Posterior urethral valves
Posterior urethral valves (PUVs), also referred as congenital obstructing posterior urethral membranes (COPUM), are the most common congenital obstructive lesion of the urethra and a common cause of obstructive uropathy in infancy.
Posterior urethral valves are congenital and only seen in male infants 2. The estimated incidence is at ~1 in 10,000-25,000 live births with a higher rate of occurrence in utero.
Clinical presentation depends on the severity of obstruction. In severe obstruction, the diagnosis is usually made antenatally. The fetus will be small for gestational age and ultrasound examination will demonstrate oligohydramnios and associated abnormalities (see below) 3. In less severe cases, the diagnosis is often not apparent until early infancy. Urinary tract infections are common in this group 3.
Posterior urethral valves result from the formation of a thick, valve-like membrane from a tissue of Wolffian duct origin (failure of regression of the mesonephric duct 5) that courses obliquely from the verumontanum to the most distal portion of the prostatic urethra. This is thought to occur in early gestation (5-7 weeks 6). The valve is a diaphragm with a central pinhole, however as it is more rigid along its line of fusion it gradually distends and becomes distended into a bilobed sail-like or windsock-like structure 1-3.
There are three types of posterior urethral valves 7:
- type 1
- most common
- occurs when the two mucosal folds extend anteroinferiorly from bottom of verumonatum and fuse anteriorly at lower level
- type 2
- no longer considered as a valve but a normal variant
- mucosal folds extend along posterolateral urethral wall from ureteric orifice to verumontanum
- type 3
- circular diaphragm with central opening in membranous urethra
- located below the verumontanum and occurs due to abnormal canalization of urogenital membrane
The vast majority of cases are sporadic, although rare examples of PUVs occurring in families have been reported 5.
Posterior urethral valves are also seen in association with other congenital abnormalities including 2:
On antenatal ultrasound, the appearance is that of:
- marked distention and hypertrophy of the bladder
- hydronephrosis and hydroureter may or may not be present
- in severe cases oligohydramnios and renal dysplasia 2 (assessing the degree of renal dysplasia is difficult antenatally, although some authors believe that significantly increased echogenicity of the kidneys is an indication of poor function 2,5)
- keyhole sign may be seen on ultrasound due to the distention of both the bladder and the urethra immediately proximal to the valve
Unfortunately, such findings are generally not seen before 26 weeks of gestation, and as such are not frequently identified on routine morphology screening, usually carried out around 18 weeks gestation 5.
Following birth, findings are the same as those on antenatal ultrasound, although as patients who present after birth usually have less severe obstruction, the features may be less evident:
- bladder is typically thick-walled and trabeculated with an elongated and dilated posterior urethra (keyhole sign) 5
- kidneys in most cases are hydronephrotic, although it is important to note that in up to 10% of cases they appear normal 5
- kidneys may also be hyperechoic with loss of the normal corticomedullary differentiation, a manifestation of renal dysplasia 5
- examination of the posterior urethra can be performed longitudinally through the perineum. Ideally, this is performed during micturition (which may take some patience) at which time the proximal urethra can be seen to dilate 3,5. Diameter of more than 6 mm is considered abnormal and is highly specific and sensitive to the diagnosis (sensitivity 100%, specificity 89%, positive predictive value 88%) 5
- additionally, the valve may be seen as an echogenic line 5
In some cases high pressure generated during attempted micturition can result in rupture of the collecting system with accumulations of urine in various compartments, including 3,5:
- rupture of a calyceal fornix: result in pararenal urinomas, seen as anechoic fluid collections around the kidney
- intraperitoneal bladder rupture: intraperitoneal fluid, indistinguishable ultrasonographically from ascites
Voiding cystourethrogram (VCUG)
Voiding cystourethrogram (VCUG) is the best imaging technique for the diagnosis of posterior urethral valves. The diagnosis is best made during the micturition phase in lateral or oblique views, such that the posterior urethra can be imaged adequately 1,4. Findings include 1:
- dilatation and elongation of the posterior urethra (the equivalent of the ultrasonographic keyhole sign)
- linear radiolucent band corresponding to the valve (only occasionally seen)
- vesicoureteral reflux (VUR): seen in 50% of patients 1
- bladder trabeculation / diverticula
Treatment and prognosis
Antenatal treatment is possible, consisting of vesicoamniotic shunting (allowing urine to exit the bladder via the shunt, bypassing the obstructed urethra). Essentially this procedure consists of a supra-pubic catheter performed under ultrasound guidance. The efficacy of this procedure is controversial, as often despite this significant renal and pulmonary morbidity exist 2.
Postnatally, definitive treatment is simple and involves transurethral ablation of the offending valve 3.
The overall prognosis is most affected by the degree and duration of obstruction. Severe cases with obstructive cystic renal dysplasia, oligohydramnios and pulmonary hypoplasia are often incompatible with life 2.
In the correct age group and with clear dilatation of the posterior urethra there is usually little differential other than urethral atresia, which is far less common 2.
When only the bladder is clearly abnormal - thick walled and trabeculated, other conditions to be considered include 5:
- 1. Berrocal T, López-pereira P, Arjonilla A et-al. Anomalies of the distal ureter, bladder, and urethra in children: embryologic, radiologic, and pathologic features. Radiographics. 22 (5): 1139-64. Radiographics (full text) - Pubmed citation
- 2. Chudleigh P, Thilaganathan B, Chudleigh T. Obstetric ultrasound, how, why and when. Churchill Livingstone. (2004) ISBN:0443054711. Read it at Google Books - Find it at Amazon
- 3. Bruyn RD. Pediatric ultrasound, how, why and when. Churchill Livingstone. (2005) ISBN:0443072752. Read it at Google Books - Find it at Amazon
- 4. Haller JO, Slovis TL, Joshi A. Pediatric radiology. Springer Verlag. (2005) ISBN:3540213546. Read it at Google Books - Find it at Amazon
- 5. Blews DE. Sonography of the neonatal genitourinary tract. Radiol. Clin. North Am. 1999;37 (6): 1199-208, vii. - Pubmed citation
- 6. Entezami M, Albig M, Knoll U et-al. Ultrasound Diagnosis of Fetal Anomalies. Thieme. (2003) ISBN:1588902129. Read it at Google Books - Find it at Amazon
- 7. Dähnert W. Radiology Review Manual. LWW. (2011) ISBN:1609139437. Read it at Google Books - Find it at Amazon
- 8. Puri P. Newborn surgery. CRC Press. ISBN:1444102834. Read it at Google Books - Find it at Amazon