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Posterior Urethral Valve Imaging and Diagnosis

Practice Essentials

A posterior urethral valve (PUV) 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
 and has undergone modification over time based on clinical observation and a better understanding of the embryologic events that lead to normal urethral development.

Young’s classification is based on the orientation of the valves within the urethra, as follows
:

Type I (95%): Posterior urethral folds (plicae colliculi) arise from the caudal verumontanum along the lateral margins of the urethra and fuse anteriorly, causing an obstruction.

Type II: Membranes cranially attached to the bladder neck originating from the verumontanum. (These are now thought to be caused by hypertrophy of the plicae colliculi and not obstructive valves.)

Type III (5%): Round membrane at the caudal verumontanum with a hole in the middle that is either above (type IIIa) the verumontanum or below it (type IIIb). (The holes do no communicates directly with the verumontanum.

Regardless of the type of valve, however, all valves essentially obstruct normal bladder emptying. This anatomic 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 leading to neonatal demise to mild obstructive symptoms of voiding dysfunction, may be noted.

Posterior urethral valves account for 17% of pediatric end-stage renal disease.

Imaging modalities

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. Voiding cystourethrography (VCUG) is necessary to confirm the diagnosis and to assess the bladder for associated findings of trabeculation, diverticula, and vesicoureteral reflux.

Contrast-enhanced serial voiding urosonography (SVU) has been proposed as a useful complementary test in pediatric patients with PUV. The advantages of SVU include absence of radiation, high sensitivity, and real-time imaging.

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.

VCUG may miss late-presenting cases of PUV.
 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 nuclear medicine renal scanning are not diagnostic for posterior urethral valves, although each modality may add details regarding the structure or function of the urinary tract.

With routine obstetric ultrasonography, the prenatal diagnosis of posterior urethral valve is becoming increasingly common in at least one third of cases,
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. Hydronephrosis is demonstrated in the sonograms below.

 

Prenatal longitudinal sonogram of the right kidney

Prenatal longitudinal sonogram of the right kidney. This image demonstrates significant hydronephrosis with possible renal cortical thinning. The kidney is larger than expected for the patient’s gestational age.

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Prenatal longitudinal sonogram of the left kidney.

Prenatal longitudinal sonogram of the left kidney. This image demonstrates significant hydronephrosis with possible renal cortical thinning. As is the case with the right kidney, the left kidney is longer than expected for the patient’s gestational age (same patient as in the previous image).

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Pathophysiology

PUV 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 anatomic 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. Renal tubular function may be affected by high pressures that result in poor urinary concentrating ability, with resultant obligatory polyuria 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 so that the boy develops renal failure as his body grows. 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 PUV and 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 a posterior urethral valve and is often thought to be physiologic, secondary to high bladder pressures overcoming the competence of the ureterovesical junction. Reflux may also be anatomic, secondary to abnormal ureteral orifice position resulting from abnormal ureteral bud development during embryogenesis.

Irreversible bladder dysfunction may occur due to alterations in collagen deposition in addition to the development of hypertrophy 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 due to high detrusor pressure with incomplete emptying and, possibly, vesicoureteral reflux.

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 a posterior urethral valve; these 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 [VURD] syndrome), bladder diverticula, and urinary ascites.

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. However, PUV remains a common cause of chronic kidney disease (CKD). In a cohort study of 173 patients with PUVs, 37.6% developed stage 3 or higher CKD.  The study found that baseline creatinine, nadir creatinine, and proteinuria were predictors of CKD.

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