Congenital Anomalies of the Kidneys and Urinary Tract

Congenital anomalies of the kidneys and urinary tract are a broad, heterogeneous group of structural developmental abnormalities of the kidneys and urinary collecting system. They are the commonest category of anomaly identified on antenatal ultrasound screening and a leading cause of chronic kidney disease in children.

They include renal agenesis, renal hypoplasia and dysplasia, multicystic dysplastic kidney, duplex collecting system, horseshoe kidney, and pelviureteric junction obstruction. The related, more specific conditions are megaureter and vesicoureteral reflux.

Management is increasingly guided by structured antenatal and early postnatal detection through routine ultrasound screening. Early identification and risk-stratified follow up address the subset of anomalies with clinically significant functional consequence.

Classification and pathophysiology

• Renal agenesis is the complete absence of one or, rarely, both kidneys. It reflects failure of normal early nephrogenic development.
• Unilateral agenesis is generally compatible with normal life, given adequate compensatory function of the remaining, solitary kidney.
• Bilateral agenesis is incompatible with extrauterine survival, because of the resulting severe oligohydramnios and associated pulmonary hypoplasia (see Potter sequence).
• Multicystic dysplastic kidney reflects abnormal, disorganised renal development, with replacement of normal parenchyma by multiple, non-communicating cysts. It typically affects one kidney, and the contralateral kidney is usually normal.
• The affected kidney is generally non-functional and, in the great majority of cases, involutes gradually and spontaneously over subsequent years without intervention. Assess the contralateral kidney carefully, because of the increased association with contralateral anomalies including pelviureteric junction obstruction or vesicoureteral reflux.
• Duplex collecting system is partial or, less commonly, complete duplication of the collecting system and ureter on one side.
• Complete duplication has additional relevance. The ureter draining the lower pole moiety is associated with vesicoureteral reflux, and the ureter draining the upper pole moiety with ureterocele or ectopic ureteric insertion. Each carries its own management considerations.
• Pelviureteric junction obstruction is a functional or, less commonly, anatomical obstruction at the junction between the renal pelvis and ureter, producing varying degrees of hydronephrosis. It is one of the commonest antenatally identified urinary tract anomalies. Clinical significance and need for intervention relate directly to the severity of obstruction and its impact on differential renal function.
• Horseshoe kidney is fusion of the two kidneys across the midline, typically at the lower poles. Overall renal function is generally normal or near-normal.
• It carries a modestly increased risk of urinary tract infection, stone formation, and pelviureteric junction obstruction, reflecting the abnormal anatomical configuration and drainage pattern.

• The great majority of congenital urinary tract anomalies are now identified antenatally through routine ultrasound screening, as an incidental finding in an otherwise asymptomatic pregnancy.
• Postnatal presentation, where it occurs, includes urinary tract infection. This is a common, important presenting feature in infants and young children with an unidentified structural anomaly, and warrants proactive imaging after a first febrile urinary tract infection in this age group.
• Other postnatal presentations are an abdominal mass (particularly with significant hydronephrosis or a large multicystic dysplastic kidney) and, in more severe cases with significant bilateral involvement, features of chronic kidney disease including faltering growth.
• Severe, bilateral, or significantly obstructive anomalies, particularly with significantly reduced amniotic fluid volume (oligohydramnios, reflecting reduced fetal urine output), have important implications for fetal pulmonary development. Adequate amniotic fluid is essential for normal fetal lung development.
• Severe, early oligohydramnios carries a significant risk of pulmonary hypoplasia and, in the most severe cases, incompatibility with extrauterine survival.
• Structured antenatal counselling is an important part of care for the family. It covers the identified anomaly, its anticipated postnatal significance, and the planned postnatal assessment and follow up.

• Antenatal ultrasound is the primary mode of identification for the great majority of these anomalies.
• Its findings inform the anticipated postnatal significance and the planned postnatal assessment. They include the degree of any hydronephrosis, using standardised grading systems, and the amniotic fluid volume as a marker of ongoing fetal renal function.
• Postnatal renal ultrasound is performed at a standardised interval after birth, commonly at forty-eight hours or later. Imaging too early can give a falsely reassuring appearance, because of the physiological, transient reduction in urine output in the immediate newborn period.
• It confirms and further characterises the anomaly identified antenatally, or identifies a new abnormality when the postnatal presentation is with infection or another clinical concern.
• Micturating cystourethrography is the specific investigation for suspected vesicoureteral reflux. Contrast is instilled into the bladder via a urethral catheter, with imaging during voiding (see vesicoureteral reflux).
• Nuclear medicine renal scanning provides functional information relevant to management decisions in appropriately selected cases.
• MAG3 assesses differential renal function and drainage, particularly for suspected pelviureteric junction obstruction.
• DMSA assesses renal cortical scarring, particularly with recurrent urinary tract infection or confirmed vesicoureteral reflux.

• The great majority of mild to moderate antenatally identified anomalies, including mild to moderate hydronephrosis and unilateral renal agenesis with a normal contralateral kidney, need only structured, periodic ultrasound surveillance rather than active intervention. The natural history is generally favourable, and many mild abnormalities improve or resolve spontaneously.
• More significant, obstructive anomalies warrant surgical intervention. These include significant pelviureteric junction obstruction with deteriorating differential renal function on serial nuclear medicine assessment, and significant, progressive hydronephrosis.
• Pyeloplasty is typical for confirmed, clinically significant pelviureteric junction obstruction, aiming to relieve the obstruction and preserve or improve renal function.
• Antibiotic prophylaxis is considered in selected infants with significant hydronephrosis or another anomaly carrying an increased risk of urinary tract infection, pending definitive assessment and management planning. Indications and evidence continue to evolve and vary with current guidance and clinical context.
• Multicystic dysplastic kidney is generally managed by observation, given the typical spontaneous involution over time.
• Reserve nephrectomy for the uncommon scenario of significant, symptomatic enlargement, associated hypertension, or diagnostic uncertainty. Continue surveillance of the contralateral kidney because of the association with contralateral anomalies.
• Provide long-term paediatric nephrology and urology follow up, with structured monitoring of renal function, blood pressure and growth.
• Investigate and treat any urinary tract infection promptly, given its particular relevance to renal scarring risk with an underlying structural anomaly.
• Management of this heterogeneous group is individualised and risk-stratified. The overarching goal is preservation of long-term renal function and prevention of complications (hypertension, recurrent infection, and progressive chronic kidney disease) that can arise from inadequately monitored or managed significant structural anomalies.