Congenital Anomalies of the Kidney

Intro & Embryology - Genesis Gone Wrong

• CAKUT (Congenital Anomalies of Kidney & Urinary Tract): Defect spectrum; primary cause of pediatric CKD.
• Embryogenesis (Metanephros - permanent kidney, from 5th week):
  • Ureteric Bud (mesonephric duct origin): Forms collecting system (ureter, pelvis, calyces, collecting ducts).
  • Metanephric Mesenchyme: Forms nephrons (glomerulus to DCT).
  • Reciprocal induction is key.

⭐ CAKUT is the leading cause of pediatric Chronic Kidney Disease (CKD), accounting for ~40-50% of cases.

Number & Position Anomalies - Kidneys Hide & Seek

• Renal Agenesis:
  • Bilateral: Potter sequence (oligohydramnios, pulmonary hypoplasia, typical facies, limb defects). Fatal. 📌 POTTER: Pulmonary hypoplasia, Oligohydramnios, Twisted face/skin, Extremity defects, Renal agenesis.
  • Unilateral: Asymptomatic; contralateral hypertrophy.
• Renal Hypoplasia:
  • Small kidney, ↓ nephrons. Risk of HTN, CKD.
• Ectopic Kidney: (e.g., pelvic)
  • ↑ risk: VUR, UPJ obstruction, stones.
• Horseshoe Kidney:
  • Fusion of lower poles (commonest), anterior to great vessels; IMA traps isthmus.
  • Associations: Turner Syn., Trisomy 18, VACTERL.
  • Complications: Hydronephrosis, stones, infection, Wilms' tumor, RCC. ⭐ > Horseshoe kidney is the most common renal fusion anomaly.

Structural & Form Anomalies - Shapeshifting Kidneys

• Horseshoe Kidney: Most common fusion (1:400). Isthmus anterior to great vessels.
  • Complications: UPJ obstruction (~33%), stones, VUR, infections, ↑trauma, ↑Wilms' tumor.
  • Assoc: Turner syndrome, Trisomy 18.
• Renal Ectopia: Kidney in abnormal position.
  • Pelvic kidney: failed ascent. Short ureter, anomalous vessels.
  • Crossed fused ectopia: both kidneys one side, fused. Crossed ureter orthotopic.
• Duplex Collecting System: Common (1%).
  • Complete (2 ureters, 2 ostia) or Incomplete (bifid pelvis/ureter, 1 ostium).
  • 📌 Weigert-Meyer Rule (complete duplication):

• Multicystic Dysplastic Kidney (MCDK): Non-hereditary, usually unilateral.
  • Kidney replaced by non-communicating cysts; no functional parenchyma.
  • Often involutes. Contralateral anomalies (VUR/UPJO ~30-40%).

  ⭐ Bilateral MCDK is fatal: Potter sequence (pulmonary hypoplasia) from oligohydramnios.

Obstructive Uropathies - Plumbing Problems

Commonest cause of antenatal hydronephrosis (ANH). Early diagnosis prevents renal damage.

• Pelvi-Ureteric Junction Obstruction (PUJO)
  • Most common pediatric upper urinary tract obstruction. Often unilateral (L>R).
  • Site: Renal pelvis-ureter junction.
  • Sx: Intermittent flank pain, UTI, mass.
  • Dx: USG, MCU, Diuretic Renogram (MAG3).
  • Rx: Pyeloplasty.
• Vesico-Ureteric Junction Obstruction (VUJO)
  • Obstruction: Ureter entry into bladder. Primary/Secondary.
  • Causes hydroureteronephrosis.
  • Rx: Ureteric reimplantation.
• Posterior Urethral Valves (PUV)
  • Males only. Membranous folds in posterior urethra.
  • Sx: Poor stream, bilateral hydronephrosis, palpable bladder, VUR common.
  • Dx: MCU (key: dilated posterior urethra).
  • Rx: Endoscopic valve ablation.

  ⭐ PUV: Most common cause of bladder outlet obstruction in male infants.

High‑Yield Points - ⚡ Biggest Takeaways

• MCDK: Most common neonatal abdominal mass; unilateral, non-functional.
• ARPKD: Infancy, bilateral enlarged kidneys, hepatic fibrosis (Caroli), PKHD1 gene.
• ADPKD: Usually adult onset (PKD1/PKD2 genes); can present in childhood.
• Horseshoe kidney: Fused lower poles, trapped by IMA; associated with Turner syndrome, ↑ risk of Wilms tumor.
• PUV: Most common bladder outlet obstruction in male infants; bilateral hydronephrosis.
• VUR: Common cause of recurrent UTIs and renal scarring; diagnosed by VCUG.
• Unilateral renal agenesis: Often asymptomatic; compensatory hypertrophy of other kidney.