, the lumen is compressed by an accessory artery. Diagnosed using diuretic renography.
- Retrocaval ureter. The right ureter courses posterior to the inferior vena cava due to a developmental anomaly of the latter, leading to hydronephrosis and pain.
Other pathologies requiring differential diagnosis
When studying imaging patterns of renal anomalies, one should also pay attention to:
- Congenital megaureter. Ureteral dilation greater than 7 mm. Classified into refluxing/non-refluxing and obstructed/non-obstructed types.
- Autosomal recessive polycystic kidney disease (ARPKD). Caused by a mutation in the PKHD1 gene. On ultrasound in early stages, it manifests as large, hyperechoic kidneys due to microcysts (up to 2 mm) in the collecting tubules. Associated with fetal oligohydramnios.
Briefly: answers to topical questions
What CT findings are characteristic of a horseshoe kidney?
A horseshoe kidney on computed tomography is characterized by medial deviation and fusion of the lower poles.
- Medial deviation — the lower poles of both kidneys are directed toward the midline.
- Presence of an isthmus (isthmus of the kidney) — the lower poles are fused together via a parenchymal or fibrous band.
- Vascular topography — sagittal slices show the inferior mesenteric artery (inferior mesenteric artery) crossing over the renal isthmus.
What causes ureteropelvic junction obstruction?
Ureteropelvic junction obstruction is caused by intrinsic and extrinsic etiological factors.
- Intrinsic causes — a congenital developmental disorder of the muscular layer of the proximal ureter at the pelviureteric junction, leading to narrowing and obstruction of urine flow.
- Extrinsic causes — compression of the proximal ureter by an aberrant or accessory renal artery.
What ultrasound findings manifest in autosomal recessive polycystic kidney disease in a fetus and newborn?
On ultrasound, autosomal recessive polycystic kidney disease manifests as bilateral enlargement and altered parenchymal echogenicity.
- Renal changes — bilateral enlargement, hyperechogenicity, preservation of the reniform shape, and the presence of multiple small hypoechoic cysts (cysts may not be visualized in early stages).
- Indirect fetal signs — development of oligohydramnios and non-visualization of the urinary bladder.
The typical sonographic picture may be absent until the 3rd trimester of pregnancy.
Which chromosomal abnormalities and genetic syndromes are most frequently associated with a horseshoe kidney?
A horseshoe kidney is most frequently associated with two chromosomal abnormalities.
- Turner syndrome (45,X0) — monosomy accompanied by an increased incidence of urinary tract anomalies, including a horseshoe kidney.
- Edwards syndrome — a chromosomal disorder caused by trisomy 18.
Additionally, a horseshoe kidney carries a somewhat elevated risk of developing a Wilms tumour.
What types of renal fusion anomalies exist other than the horseshoe kidney?
Aside from the horseshoe kidney, the following types of fusion anomalies are distinguished:
- Crossed-fused ectopia (crossed-fused ectopy) — the dystopic kidney crosses the midline and fuses with the normally positioned contralateral kidney. The most frequent variant forms an L-shape (fusion of the upper pole of the dystopic kidney with the lower pole of the normal one).
- Pancake kidney (pancake kidney) — an anomaly that forms as a result of the fusion of two pelvic kidneys.
Which vascular structure prevents the normal ascent of a horseshoe kidney during embryogenesis?
The normal ascent of a horseshoe kidney during embryogenesis is hindered by the inferior mesenteric artery.
- Inferior mesenteric artery (a. mesenterica inferior) — the vascular structure that mechanically limits the ascension of the fused kidneys. As a result, the isthmus of the horseshoe kidney lies posterior to this artery, but anterior to the aorta and inferior vena cava.
What embryonic developmental anomaly of the venous system leads to the formation of a retrocaval ureter?
The formation of a retrocaval ureter is caused by a disruption in the normal embryogenesis of the inferior vena cava.
- Persistence of the right subcardinal vein — a developmental anomaly in which the right ureter (circumcaval ureter) passes posterior to the inferior vena cava (IVC) and then emerges anterior to the pelvic vascular structures. This can lead to ureteral compression and impaired urine outflow.
The Weigert-Meyer rule in duplication: "The upper goes lower, the lower goes higher." The ureter from the upper moiety of the kidney inserts distally (lower), while that from the lower moiety inserts orthotopically (higher).
Class and exam questions
What does a horseshoe kidney look like on a CT scan?
Contrast-enhanced CT shows that the lower poles of both kidneys are medially deviated and connected by a parenchymal isthmus.
What is the essence of the Weigert-Meyer rule?
It states that in urinary tract duplication, the ureter from the upper renal moiety always inserts into the bladder more medially and inferiorly (distally) than the ureter from the lower moiety.
What is the most common cause of ureteropelvic junction obstruction?
Most often this is an intrinsic factor—a congenital developmental disorder of the muscular layer in the proximal ureter.
What is characteristic of a retrocaval ureter?
The right ureter passes posterior to the inferior vena cava due to a developmental anomaly of the latter, which can cause urinary stasis and hydronephrosis.
What else to study on this topic
- Weigert-Meyer law: detailed anatomy and complications
- Ureteropelvic junction obstruction: etiology and imaging findings
- Classification and diagnosis of congenital megaureter
- Retrocaval ureter: anatomical features
- ARPKD (Autosomal recessive polycystic kidney disease): ultrasound appearance and genetics
Master the entire topic in the "Diagnostic Radiology" course
This is a summary. The course covers everything tested in classes and exams.
- Complete study guide on "Horseshoe kidney" and 2,199 more pages of the subject
- Assignments and quizzes after every topic — just like in a colloquium
- AI tutor answers questions on the topic anytime
- 10 video lessons on the subject
Other topics in the section "Diagnostic Radiology of the Abdominal Organs"
Prepared by WhiteCoat Pro faculty in accordance with the medical school curriculum. For educational purposes only.