Abstract

Congenital anomalies of the kidneys and urinary tract (CAKUT) are one of the most common malformations diagnosed in newborns. Even though most cases are initially asymptomatic, up to 50% of children with end-stage renal disease have an underlying CAKUT. For this reason, early identification of these malformations is essential in order to prevent renal damage. The pathogenesis of CAKUT is multifactorial; both specific genes and environmental factors (e.g., in utero exposure to ACE inhibitors) have been implicated in the development of CAKUT.

Etiology

Disturbance of embryonic development
Environmental factors
- In utero exposure to ACE inhibitors or angiotensin II receptor blockers → juxtaglomerular hyperplasia, absent proximal convoluted tubules, or renal fibrosis

References:^[1]

Clinical features

Most CAKUT are incidental findings with no clinical significance.
CAKUT are considered clinically significant if they can lead to chronic kidney disease, vesicoureteral reflux, urinary tract infections, hydronephrosis, or urinary incontinence.
Signs that an underlying clinically significant CAKUT may be present include:
- Hydronephrosis on prenatal ultrasound
- Urinary tract infection in children < 24 months of age
- Recurrent urinary tract infections in children of any age

References:^[1]

Diagnostics

Ultrasound imaging of the kidneys and bladder
- First-line imaging modality
- Detects anatomical abnormalities
- Renal ultrasound is warranted for all children with recurrent UTIs or UTI in children < 24 months of age.
Voiding cystourethrogram
- Detects retrograde movement of urine from the bladder to the urinary tract
- Although the test is invasive and involves radiation exposure, it is warranted in patients with recurrent UTIs or single UTI and abnormal findings on renal ultrasound.
Intravenous pyelogram: detects anatomic abnormalities; has largely been replaced by ultrasound and CT urography
CT scan: detects anatomic abnormalities; can complement renal ultrasound if findings on ultrasound are equivocal

References:^[2]

Differential diagnoses

Floating kidney

Description
- Downward displacement of the kidney due to reduced perirenal fat (thin individuals, anorexia nervosa)
- Hypermobility of the kidney
Symptoms
- Flank and lower abdominal pain with exacerbation in upright position (nephroptosis causes painful strain)
- Alleviation of pain in supine position
Diagnostics
- Renal ultrasound in upright and supine position
- Intravenous pyelogram in upright and supine position
Treatment
- Conservative: abdominal muscle training and stabilization with abdominal wall binders
- Surgical: laparoscopic nephropexy

References:^[3]

The differential diagnoses listed here are not exhaustive.

Overview

Kidney

Condition	Anomaly	Important notes
Duplex kidney	Kidney with a duplication of the collection system	The extent of clinical manifestation depends on the concurrent ureter malformation (see table below): Complete duplication: ureter duplex Incomplete duplication: ureter fissus, bifidus pelvis
Renal agenesis	Absence of the kidney ♂ > ♀	Interdisciplinary treatment in cases of bilateral agenesis
Renal aplasia	Incompletely developed or absent kidney
Renal dysplasia	Abnormal development of the kidney (see Potter classification) Disorganized nephrons, ↓ number of nephrons, inappropriate differentiation of renal tissue Associated with collecting system anomalies that predispose to UTI	Unilateral dysplasia is usually asymptomatic; bilateral dysplasia leads to early renal insufficiency and may lead to renal failure.
Renal hypoplasia	Underdevelopment of the kidney ↓ Number of nephrons
Renal dystopia/renal ectopia	Displacement of the kidney within the retroperitoneum
Malrotation	Torsion of the orthotopic kidney
Crossed dystopia	Displacement of the kidneys; both kidneys are located on the same side of the spine
Horseshoe kidney	Fusion of the left and right inferior renal poles	Usually asymptomatic; urinary tract obstruction may occur Rarely requires treatment Typically seen incidentally on abdominal imaging for other purposes Normal ascent interrupted as fused kidney gets caught on inferior mesenteric artery

Ureter

Condition	Anomaly	Important notes
Bifid Ureter	Duplication of the ureter; the distal ureters converge and share a single ostium into the bladder	Possible yo-yo effect of the urine in the Y-shaped ureter (uretero-ureteral reflux)
Double Ureter	Duplication of the ureter with two separate orifices merging into the bladder Meyer-Weigert law: In cases of double ureter, the ureter from the upper pole of the kidney always inserts medial and inferior to the insertion of the ureter from the inferior pole of the kidney!
Bifid collecting system	Duplication of the renal pelvis; the two merge directly at the ureteropelvic junction
Ectopic ureter	The ureteral opening is located caudal to the normal insertion site on the bladder.	Prenatal hydronephrosis Postnatal UTI In females, the ureter may bypass the external sphincter and insert directly in the vagina, resulting in incontinence.
Ureterocele	Abnormal dilation of the terminal portion of the ureter, which may occur in the bladder or in the urethra	Prenatal hydronephrosis Postnatal UTI

Treatment

The majority of cases do not require treatment.
If there is vesicoureteral reflux → antibiotic prophylaxis (see vesicoureteral reflux)
If there is vesicoureteral reflux and recurrent cystitis or pyelonephritis → surgical correction
- Abnormalities of the ureter or its insertion into the bladder (e.g., ureterocele or ectopic ureter) are treated with resection of the distal ureter and reimplantation of the ureter into the bladder.
Laparoscopic nephrectomy may be indicated in cases with recurrent pyelonephritis and loss of kidney function.