Urinary Tract Obstruction

General Information

• Most cases of hydronephrosis are transient with little long-term significance, according to the American College of Radiology 1
• The American College of Radiology recommends ultrasound with Color Doppler of kidneys, bladder, and retroperitoneum as the first-line imaging modality to assess degree of collecting system dilation, evaluate renal parenchymal thickness, check for presence of hydroureter, identify bladder abnormalities, evaluate ureteral jets, measure postvoid residual volume, and assess for prostatomegaly in males 2
• Ideally, scan the bladder before voiding and kidneys after voiding, and use a curved array transducer (2-5 MHz) for adults, as suggested by the American College of Radiology and the American College of Emergency Medicine 2, 3

Diagnosis and Grading

• The Society for Fetal Urology (SFU) grading system categorizes hydronephrosis as Grade 1-2: Mild hydronephrosis, and Grade 3-4: Moderate to severe hydronephrosis, with the following table:

1

• Anteroposterior renal pelvic diameter (APRPD) can be used to assess severity, with <15 mm indicating mild to moderate hydronephrosis, and >15 mm indicating severe hydronephrosis 2

Follow-up and Monitoring

• If mild hydronephrosis is identified on ultrasound, follow-up in 3-6 months is recommended, and if etiology remains unclear, consider CT Urography (CTU) without and with IV contrast, MR Urography (MRU), or MAG3 renal scan (if obstruction is suspected) 2
• Initial postnatal ultrasound should be performed 48-72 hours after birth for all newborns with antenatal hydronephrosis, with earlier imaging for severe bilateral hydronephrosis, bladder abnormalities, or oligohydramnios, as recommended by the American College of Radiology 1
• Follow-up ultrasound is recommended in 1-6 months, with no immediate intervention required, and a resolution rate of 64-73% can be expected for mild hydronephrosis, while severe hydronephrosis has a resolution rate of only 29% 1
• Regular follow-up ultrasound is essential, with frequency depending on severity and stability of hydronephrosis 2

Indications for Referral and Intervention

• Indications for urological referral include progressive worsening of hydronephrosis, development of symptoms (pain, infection), impairment of renal function, or severe hydronephrosis (SFU grade 3-4 or APRPD >15mm) 2
• Voiding cystourethrography (VCUG) is indicated for bilateral high-grade hydronephrosis, duplex kidneys with hydronephrosis, ureterocele, ureteric dilatation, abnormal bladder, or history of febrile UTIs, as recommended by the American College of Radiology and European Urology 1, 4
• Surgical options include pyeloplasty for UPJO, valve ablation for PUV, and ureteral reimplantation for severe VUR or obstructive megaureter, and bilateral pyeloplasty can often be avoided with careful monitoring and selective unilateral intervention when necessary 1

Special Considerations

• Pregnancy-related hydronephrosis occurs in 70-90% of pregnant patients, typically asymmetrically prominent on the right, and for imaging during pregnancy, ultrasound or MRU without contrast is preferred to avoid ionizing radiation and gadolinium contrast, as recommended by the Journal of the American College of Radiology 2
• Patients with hydronephrosis should be followed by a multidisciplinary team that includes a pediatric nephrologist, dietitian, psychologist, social worker, and urologist, as stated in Nature Reviews Nephrology and recommended by the Society for Fetal Urology 5
• Regular kidney ultrasound at least once every 2 years is recommended to monitor for dilatation of the urinary tract and/or bladder dysfunction caused by polyuria, as suggested by the guidelines from Nature Reviews Nephrology 5