What is Hydroureter?
Hydroureter is the abnormal dilation of the ureter, typically defined as a ureteral diameter ≥7 mm in infants and children, or ≥4 mm in some pediatric contexts, representing pathological distension of the ureteral lumen that may indicate obstruction, reflux, or other urological abnormalities. 1, 2
Anatomical Definition and Measurement
Hydroureter refers to dilation of the ureter itself, distinct from hydronephrosis (dilation of the renal pelvis and calyces), though these conditions frequently coexist as "hydroureteronephrosis." 3
The clinically significant threshold for hydroureter is ≥7 mm in diameter based on prospective multicenter data showing this cutoff identifies patients at nearly three times the risk of urinary tract infection. 1
Some studies use ≥4 mm as the definition for hydroureter in pediatric populations with postnatal hydronephrosis, though this lower threshold may be less clinically relevant. 2
Common Causes and Clinical Context
In Pediatric Patients
Vesicoureteral reflux (VUR) is a major cause, with hydroureter ≥7 mm strongly associated with high-grade VUR (72.7% of cases with hydroureter had high-grade VUR versus only 3.5% low-grade VUR in those without hydroureter). 2
Primary obstructing megaureter accounts for 5-10% of antenatal hydronephrosis, diagnosed by persistent ureteral dilation >7 mm, and most cases resolve spontaneously. 3
Posterior urethral valves in male infants can cause hydroureter along with bladder wall thickening and dilated posterior urethra, requiring urgent evaluation. 3, 4
Duplicated collecting systems may present with hydroureter of the upper-pole moiety, sometimes reaching massive proportions. 5
In Adult Patients
Obstructive uropathy from stones, strictures, or masses is the most common cause in adults. 6
Rare congenital anomalies like retrocaval ureter (where the ureter passes behind the inferior vena cava) can cause hydroureter and hydronephrosis, often presenting in the third or fourth decade. 6
Malignancy, particularly upper urinary tract urothelial carcinoma, is associated with hydroureteronephrosis as a marker of advanced disease and poor prognosis. 3
Clinical Significance and Risk Stratification
Hydroureter ≥7 mm identifies high-risk patients for urinary tract infection who benefit from continuous antibiotic prophylaxis (hazard ratio 2.7 for UTI risk, with prophylaxis reducing risk by 50%). 1
In prenatal hydronephrosis, the presence of hydroureter on initial neonatal ultrasound indicates need for voiding cystourethrogram (VCUG) to exclude VUR and posterior urethral valves, particularly in male infants. 3, 2
Hydroureter without VUR (<7 mm) may be managed conservatively without routine VCUG or antibiotic prophylaxis based on multicenter registry data. 1
In upper tract urothelial carcinoma, hydroureteronephrosis is independently associated with worse overall survival, cancer-specific survival, and disease-free survival, though it should be considered alongside other high-risk factors rather than used alone for risk stratification. 3
Diagnostic Approach
Ultrasound is the primary imaging modality for detecting hydroureter, measuring ureteral diameter, and assessing associated hydronephrosis or bladder abnormalities. 3
CT urography provides superior anatomical detail for identifying the cause and extent of hydroureter, particularly in adults with suspected obstruction or malignancy. 3
MAG3 renal scan is indicated for severe hydroureter with hydronephrosis to assess differential renal function and drainage, with surgery recommended if function <40%, deterioration >5%, or T1/2 washout >20 minutes. 3, 7
Common Pitfalls
Do not confuse hydroureter with normal physiologic ureteral peristalsis, which may cause transient ureteral dilation on ultrasound without pathological significance. 3
Bilateral hydroureter with distended bladder may indicate bladder outlet obstruction rather than upper tract pathology, as seen with inadequate catheter drainage or posterior urethral valves. 8
Hydroureter in pregnancy or with full bladder may be physiologic and should not be misinterpreted as pathological without clinical correlation. 4