Congenital anomalies of the kidney and urinary tract often coexist with genital tract and vascular abnormalities because they originate from closely related embryologic structures.
Unilateral renal agenesis, ectopic kidneys, aberrant renal arteries, and cross-fused kidneys are all part of this spectrum.
Unilateral Renal Agenesis (URA)
Unilateral renal agenesis means complete absence of one kidney due to failure of the ureteric bud to develop or interact with the metanephric blastema.
Incidence - 1 in 1000–2000 births.
Embryology
Kidney and reproductive tract arise from the intermediate mesoderm.
Developmental sequence:
- Mesonephric duct forms
- Ureteric bud arises from it
- Bud interacts with metanephric blastema → kidney
Failure of this step leads to renal agenesis.
Because the mesonephric and paramesonephric systems are interconnected, anomalies often affect:
- reproductive tract
- ureters
- vasculature
Prenatal ultrasound diagnosis
Typical findings:
- One renal fossa empty
- No renal artery on Doppler
- Contralateral kidney enlarged (compensatory hypertrophy)
- Normal amniotic fluid
Important to confirm true absence vs ectopic kidney.
Associations with unilateral renal agenesis
Müllerian duct anomalies (females)
There is a strong link between URA and Müllerian duct anomalies.
This occurs because the mesonephric duct induces development of Müllerian structures.
Common associations:
| Uterine anomaly | Frequency |
|---|---|
| Unicornuate uterus | most common |
| Uterus didelphys | common |
| Obstructed hemivagina | possible |
Mesonephric duct abnormalities (males)
In males the mesonephric duct forms:
- vas deferens
- seminal vesicle
- epididymis
Possible associations:
- absent vas deferens
- seminal vesicle cysts
- ejaculatory duct obstruction
Single umbilical artery (SUA)
URA has an association with Single umbilical artery.
Umbilical artery anomalies often reflect abnormal vascular development, which can also affect renal formation.
When SUA is present, detailed evaluation for:
- renal anomalies
- cardiac anomalies
- skeletal anomalies
URA may occur with:
- VACTERL association
- cardiac anomalies
- vertebral defects
- anal anomalies
Long-term prognosis of unilateral renal agenesis
Most individuals do well because of compensatory hypertrophy of the remaining kidney.
However long-term risks exist:
| Risk | Explanation |
|---|---|
| Hypertension | hyperfiltration injury |
| Proteinuria | glomerular stress |
| Chronic kidney disease | possible later in life |
Ectopic kidney
An Ectopic kidney occurs when the kidney fails to ascend to the lumbar region.
| Type | Location |
|---|---|
| Pelvic kidney | most common |
| Iliac kidney | intermediate |
| Thoracic kidney | rare |
Aberrant renal arteries
During normal ascent, kidneys receive sequential arterial branches from the aorta.
If ascent stops early:
- earlier vessels persist
- multiple arteries may remain
These are aberrant renal arteries.
Consequences:
- usually asymptomatic
- may predispose to hydronephrosis if crossing ureter
Long-term function of ectopic kidneys
Most ectopic kidneys function normally but have higher risk of:
| Complication | Reason |
|---|---|
| Hydronephrosis | abnormal ureter course |
| Vesicoureteral reflux | abnormal ureter insertion |
| Stones | urinary stasis |
| Infection | impaired drainage |
Overall renal function is usually preserved.
Cross-fused renal ectopia
One kidney crosses to the opposite side and fuses with the other kidney.
Incidence: About 1 in 1000–7500 births.
Types
The most common form is S-shaped kidney.
Features:
- both kidneys on same side
- ureters enter bladder normally on their own sides
- fused renal parenchyma
This configuration creates an S-shaped appearance.
Mechanism
Occurs during embryonic ascent when:
- one kidney migrates across the midline
- fusion occurs before ascent completes
Clinical implications
Most patients remain asymptomatic.
Possible complications:
| Issue | Cause |
|---|---|
| Hydronephrosis | abnormal ureter path |
| Stones | urinary stasis |
| Infection | impaired drainage |
Distinguishing renal agenesis vs ectopic kidney on ultrasound
Clues suggesting true agenesis:
- absent renal artery on Doppler
- empty renal fossa
- no kidney in pelvis
- compensatory hypertrophy of other kidney
Clues suggesting ectopic kidney:
- kidney seen in pelvis
- renal artery arises from iliac or distal aorta
Practical fetal medicine counseling points
If unilateral renal agenesis is isolated:
- prognosis generally good
- normal amniotic fluid
- normal survival expected
However evaluation should include:
- detailed anatomy scan
- umbilical cord vessel count
- contralateral kidney morphology
- genital tract evaluation
Kidney and reproductive tract share a common embryologic origin, so renal agenesis frequently coexists with genital tract anomalies, vascular variations, and other urinary tract malformations.