eMedicine Specialties > Radiology > Pediatrics

Posterior Urethral Valve

Author: John S Wiener, MD, FACS, FAAP, Clinical Assistant Professor, Division of Urology, University of North Carolina at Chapel Hill; Associate Professor of Surgery and Associate Residency Program Director, Division of Urologic Surgery, Associate Professor of Pediatrics, Duke University School of Medicine
Coauthor(s): Anamaria Gaca, MD, Clinical Instructor, Division of Pediatric Radiology, Duke University Medical Center; Jeffrey Sekula, MD, Staff Physician, Chief Resident in Urology, Department of Urology, Duke University Medical Center
Contributor Information and Disclosures

Updated: Aug 7, 2007

Introduction

Background

A posterior urethral valve is an abnormal congenital obstructing membrane that is located within the posterior male urethra; this valve is the most common cause of bladder outlet obstruction in male children.

The valve is believed to result from abnormal embryologic development of the fetal posterior urethra. The classic categorization of posterior urethral valves into types I, II, and III was developed by Young et al in 1919¹ (see Anatomy, Types of posterior urethral valves) and has undergone modification over time based on clinical observation and better understanding of the embryologic events that lead to normal urethral development.

Regardless of the type of valve, however, all valves essentially obstruct normal bladder emptying. This mechanical obstruction increases voiding pressures and may alter normal development of the fetal bladder and kidneys. Typically, children with higher degrees of obstruction present earlier with the most severe symptoms. A spectrum of signs and symptoms ranging from severe obstruction with resultant renal failure and pulmonary hypoplasia to mild obstructive symptoms of voiding dysfunction may be noted.

With routine obstetric ultrasonography, the prenatal diagnosis of posterior urethral valve is becoming increasingly common, leading to investigative efforts at improving bladder drainage in utero. Postnatal diagnostic modalities and treatment algorithms are fairly well established, and valve management has become less invasive with the development of pediatric endoscopic instruments. The final goal of all therapeutic intervention for posterior urethral valve is proper urinary tract function with protection of the renal units. As a result of progress in the diagnosis, treatment, and surveillance of this condition, morbidity and mortality rates have markedly diminished over the past several decades.

For excellent patient education resources, visit eMedicine's Men's Health Center. Also, see eMedicine's patient education article Understanding the Male Anatomy.

Pathophysiology

Posterior urethral valves can appear at the earliest stage of urinary tract development; therefore, the entire urinary tract develops in an abnormal environment of high intraluminal pressure due to the mechanical obstruction. Permanent defects in the function of the kidneys, ureters, and bladder may result from prenatal maldevelopment — despite adequate decompression of the urinary tract after birth.

Renal function may be impaired for several reasons. Renal parenchymal dysplasia is common and may be related to maldevelopment of the metanephric blastema (renal precursor tissue) in an environment of high intraluminal pressure. In other cases, renal tubular function may be affected by high pressures that result in poor urinary concentrating ability, with resultant diuresis and the development of ureteral and bladder dysfunction due to high urinary production. The affected kidneys may function well initially, but they have a reduced renal reserve. Renal deterioration may also occur due to hyperfiltration injury that causes glomerulosclerosis, chronic pyelonephritis associated with vesicoureteral reflux, urinary stasis, or incomplete bladder emptying — all of which are common in boys with posterior urethral valves and which can cause further insult to the developing kidneys.

Hydronephrosis is common and may be due to a variety of causes. First, bladder dysfunction with high back pressures on the ureter may be causative. Second, the ureter itself may develop an abnormally deficient musculature due to chronic distention from high pressure or high urine flow. Third, high urinary flow due to the lack of urinary concentrating ability of the nephron can dilate the kidneys and ureters. Finally, there may be abnormalities of the vesicoureteral junction such as reflux or, rarely, ureterovesical obstruction.

Vesicoureteral reflux is present in one half of male patients with posterior urethral valves and is often thought to be physiologic as a result of high bladder pressures that overcome the competence of the ureterovesical junction. Reflux may also be anatomic, secondary to abnormal ureteral orifice position that is caused by an abnormal ureteral bud development during embryogenesis.

Bladder dysfunction is thought to be caused by alterations in collagen deposition and the development of detrusor smooth muscle cells. Several authors have noted high pressure due to poor compliance or uninhibited contraction of the detrusor muscle and eventual myogenic failure. In mild cases, incontinence may be present; in severe cases, ongoing deterioration of renal function occurs. Bladder dysfunction often improves over time after definitive treatment of the obstruction. Management of the persistently hostile bladder is imperative to diminish further renal impairment and the risk of urinary tract infection (UTI), persistent hydronephrosis or vesicoureteral reflux, and incontinence.

Several protective mechanisms may develop in boys with posterior urethral valves, which may lower intraluminal pressures and allow at least one renal unit to develop more normally. These mechanisms include massive unilateral vesicoureteral reflux (usually associated with an ipsilateral dysplastic kidney, known as vesicoureteral reflux and dysplasia syndrome), bladder diverticula, and urinary ascites.

Frequency

United States

A posterior urethral valve occurs in approximately 1 in 5000 live male births. The prevalence is in the range of 1:2375 to 1:8000.

International

The incidence rates are similar in countries worldwide.

Mortality/Morbidity

The early diagnosis and management of posterior urethral valves in boys has led to significant improvements in the morbidity and mortality rates for this condition.

• Perinatal mortality, formerly around 50%, was usually secondary to pulmonary hypoplasia and sepsis. Mortality rates have declined to <10% with improvements in perinatal care, as well as increases in the elective termination of pregnancies following prenatal detection of the condition.
• The long-term prognosis and morbidity of affected boys depend on the degree of parenchymal dysfunction, which may be prenatally determined. In addition, one frequently cited indicator is a nadir serum creatinine level less than 0.8 mg/dL at age 1 year; this suggests a good prognosis for long-term renal function. Proper management of the adverse urinary tract characteristics is critical to optimize renal function, even when renal function is impaired.
• Renal transplantation has been instrumental in the treatment of patients with posterior urethral valve and end-stage renal disease. Several studies have shown that transplantation is an effective therapy; however, hostile bladder parameters should be ruled out or corrected to ensure allograft survival.

Race

No defined racial predilection has been identified.

Sex

Posterior urethral valve affects only males.

Age

The posterior urethral valve defect is congenital, and as noted previously (see Introduction, Background), those males with more serious obstruction manifest symptoms earlier. If the condition is not diagnosed during prenatal ultrasonography, patients with severe obstruction present within days of birth, or toddlers present with voiding dysfunction and UTI. Incontinence is a rare presenting symptom in older boys with mild obstruction.

Anatomy

Structure and development of posterior urethral valves

The male urethra is composed of posterior and anterior urethral segments. The posterior urethra consists of the prostatic urethra (the urethra that traverses the prostate from the bladder neck to the urogenital diaphragm) and the membranous urethra (the segment that traverses the urogenital diaphragm).

Embryologically, posterior urethral valves are thought to arise due to an abnormally anterior insertion of the mesonephric (wolffian) duct on the cloaca before its division into the urogenital sinus and the anorectal canal. Remnants of normal insertion are noted in normal male urethras as plicae colliculi, folds that run distal and lateral to the verumontanum (a midline prominence in the midprostatic urethra where the ejaculatory ducts enter). This anterior insertion is thought to exaggerate the normal folds, with the distinction of being thicker, more prominent, and fused anteriorly.

Types of posterior urethral valves

Young et al initially categorized posterior urethral valves into 3 types.¹

The type I valve is a bicuspid valve that radiates distally from the posterior edge of the verumontanum to the anterior proximal membranous urethra. There is a variable aperture to allow urine flow during voiding; however, the fused portion fills with urine and bulges into the membranous urethra. This gives the characteristic sail-in-the-wind finding commonly seen on voiding cystourethrography (VCUG). Type I valves account for 95% of all valves.

The type II valve is no longer considered an obstructing valve; rather, it is thought to be a sequela of voiding dysfunction. It was described as a hypertrophic band of superficial muscle that runs along the posterolateral wall of the urethra from the ureteral orifice to the verumontanum. Therefore, these valves can be differentiated from type I and type III valves by their location proximal to the verumontanum. However, although this muscular hypertrophy may present with the anatomic obstruction of a posterior urethral valve, it may also occur with other nonobstructive causes of high-pressure voiding.

The type III valve is a circumferential membrane or diaphragm that is located at the membranous urethra, which is thought to result from the incomplete regression of the urogenital membrane during embryogenesis. There is a central aperture, and the central portions of the ring may prolapse into the more distal urethra during voiding, which results in a wind-sock appearance on VCUG. Type III valves account for almost 5% of all valves.

Despite the former classification of posterior urethral valves, a theory holds that there is most likely a single obstructive membrane that is altered by the passage of urethral catheters or cystoscopes, which results in variable tears of the membrane; these membranes may be perceived as a type I or type III valve. This concept of a single type of valve is referred to as congenital obstructing posterior urethral membrane.²

Presentation

The clinical presentation of posterior urethral valve is extremely variable. Increasingly, these valves are identified on antenatal ultrasonography. A history of oligohydramnios, bilateral hydronephrosis, and incomplete emptying of a thick-walled bladder may be elicited. Severely affected newborns may exhibit respiratory difficulties due to pulmonary hypoplasia, Potter facies, ascites, and a palpable abdominal mass (eg, involving the bladder or ureter).

It is common for newborns to not void for the first 24 hours of life; thus, a history of anuria may be misleading. A presentation in the neonatal period of abdominal distention due to a dilated bladder, ureters, and kidneys or due to urinary ascites has also been recognized.

If the diagnosis of posterior urethral valve is not recognized at birth, within weeks of birth, severely affected boys often present with urosepsis, dehydration, electrolyte abnormalities, or failure to thrive. A poor, dribbling urine stream may be noted. Toddlers often present with voiding dysfunction or UTI, and school-aged boys usually come to the clinician's attention because of urinary incontinence.

Preferred Examination

Renal ultrasonography in the male newborn can confirm the antenatal findings of hydroureteronephrosis with a dilated, thick-walled bladder and a dilated posterior urethra. In symptomatic older boys, ultrasonography is useful to screen for these findings of posterior urethral valves. VCUG is necessary to confirm the diagnosis and to assess the bladder for associated findings of trabeculation, diverticula, and vesicoureteral reflux.

Limitations of Techniques

VCUG is considered the diagnostic criterion standard imaging modality for posterior urethral valves, but normal mucosal folds (plicae colliculi) may appear as lucencies on VCUG and suggest the presence of valve leaflets. Conversely, valve leaflets may not be visible on VCUG; however, other associated findings with valves should raise suspicions. Also, improper VCUG technique can cause the diagnosis to be missed by omission of adequate urethral views during voiding or failure to remove the urinary catheter, which can stent the valves open during voiding.

Findings from renal ultrasonography, computed tomography (CT) scanning, intravenous pyelography (IVP), and renal scanning are not diagnostic for posterior urethral valves, although each modality may add details regarding the structure or function of the urinary tract.

Differential Diagnoses

Other Problems to Be Considered

Anatomic obstructive disorders

Urethral atresia
Anterior urethral valves
Syringocele
Megalourethra
Congenital urethral polyp
Urethral duplication
Congenital urethral stricture (Cobb collar)
Prune-belly syndrome (Eagle-Barrett syndrome or triad syndrome)
Plicae colliculi (normal anatomic structure)

Functional voiding disorders

Neuropathic bladder
Detrusor sphincter dyssynergy/dyssynergia
Small-capacity hypertonic bladder
Detrusor hyperreflexia
Nonneurogenic neurogenic bladder (Hinman syndrome)

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