Causes and Differential Diagnosis
Pediatric urinary tract pathology is fundamentally divided into unilateral and bilateral.
- Unilateral pathology: often resolves spontaneously and has a relatively benign course. It is caused by vesicoureteral reflux (VUR), multicystic dysplastic kidney (MCDK), complicated renal duplication, or pelviureteric junction obstruction (PUJO).
- Bilateral pathology: is always suspicious for serious anomalies. The most severe of these is posterior urethral valves (PUV), causing bladder outlet obstruction. Neurogenic bladder, bilateral reflux, and polycystic kidney disease also lead to bilateral involvement.
Postnatal Diagnostic Imaging Algorithm
If antenatal pelvicalyceal dilation is detected, the key factor is selecting the right timing for the first postnatal ultrasound. Performing it within the first 24 hours is uninformative: transient physiological dehydration in the newborn masks hydronephrosis. The optimal timeframe is from days 5 to 7 of life.
- Mild dilation (renal pelvis less than 7 mm): a follow-up ultrasound is scheduled at 1 month. If findings remain normal, monitoring is discontinued.
- Moderate to severe dilation (renal pelvis 7 mm or greater, calyceal dilation): micturition cystourethrography (MCUG) or contrast-enhanced urosonography (ce-VUS) is indicated to rule out reflux and urethral valves.
- Severe or bilateral dilation: requires an urgent ultrasound and MCUG to determine the need for early bladder drainage.
Renal Function Assessment and Current Trends
The functional status of the kidney (absence of impairment) is more important than the anatomical size of the renal pelvis. Isotope studies are used to assess renal function and urine drainage.
- In newborns and cases of severe collecting system dilation, it is preferable to use the radiopharmaceutical MAG3 rather than DMSA (DMSA is postponed until 3–6 months of age).
- Controversial issues: modern radiology aims to reduce radiation exposure. The role of MCUG in mild cases is decreasing, as low-grade VUR often resolves spontaneously and does not lead to scarring.
- Paradigm shift: congenital reflux is increasingly viewed not as the primary cause of renal damage during urinary tract infections (UTI), but as a marker of underlying congenital renal parenchymal dysplasia. Antibiotic prophylaxis for unilateral anomalies lacks robust evidence.
Acute Kidney Injury and Thrombosis
Acute kidney injury in newborns can result from birth hypoxia, genetic anomalies, or renal vein thrombosis (RVT). The main objective of duplex ultrasound is to differentiate prerenal, renal (intrinsic), and postrenal causes.
In renal vein thrombosis, ultrasound demonstrates enlarged, echogenic kidneys with loss of corticomedullary differentiation. In 75% of cases, the process is asymmetric. Power Doppler shows absent venous flow, while spectral Doppler reveals reversed diastolic flow in the renal artery (high resistance).
Briefly: answers to topical questions
What are the grades of hydronephrosis in children according to the SFU ultrasound classification?
According to the SFU ultrasound classification, there are six grades of pelvicalyceal system dilation.
- PCD Grade 0 — collecting system is not visualized or barely noticeable.
- PCD Grade I — only the renal pelvis is clearly visualized (size less than 7 mm), calyces are not identified.
- PCD Grade II — renal pelvis size is less than 10 mm, some calyces are visualized, and the shape of fornices and papillae is normal.
- PCD Grade III — marked dilation of calyces and pelvis (greater than 10 mm), papillae are flattened, fornices are rounded, without parenchymal thinning.
- PCD Grade IV — massive dilation of the collecting system combined with renal parenchymal thinning.
- PCD Grade V — extreme degree, where only a thin membrane-like rim of renal parenchyma remains.
What specific sonographic signs are identified in posterior urethral valves (PUV)?
Ultrasound examination for posterior urethral valves reveals:
- dilation of the upper urinary tract;
- renal dysplasia;
- urinomas resulting from calyceal rupture due to high pressure;
- bladder wall thickening;
- dilation of the posterior urethra;
- bladder neck hypertrophy.
What are the grades of vesicoureteral reflux (VUR) according to micturition cystourethrography?
Based on micturition cystography, there are five grades of severity of vesicoureteral reflux (Vesicoureteric Reflux).
- Grade I — reflux is confined to the ureter, urine backflow does not reach the renal pelvis.
- Grade II — total backflow reaches the pelvicalyceal system, but there is no dilation of the ureter and pelvis, calyceal fornices retain their normal shape.
- Grade III — mild dilation of the ureter and pelvicalyceal system appears, calyceal fornices may be mildly deformed.
- Grade IV — marked dilation and tortuosity of the ureter, calyceal fornices are blunted, but impressions from renal papillae are still traceable.
- Grade V — massive dilation and tortuosity of the ureter, severe dilation of the collecting system, papillary impressions are not visualized, intrarenal reflux is possible.
To remember the ultrasound algorithm: "First 24 hours are dry." Do not evaluate hydronephrosis in the first 24 hours due to physiological fluid loss in newborns; wait 5–7 days.
Class and exam questions
Why shouldn't a renal ultrasound be performed to assess hydronephrosis on the first day of life?
During the first 24 hours, newborns are physiologically dehydrated. This can artificially reduce the size of the renal pelvis and yield a false-negative result.
In which cases is micturition cystourethrography (MCUG) strictly mandatory?
MCUG is mandatory in severe bilateral dilation, suspected posterior urethral valves, bladder wall thickening, and in boys with ureteral dilation.
Which radiopharmaceutical is best to choose in severe hydronephrosis?
MAG3 is preferred because it better evaluates urine drainage and tubular function in dilated systems. DMSA scans are performed no earlier than 6 weeks of age.
What else to study on this topic
- Criteria for declining renal function: when to transition from ultrasound monitoring to surgery.
- Differential diagnosis of obstructive and non-obstructive megaureter.
- The role of magnetic resonance urography (MRU) in evaluating complex urinary tract anatomy.
- Imaging features in multicystic dysplastic kidney (MCDK).
- Vesicoureteral reflux (VUR): a marker of dysplasia or a cause of scarring?
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