Congenital anomalies of the kidney and urinary tract: Viva

Branch 1: Classification and embryology

The candidate should define CAKUT as the spectrum of congenital structural malformations of the kidney and urinary tract that arise from errors in embryological development. A strong candidate classifies the spectrum into three anatomical groups: the kidney and parenchymal anomalies including renal agenesis, hypoplasia, dysplasia, and the multicystic dysplastic kidney, the ureteric and collecting system anomalies including vesicoureteral reflux, megaureter, ureterocele, and duplex systems, and the bladder and urethral anomalies including posterior urethral valves, prune belly syndrome, and neurogenic bladder. [1]

If the examiner presses on the embryology, the candidate should describe the ureteric bud and the metanephric mesenchyme interaction. The ureteric bud grows from the Wolffian duct and invades the metanephric mesenchyme, branching to form the collecting system while the mesenchyme forms the nephron. The GDNF-RET pathway is the master regulator, and disruption produces agenesis, dysplasia, or ureteric anomalies depending on the stage and the nature of the disruption. A strong candidate names the Weigert-Meyer rule for duplex systems, in which the upper-pole ureter inserts ectopically and distally. [1]

Branch 2: The investigation pathway

If asked about the investigations, the candidate should describe the sequential image-led pathway. The first test is the postnatal renal ultrasound, performed at 5 to 7 days to avoid the false-negative of neonatal dehydration. The micturating cystourethrogram, or MCUG, detects vesicoureteral reflux and posterior urethral valves and is reserved for children with a high pre-test probability. The DMSA scan maps the cortical scarring and the split renal function, and the MAG3 renogram assesses the obstruction and the drainage. [7]

A strong candidate cites the meta-analysis by Lee and colleagues, which showed that antenatal hydronephrosis predicts a postnatal CAKUT in a significant proportion of cases, with the risk proportional to the degree of dilation. The candidate should state that the threshold for the postnatal investigation is low, because even mild antenatal hydronephrosis carries a risk of postnatal pathology. The candidate should start prophylactic antibiotics, such as trimethoprim at 2 mg per kg at night, while the investigations are completed, to prevent the urinary tract infection that would cause renal scarring. [7]

Branch 3: Management and long-term implications

If the examiner moves to the management, the candidate should tailor it to the specific anomaly. For a ureteropelvic junction obstruction confirmed on the MAG3 renogram, the management is conservative with surveillance if the function is preserved, and a pyeloplasty if the function declines or the obstruction is severe. For vesicoureteral reflux, low-grade reflux is managed with prophylactic antibiotics and surveillance, and high-grade reflux with recurrent infection may warrant endoscopic or surgical correction. [6]

A strong candidate discusses the long-term CKD risk. CAKUT is the single largest cause of paediatric end-stage kidney disease in registry data, and every child with a confirmed CAKUT needs life-long nephrology surveillance with blood pressure monitoring, serial creatinine, and urinalysis for proteinuria. The multicystic dysplastic kidney involutes in the majority of cases, as shown by Aslam and Watson, and the nephrectomy is no longer routine. The solitary functioning kidney from agenesis or MCDK carries a risk of hyperfiltration injury, as shown by Matsell and colleagues, which is why the surveillance is essential. [6]