Congenital anomalies of the kidneys

Congenital anomalies of the kidneys and urinary tract (CAKUT) are one of the most common malformations diagnosed in newborns. 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. 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.

Anomalies of the collecting system

• Duplex collecting system: a complete or incomplete duplication of the collecting system (pelvicalyceal system and ureter); has different clinical manifestations depending on the degree of fusion
  • The most common congenital anomaly of the urinary tract
  • Complete duplication: an additional ureteric bud forms resulting in two separate ureteric buds that interact with the same metanephric blastema; the affected kidney has ; two separate pelvicalyceal systems and two ureters that enter the bladder at different points (double ureter).
  • Incomplete duplication: the ureteric bud splits before interacting with the metanephric blastema; the affected kidney has two collecting systems that merge between the ureteropelvic junction and the urinary bladder, resulting in a single ostium in the bladder
    • Bifid ureter; : The affected kidney has two separate pelvicalyceal systems and two ureters that merge and share a single ostium into the bladder (Y-shaped ureter).
    • Bifid pelvis: The affected kidney has two separate pelvicalyceal systems that merge into a single ureter at the ureteropelvic junction.
  • Associated with vesicoureteral reflux, ureteral stricture, hydronephrosis, and UTI
  • Ureteroureteral reflux may occur due to Y-shaped ureter.
• Ectopic ureter: The ureter opening is located caudal to the normal insertion site on the bladder.
  • Prenatal hydronephrosis
  • Postnatal UTI
  • In female individuals, 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 bulges into the urinary bladder or extends to the bladder neck and the urethra; often appears balloon-shaped on cystoscopy
  • Prenatal hydronephrosis
  • Postnatal UTI
• Ureteropelvic junction obstruction: See learning card on “Urinary tract obstruction.”

Malformation of the renal parenchyma

Renal dysgenesis

• Renal agenesis: the absence of the kidney and the ureter
  • ♂ > ♀
  • Pathophysiology: ureteric bud does not develop → complete lack of stimulating signals for differentiation of metanephric blastema → kidney fails to develop.
  • Unilateral renal agenesis; : usually asymptomatic, depending on the condition of the other kidney (see congenital solitary kidney)
  • Bilateral renal agenesis: causes Potter sequence
• Renal hypoplasia: underdevelopment of the kidney characterized by a reduced number of nephrons
• Renal dysplasia: abnormal development of the kidney that results in a nonfunctional kidney with abnormal features (e.g., cysts, increased amount of connective tissue)
  • Pathophysiology: Ureteric bud does not properly stimulate differentiation of metanephric blastema → disorganized nephrons, reduced number of nephrons, and abnormal cells
  • Unilateral dysplasia is usually asymptomatic. (see congenital solitary kidney)
  • Bilateral dysplasia leads to early renal insufficiency and Potter sequence if present in utero.
  • Examples: polycystic kidney disease, multicystic dysplastic kidneys, obstructive cystic dysplasia, medullary sponge kidney
  • Associated with anomalies of the collecting system that predispose to UTI
• Renal aplasia: kidney with only rudimentary renal parenchyma present with no residual function

Congenital solitary kidney

• Definition: the presence of a single functioning kidney due to absence or anomalies of the contralateral kidney
  • Anatomical congenital solitary kidney: the absence of one kidney due to renal agenesis
  • Functional congenital solitary kidney: a nonfunctional kidney due to e.g., renal aplasia, severe renal dysplasia or hypoplasia
• Often asymptomatic and diagnosed during prenatal ultrasound screening
• A solitary kidney is usually hypertrophic to compensate for the contralateral kidney
• Higher incidence of anomalies (e.g., vesicoureteral reflux)
• Increased risk of hypertension, renal insufficiency, and progression to end-stage renal disease

Anomalies of kidney migration

• Horseshoe kidney: Fusion of the left and right inferior renal poles
  • Normal ascent interrupted as fused kidney gets caught on the inferior mesenteric artery (IMA).
  • Increased incidence in patients with chromosomal aneuploidy (e.g., trisomies 13, 18, 21, and Turner syndrome)
  • Usually asymptomatic; typically diagnosed incidentally on abdominal imaging for unrelated conditions
  • Rarely requires treatment
  • Increased risk of renal stones, ureteropelvic junction obstruction, hydronephrosis, infections, and renal cancer
• Renal dystopia and renal ectopia: displacement of the kidney within the retroperitoneum
• Crossed dystopia: displacement of the kidneys; both kidneys located on the same side of the spine
• Malrotation: Torsion of the orthotopic kidney

• 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

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