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Congenital anomalies of the urinary tract encompass a diverse range of conditions resulting from abnormalities in the development and morphogenesis of the urinary system. These anomalies can manifest as obstructive or non-obstructive dilatation of the urinary tract, often accompanied by variations in the number, size, or position of the kidneys.
Renal parenchymal malformations, including renal dysplasia, renal agenesis, renal tubular dysgenesis, and polycystic renal diseases, can also be observed. Additionally, abnormalities related to the embryonic migration of the kidneys, such as renal ectopy (e.g., pelvic kidney) and fusion anomalies like horseshoe kidney, contribute to this spectrum of conditions.
Disorders involving the development of the urinary collecting system, including duplicate collecting systems, posterior urethral valves, and ureteropelvic junction obstruction, further contribute to the complexity of congenital urinary anomalies.
It is important to note that congenital anomalies of the kidneys or lower urinary tract (CAKUT) are a leading cause of renal failure in children, accounting for 30 to 50 percent of cases of end-stage renal disease (ESRD). Furthermore, CAKUT, especially those affecting the bladder and lower tract, can have a significant impact on kidney graft outcomes post-transplantation.
Children with CAKUT are at increased risk of urinary tract infections (UTIs) and urinary calculi and may also experience urinary incontinence due to these congenital anomalies.
The prevalence of congenital anomalies of the urinary tract (CAKUT) is estimated to be approximately 3-6 cases per 1000 births, making it one of the most common anomalies affecting humans.
CAKUT constitutes a significant portion, accounting for around 20 to 30 percent, of all anomalies identified during the prenatal period. This underscores its prominence among congenital anomalies.
Moreover, the presence of CAKUT in a child is often associated with an increased likelihood of discovering congenital anomalies in other organ systems, further emphasizing its clinical significance.
Genes and environmental factors interact to contribute to the development of congenital anomalies of the urinary tract (CAKUT). Several genes have been identified, including PAX2, WAGR, RET, WTI genes, as well as those involved in the renin-angiotensin system, all of which play roles in nephrogenesis.
It's important to note that genetic disorders are more commonly found in syndromic CAKUT compared to non-syndromic CAKUT. The mode of inheritance typically involves multiple factors, although some cases exhibit Mendelian inheritance patterns, such as in Polycystic Kidney disease and nephronophthisis.
On the environmental front, causative factors can be challenging to pinpoint, but some recognized ones include:
- Vitamin A deficiency in the mother, which is associated with reduced kidney size and nephron numbers.
- The use of angiotensin-converting enzyme inhibitors or angiotensin receptor blockers during early pregnancy.
- Infants born to diabetic mothers are at risk of renal agenesis.
- The urinary and genital systems are closely associated embryologically and anatomically.
- They both develop from a common mesodermal ridge known as the Intermediate mesoderm.
- Initially, ducts of both systems enter into a common cavity called the Cloaca.
Embryology of the Kidneys and Urinary Tract
- The kidneys and the urinary tract develop from the primitive kidneys and the primitive collecting system.
- The primitive kidneys include the pronephros, mesonephros, and metanephros, while the mesonephric duct and ureteric bud represent the urinary collecting system.
- The major part of the bladder is formed by the urogenital sinus.
- The fetal urinary system begins to develop in the first trimester.
- There are three successive primitive renal systems in fetal life:
- Pronephros
- Mesonephros
- Metanephros (Definitive kidney)
Pronephros
- The earliest and simplest.
- A vestigial system found in the cervical region.
- Appears at 3/52 of intrauterine life.
- Save for the caudal end of the pronephric duct, it disappears at about 30/7 gestation.
- It induces the appearance of the mesonephros.
Mesonephros
- The 2nd vestigial system, a more advanced system that extends from the upper thoracic to upper lumbar (L3) segments.
- Its first excretory tubules appear early in the 4th week of intrauterine life as the pronephros regresses.
- They lengthen rapidly, form an S-shaped loop, and acquire a glomerulus at their medial extremity with the tubules forming the Bowman’s capsule.
- The capsule and the glomerulus form a renal corpuscle. Laterally, the tubule enters the longitudinal collecting duct, the Mesonephric or Wolffian duct.
- In humans, the mesonephros regresses by 16 wg in a caudal-to-cranial manner except in males, where the cranial most mesonephric tubules become the epididymis and efferent ductules and the Wolffian duct becomes the vas deferens.
The Metanephros
- The metanephros (definitive kidney) begins to form at 4.5 wg (and becomes functional at 10 wg) in humans.
- Metanephric development starts with the outgrowth of epithelial cells called the ureteric bud (UB) from the caudal Wolffian duct.
- The UB invasion of the metanephric mesenchyme (MM), in response to inductive signals from the MM, induces MM cells to aggregate around the UB tip.
- The MM cells coalesce around the branched tips to form the cap mesenchyme, hereafter referred to as nephron progenitors.
- Thereafter, the UB undergoes repetitive branching to form the elaborate collecting duct system through branching morphogenesis.
- The first few branches give rise to the major and minor calyces in humans, into which the collecting ducts from the respective papillae drain.
- All major calyces drain into the pelvis, finally leading into the ureter.
- Nephrogenesis begins when mesenchyme cells coalesce near the junction of the UB stalk and the UB tips and transition into pretubular aggregates.
- These cells undergo a series of morphogenetic stages to form the proximal nephron, consisting of the glomerulus, the proximal tubules, descending and ascending loops of Henle, distal tubules, and connecting tubules.
- The endothelial progenitors invade the cleft at the distal aspect of the S-shaped body along with the stromal cells to become the glomerular capillaries and mesangium, respectively.
- The kidney fails to form. May be unilateral or bilateral.
- BRA (Bilateral Renal Agenesis): 1 in 5000 fetuses; URA (Unilateral Renal Agenesis): 1 in 750-2900.
- In BRA, the absence of fetal urine results in severe oligohydramnios and subsequently Potter’s sequence. Most of the children die in-utero or immediately after birth from pulmonary hypoplasia.
- URA is usually asymptomatic. There is usually a compensatory hypertrophy of the contralateral kidney over the first 3-4 months of life.
- Renal function is normal.
- Abnormalities of the genital system and other organs of the boys are common in renal agenesis.
- Differential Diagnosis: Renal hypoplasia, ectopic kidney.
- Other malformation of the renal parenchyma
- Renal hypoplasia/dysplasia
- Multicystic dysplastic kidney
- Polycystic kidney diseases
Renal Dysplasis/Multicystic Dysplatic Kidney
- Renal Dysplasia/Multicystic Dysplastic Kidney is a common congenital anomaly of the urinary system (CAKUT).
- Dysplasia refers to abnormal differentiation of kidney tissue.
- Multicystic Dysplastic Kidney (MCDK) is a specific type of renal dysplasia characterized by structural disorganization of kidney tissue.
- In MCDK, many non-communicating cysts are found throughout the dysplastic kidney tissue.
- This condition results in the loss of the normal differentiation between the cortex and medulla in the kidney.
- MCDK can occur unilaterally (affecting one kidney), bilaterally (affecting both kidneys), focally (in a specific area), or segmentally (in a specific segment of one kidney).
- Usually, the ureter connected to the MCDK is atretic (non-functional).
- MCDK is typically seen unilaterally but can occasionally affect both kidneys.
- Approximately 20-30% of individuals with MCDK may also have dysplasia or vesico-ureteric reflux in the other kidney.
- The natural history of MCDK involves either complete or partial involution (shrinking) of the affected kidney during the first few years of life.
- As the MCDK involutes, the contralateral (opposite) kidney may undergo compensatory hypertrophy (enlargement) to maintain kidney function.
Abnormalities of Migration of Embryonic Kidney
- Pelvic Kidney: In this condition, the kidney fails to ascend from its original lower abdominal location and remains in the pelvic area. This is an abnormality in the migration of the developing kidney.
- Horseshoe Kidney: Horseshoe kidney is a congenital anomaly where the lower parts of the two kidneys are fused together at the midline. This fusion gives the appearance of a horseshoe-shaped kidney, and it occurs due to a failure in the ascent of the kidneys during development.
- Crossed Fused Ectopia: Crossed fused ectopia is a rare kidney anomaly where one kidney crosses over to the opposite side and fuses or partially fuses with the other kidney. This condition results from the abnormal migration of both kidneys during embryonic development.
Polycystic Kidney Disease
Autosomal Recessive Polycystic Kidney Disease
- Mode of Inheritance: Autosomal recessive.
- Associated Gene: Polycystic kidney and hepatic disease gene (PKHD1).
- Clinical Features: Progression to End-Stage Kidney Disease (ESKD) in childhood is typical. Both kidneys are ALWAYS involved. Liver abnormalities are present in 100% of cases. Kidneys are bright, enlarged, and contain microcysts.
Autosomal Dominant Polycystic Kidney Disease
- Mode of Inheritance: Autosomal dominant. Two genes involved: PKD1 and PKD2.
- Clinical Features: Typically presents in the 3rd to 4th decade with relentless enlargement of the kidneys. Large cysts are found in the kidneys. Unilateral presentation is not unlikely. Positive family history of PKD is common. Cysts may also be found in other organs such as the liver and pancreas.
Malformation of the Bladder
Trigonitis: This condition involves a transitional type of epithelium transitioning to squamous type epithelium, which can overproliferate and lead to urinary blockages.
Abnormal Attachment of the Ureters: Sometimes, the ureters can be abnormally attached to either the urethra or parts of the reproductive tracts.
Urachal Fistulas, Sinuses, and Cysts: These abnormalities occur when a remnant of the allantois persists, leading to the formation of fistulas, sinuses, or cysts in the urinary tract.
Posterior Urethral Valves
Commonest Obstructive Uropathy: Posterior Urethral Valves (PUV) are the most common cause of obstructive uropathy leading to chronic kidney disease (CKD) and end-stage kidney disease (ESKD) in children.
Incidence: PUV are found in approximately 1 in 5000 to 8000 pregnancies.
Gender: PUV are exclusively found in male children.
Nature of Obstruction: PUV represent a web of tissues emanating from the verumontanum and obstructing the posterior urethra. This obstruction can have severe destructive effects on the entire renal system, not just the bladder.
Associated Renal Anomalies: PUV are often associated with renal anomalies, including renal dysplasia (present in 60% of cases), vesicoureteric reflux (VUR, present in 30-50% of cases, which may be unilateral or bilateral), and common conditions like cryptorchidism and inguinal hernias.
Development of CKD: The development of CKD in PUV results from a vicious cycle involving high back pressure in the urinary tract, VUR, repeated acquired injury from urinary tract infections (UTIs), nephrotoxic drugs, and associated renal dysplasia.
Clinical Presentations
- Prenatally: Prenatal diagnosis may show bilateral hydronephrosis, a persistently filled bladder, and a dilated posterior urethra in a male child. Oligohydramnios may also be present.
- Postnatally: The most common presentation in developing countries.
- Neonatal: Neonates may exhibit symptoms such as abdominal distension, poor urinary stream, respiratory distress, and straining to void.
- Infants: Infants may experience recurrent urinary tract infections (UTIs) and urosepsis, abdominal distension, failure to thrive, poor urinary stream, and straining to void.
- Older Children: Older children may present with lower urinary tract symptoms like poor urinary stream, straining to void, daytime incontinence, as well as other symptoms including abdominal distension, repeated UTIs, and features of chronic kidney disease (CKD).
Imaging & Investigations
- Ultrasonography (USS): A commonly used imaging technique.
- Micturating Cystourethrogram (MCUG): Used to assess the bladder and urethra.
- Cystoscopy: Direct visualization of the bladder and urethra.
- Dimercaptosuccinic Acid Scan: A diagnostic test to evaluate kidney function.
- Urodynamics: Tests used to evaluate bladder and urethra function.
- Other Tests: Serial electrolyte, urea, and creatinine measurements, as well as assessments of acid-base status, may be performed.
Management
- Prenatal Surgery: Some centers perform prenatal surgery, typically involving vesicoamniotic shunt procedures, but this carries significant risks.
- Initial Stabilization: Stabilizing the child is the first priority.
- Temporary Urinary Diversion: Temporary diversion is often achieved by passing a urethral catheter, preferably a feeding tube size 8 Fr.
- Vesicostomy: In cases where urinary diversion through catheterization is not possible and primary valve ablation is delayed, a vesicostomy may be performed.
- Valve Ablation: All children with Posterior Urethral Valves (PUV) should undergo valve ablation as soon as possible.
- Persistent Bladder Dysfunction: Children with persistent bladder dysfunction in the presence of Vesicoureteral Reflux (VUR) and Chronic Kidney Disease (CKD) require long-term follow-up even after valve ablation. They may benefit from anticholinergics due to an overactive, poorly compliant bladder.
- Bladder Augmentation: In cases of very small bladder size, bladder augmentation may be needed.
Factors Indicating Likelihood of Progression to ESKD
- 20-30% of Children: Approximately 20-30% of children with PUV will develop End-Stage Kidney Disease (ESKD).
- Features Detected Before 24 Weeks Gestation: PUV features detected before 24 weeks of gestation may be associated with an increased risk of progression to ESKD.
- Severe Oligohydramnios: Severe oligohydramnios in the mother during pregnancy can be a concerning sign.
- Severe Bilateral Hydronephrosis: Children with severe bilateral hydronephrosis may be at higher risk.
- Nadir Serum Creatinine: A nadir serum creatinine level greater than 1.0 mg/dl can indicate potential kidney issues.
- Persistent Bladder Dysfunction: Persistent bladder dysfunction is a factor to consider.
- Delay in Valve Ablation: A delay in valve ablation can also influence the risk of progression to ESKD.
Pelvi-Ureteric Junction Obstruction (PUJO)
Overview
- Common congenital obstructive uropathy in children.
- It is a congenital narrowing, usually a malformation at the junction of the pelvis and proximal aspect of the ureter.
- Extrinsic causes may include aberrant vessels.
- It occurs more commonly on the left (2:1) and in males (2:1).
- It may be bilateral in 10-40 % of cases.
- Other genitourinary anomalies may be present in the contralateral kidney: VUR in 10-20%.
Clinical Presentations
- Antenatal hydronephrosis (AP pelvis diameter >5mm): PUJ obstruction is the commonest cause of ANH. Most children with PUJO are detected antenatally in developed countries.
- Postnatally, most children are asymptomatic.
- Common symptoms include:
- Abdominal mass
- Abdominal/flank pain: episodic, colicky, made worse by brisk diuresis and relieved by dehydration
- They may present with Diettel’s crisis: episodes of nausea, vomiting, and colicky abdominal pain
- Hematuria
- Features of urinary stones
Investigations
- KUB ultrasound scan: hydronephrosis with dilated pelvis, normal ureter and bladder
- with no improvement after furosemide administration
- VCUG: in children with UTI
- Serum EUCr: impaired in those with bilateral disease or UPJO in a solitary kidney
- Urinalysis/Urine MCS: if UTI is confirmed investigate for VUR
Not all Children with PUJO need Dismembered Pyeloplasty
A significant number of children with PUJO will remit spontaneously, so they need follow-up
Indications for Surgical Intervention:
- Split kidney function <40% or difference >10%
- Deterioration in split kidney function >5%
- Severe bilateral hydronephrosis
- AP pelvis diameter >30mm
- Severe hydronephrosis in a single kidney
- Increasing AP pelvis diameter especially with deterioration in split kidney function
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