Congenital anomalies of the male urethra
The spectrum of congenital anomalies of the male urethra is presented. The embryologic basis of each anomaly, when known, is discussed. Clinical and imaging features of each entity are presented.
KeywordsMale urethra Anomalies Embryology Congenital
The development of the male genitourinary system is complex. Numerous anomalies of the male urethra exist, either as isolated anomalies or in combination with other disorders. The current understanding of the embryology and anatomy of normal male urethral development might help explain the causes of the various anomalies as well as their relationships.
Embryology of the male urethra
Normal anatomy of the male urethra
The membranous urethra is the shortest, least distensible portion of the urethra. It is surrounded by the sphincter urethrae membranaceae (external sphincter), which in addition to vessels, nerves and deep transverse perineal muscle represents the urogenital diaphragm. The internal sphincter is located at the bladder neck.
The penile urethra is the longest portion of the urethra. The proximal portion of the penile urethra, the bulbar urethra, is dilated and is the site of drainage of Cowper’s ducts. The penile urethra is also dilated distally at the glans penis, where it forms the fossa navicularis. The midportion of the penile urethra is tethered superiorly by the suspensory ligament .
Congenital anomalies of the urethra
Posterior urethral valves
Posterior urethral valves (PUV), first described in 1717 by Morgagni, occur in one of every 5,000–8,000 male infants . PUV can lead to bilateral renal obstruction, bladder dysfunction, vesicoureteral reflux, hydronephrosis and renal damage. In 1919, Young proposed a classification system consisting of three types of valves. Type I valves (the most common) arise from the verumontanum and extend distally to attach to the lateral walls of the urethra as two leaflets (type Ia) or as a unicuspid leaflet (type Ib). Type II valves extend proximally from the verumontanum to the bladder neck, and type III valves represent a diaphragm attached to the circumference of the urethra with a central hole, distal (type IIIa) or proximal (type IIIb) to the verumontanum [1, 8]. The existence of Young’s type II valves is debated . Type III valves are considered by many to be the same entity as Cobb’s collar and are discussed in the section on congenital urethral strictures [1, 7].
The valves are composed of connective tissue interspersed with smooth muscle, similar to the tissue encasing the ejaculatory ducts, and are lined by stratified squamous epithelium rather than the transitional epithelium that lines the posterior urethra .
The embryologic defect leading to the development of PUV is not known. Many authors believe the anomaly is caused by abnormal integration of the Wolffian ducts into the urethra, while others consider it a result of persistence of the cloacal membrane [1, 7].
Autopsy studies have shown that the valves represent a single obliquely oriented diaphragm rather than a bicuspid structure as originally believed [7, 9]. The term congenital obstructive posterior urethral membrane (COPUM) has, therefore, been introduced .
Many cases of PUV are detected by prenatal sonography. Normal fetal renal parenchyma and amniotic volume do not necessarily predict good postnatal renal function . Intrauterine intervention, while improving hydronephrosis, does not prevent the development of renal dysplasia . Newborns might present with pulmonary hypoplasia secondary to oligohydramnios, or an abdominal mass caused by hydronephrosis or bladder distension. In older patients clinical signs include failure to thrive, poor urinary stream, sepsis, poor renal function or salt-losing nephropathy . Treatment is by fulguration of the valve, although in infants vesicostomy or pyeloureterostomy might be performed prior to valve ablation. Posterior urethral dilatation persists after fulguration in 20% of patients. Despite therapy, 40% of patients have poor renal growth and another 40% develop end-stage renal disease [12, 14]. A normal or near normal serum creatinine level or GFR at 1 year of life, following decompression of the urinary tract, are useful positive prognostic indicators [12, 14]. In contrast, the development of proteinuria is associated with a poor prognosis .
Prune-belly syndrome (PBS) was first described in 1950 by Eagle and Barret as a triad of deficient abdominal wall muscles, urinary tract anomalies and cryptorchidism. Associated abnormalities include cardiac, limb and intestinal abnormalities (30% of patients have malrotation). Although PBS is primarily seen in males, it has been described in females; female patients have genital abnormalities including bicornuate uterus and vaginal atresia .
The prostatic urethra is dilated secondary to or in association with prostatic aplasia or hypoplasia. (Pathologically, the verumontanum is small and the ejaculatory ducts are normal.) Unlike PUVs, the prostatic urethra in patients with PBS is high in position and its posterior wall is elongated relative to its anterior wall. A normal utricle might be present. The penile urethra might be normal, or additional urethral anomalies such as megalourethra, urethral atresia or coexisting posterior urethral valves can be present. Patients with urethral obstruction do not survive .
Congenital urethral stricture
Congenital urethral polyps
Mullerian duct remnants: enlarged prostatic utricle and Mullerian duct cyst
Midline cystic structures arising at the dorsal aspect of the prostatic urethra represent two distinct categories of Mullerian duct remnants. They are described as enlarged prostatic utricles when they communicate with the urethra and as Mullerian duct cysts when they do not. These terms have been used interchangeably, contributing to confusion in nomenclature.
Enlarged prostatic utricles are classified according to a grading system. In grade 0, the opening is located in the prostatic urethra but the utricle does not extend over the verumontanum. In grade I the utricle is larger but does not extend to the bladder neck. In grade II, it extends over the bladder neck, and in grade III, the opening of the utricle opens into the bulbous urethra rather than the prostatic urethra . Enlarged prostatic utricles are often seen in patients with intersex, hypospadias (11–14%), and cryptorchidism. There is a direct relationship between the degree of hypospadias and utricular size .
Cowper’s syringocele, a rare anomaly, occurs when there is dilatation of the main draining duct. Maizels et al.  described four types of Cowper’s syringocele: simple syringocele, in which there is reflux into a minimally dilated duct; imperforate syringocele, in which the orifice draining the dilated duct is closed and there is cystic dilatation of the distal duct at the level of the bulbourethra; perforate syringocele, in which the orifice draining the duct is patulous and there is free reflux into the duct resembling a diverticulum; and ruptured syringocele, in which the distal portion of the duct is dilated but is not in communication with the more proximal portion of the main duct . Cowper’s syringocele can also be classified as open or closed, open if it communicates with the urethra and closed if it does not .
Clinically, patients present with frequency, urgency, dysuria, post-void incontinence, hematuria or urinary tract infection. Diagnosis is made by VCUG, retrograde urethrography or urethrocystoscopy. Treatment requires marsupialization of the syringocele.
Anterior urethral valves and diverticula
Anterior urethral valves and diverticula are rare. Most anterior valves (40%) are located in the bulbous urethra, but they can also be located at the penoscrotal junction (30%) and penile urethra (30%) . They are composed of folds located on the ventral aspect of the urethra that rise during voiding, resulting in urethral obstruction . Anterior urethral diverticula communicate with the urethra and are found on the ventral aspect of the urethra between the bulbous and mid-penile urethra .
Although many authors distinguish between anterior urethral valves and diverticula, others consider them the same entity. It has been proposed that valves cause proximal urethral dilatation with the formation of a saccular diverticulum . Conversely, progressive enlargement of a diverticulum can result in a distal valve-like flap. Embryologic theories of anterior urethral diverticula formation include a developmental defect in the corpus spongiosum leading to formation of a diverticulum, cystic dilatation of urethral glands, and sequestration of an epithelial rest . McLellan et al.  have demonstrated connections between an anterior urethral diverticula and cowperian ducts, proposing that anterior urethral diverticula and valves arise from the anterior lip of a ruptured cowperian duct syringocele.
Urethral duplication is a rare anomaly frequently seen in association with other anomalies including hypospadias, epispadias, cleft lip and palate, congenital heart disease, tracheoesophageal fistula, imperforate anus and musculoskeletal anomalies .
Duplication commonly occurs along the sagittal plane. The ventral urethra is the more functional urethra and contains the verumontanum and sphincters . When urethral duplication is present along the coronal plane, bladder duplication is always present .
Urethral duplication can be classified into three types using Effmann’s classification . In type I, there is partial duplication of the urethra. In type II, there is complete duplication of the urethra. Type II urethral duplication can be subclassified as IIA1 if both urethras arise from separate bladder necks, type IIA2 if one channel arises from the other, and type IIB if there is duplication with one meatus. IIA2 Y-duplication occurs when one urethra arising from the bladder neck or posterior urethra opens to the perineum. This type of urethral duplication often coexists with stenosis of the anterior portion of the normally positioned urethra  and other severe congenital anomalies . Type III urethral duplication consists of complete duplication of the urethra and bladder.
Embryologically, urethral duplication can be caused by abnormal Mullerian duct termination and growth arrest of the urogenital sinus  or misalignment of the termination of the cloacal membrane with the genital tubercle .
Congenital urethroperineal fistula
Although congenital urethroperineal fistula (CUF) resembles Y-type urethral duplication, it should be considered a separate entity. In Y-type urethral duplication the ventral urethra opens to the perineum and, as in all urethral duplications, is the functional urethra. In contrast, in CUF, the dorsal urethra is the functional urethra and the ventral urethra (fistula) is hypoplastic [41, 45]. The differentiation between Y-type duplication and CUF is particularly important in the surgical management of these patients. In Y-duplication the functional ventral channel should not be resected, while in the CUF resection of the ventral channel is curative .
The embryology of CUF is not clear. Diagnosis is made by retrograde urethrography or VCUG to determine the dominant urethra. In the case of congenital urethroperineal fistula, voiding will be predominantly through the dorsal urethra. Urodynamics is important to determine the position of the external urethral sphincter .
Congenital anorectal malformation represents a complicated spectrum of anomalies that range from anal stenosis to imperforate anus with communication with the urogenital system . In males, the anomaly is characterized by gastrointestinal anomalies, imperforate anus and frequently other congenital anomalies including tracheoesophageal fistula, heart disease, limb anomalies and radial and renal abnormalities (VACTERL complex) . The malformation can be classified as low or high/intermediate. Fistulous communications between the blind-ending rectum and the urinary tract are more common in males with high/intermediate anomalies. Rectobulbocutaneous fistulous tracts have also been described in which a fistula arises from the rectum, communicates with the bulbous urethra at its mid-portion and continues ventrally to terminate at the scrotum wall [48, 49]. A concurrent rectourethral fistula to the posterior urethra or, less commonly, the bladder, has been described in 28% of patients of imperforate anus  and there is a high incidence of other urethral anomalies as well [39, 40].
Embryologically the normal sequence of cloacal division by the urorectal septum into the urogenital sinus and rectum is disrupted. Preexisting fusion of mesoderm at the dorsal aspect of the cloacal folds might be present, which prevents normal caudal descent of the urogenital septum and results in a urorectal septal defect. Recent reports describe a possible genetic link between abnormalities of the Sonic Hedgehog gene (Shh) and midline developmental abnormalities such as those seen with the VATERL association [40, 47, 51].
The development of the male urethra is complex. Although understanding its embryology can help explain certain anomalies, there are others in which the embryologic defect has not been elucidated. Imaging, including VCUG and retrograde urethrography, is helpful in defining the anatomy and identifying associated findings.