The male urethra is a narrow fibromuscular tube that conducts urine and semen from the bladder and ejaculatory ducts, respectively, to the exterior of the body (see the image below). Although the male urethra is a single structure, it is composed of a heterogeneous series of segments: prostatic, membranous, and spongy.
Male urethra and its segmentsMost proximally, the prostatic urethra is responsible for involuntary continence, transmission of semen into the common genitourinary tract, and the most common site of bladder outlet obstruction in the Western world. The membranous urethra is critical to voluntary continence and, because of its rigid attachments, is highly susceptible to injury in pelvic trauma. The spongy urethra is surrounded by the corpus spongiosum and forms the terminal conduit communicating with the outside of the body.
Knowledge of male urethral anatomy is essential for all health professionals because urethral catheterization is one of the most commonly performed procedures in health care. The male urethra is susceptible to a variety of pathologic conditions, ranging from traumatic to infectious to neoplastic. Pathophysiologic variants of the urethra may have devastating consequences, such as renal failure and infertility.
The posterior male urethra forms from the urogenital sinus (see the image below). This sinus derives from the endoderm-derived cloaca, which is separated from the anorectal canal by the growth of the urorectal septum in the fourth week of gestation. The spongy urethra is formed after the seventh week by tubularization of the urethral folds along the urethral groove under the influence of dihydrotestosterone. The most distal portion of the urethra is likely formed by invagination of an epithelial tag at the distal end of the genital tubercle. [1]
Embryologic development of pendulous urethra.The male urethra is a fibromuscular tube. It has distinct longitudinal folds that protrude into its lumen and make it readily identifiable on cross-section. The lining of the urethra varies from segment to segment but transitions from the urothelium of the bladder to the keratinized stratified squamous epithelium of the glans. The prostatic urethra is lined with transitional cell epithelium (urothelium). The membranous urethra is lined with stratified columnar and pseudostratified epithelium. Also, a rich vascular submucosa exists in the membranous urethra.
Finally, the penile urethra is enclosed by the corpus spongiosum and lined with stratified columnar and pseudostratified epithelium with stratified squamous epithelium distally. The entire posterior urethra is lined with a submucosa and a series of muscular sphincters. The urethra is lined on the dorsal surface by the glands of Littre, which are concentrated more distally. Additionally, small diverticula, called lacunae of Morgagni, and a larger lacuna magna can be found at the fossa navicularis. [3]
Perianal Lesions
The location of perianal lesions is described in relation to a clock (as seen in the supine position), eg, 2 o’clock, 7 o’clock. Sites of perianal lesions include the following:
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Perianal skin – Abscess, hematoma (erroneously called thrombosed external hemorrhoids), external opening of fistula-in-ano, skin tag (in chronic fissure-in-ano)
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Anal canal skin (anoderm, below dentate line) – Fissure-in-ano, externalhemorrhoids, cancer
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Anal canal mucosa (above pectinate line) – Internal hemorrhoids, cancer
The pectinate line cannot be felt on rectal examination but is seen on anoproctoscopy; under anesthesia, the pectinate line can be seen on retraction of the perianal skin. The anorectal flexure can be palpated on rectal examination (but not under anesthesia when the muscles relax).
Infection of an anal gland is considered the initial event in the formation of a perianal abscess and then fistula-in-ano. Fissure-in-ano is an ulcer in the sensitive anal canal skin and is a very painful condition. Fistula-in-ano can be intersphincteric, trans-sphincteric, or suprasphincteric. The internal opening of fistula-in-ano can be in the anal canal or rectum.
External hemorrhoids are in located below the pectinate line on sensitive anal canal skin and are painful, while internal hemorrhoids are located above the pectinate line in insensitive anal canal mucosa and are painless (unless complicated). For the same reason, internal hemorrhoids can be intervened (injected with sclerosant or ligated with rubber band) without anesthesia.
During posterior or lateral sphincterotomy for fissure in ano, it is only the internal sphincter that is divided.
A cruciate incision in the perianal skin lateral to the anal verge provides easy and direct access to ischioanal fossae for drainage of an abscess.
Surgical considerations
Intersphincteric resection of the rectum (eg, for ulcerative colitis) follows the plane between the external and internal sphincters; external anal sphincter, levator ani, and puborectalis muscles are preserved.
In hand-sewn ileal pouch anal anastomosis (IPAA), also called restorative proctocolectomy (RPC) for ulcerative colitis, the ileal pouch is anastomosed to the pectinate line, which is exposed perianally. For stapled IPAA, the surgical anal canal is divided 1-2 cm above the pectinate line using a linear stapler; the ileal pouch is then anastomosed to the anal canal stump using a circular stapler.
Cancers of anal canal below the pectinate line are usually squamous cell carcinoma (or basal cell carcinoma and melanoma), whereas those of the anal canal above pectinate line (and of the rectum) are adenocarcinoma. Anal canal and low rectal cancers can infiltrate the anorectal ring and cause incontinence—a contraindication for sphincter preservation (by chemoradiotherapy for squamous cell carcinoma and low-anterior resection for adenocarcinoma). Anal canal cancer (or rectal cancer infiltrating into the anal canal) spreads to the superficial inguinal lymph nodes.
Imaging considerations
Magnetic resonance imaging (MRI) has become the imaging modality of choice for delineation of anal and perianal anatomy in diseases such as fistula-in-ano, incontinence, and anorectal cancer, among others.
Although the male urethra varies in length and angulation, no common natural variants in urethral anatomy are recognized. Pathophysiologic variants include duplication, urethrorectal fistulae, congenital strictures, hypospadias, epispadias, and posterior or anterior urethral valves.
Duplication
Urethral duplication is a rare anomaly that typically occurs in the sagittal plane. Incontinence and infection are presenting symptoms; these anomalies can usually be picked up on newborn examination. A dorsal duplication is associated with a normal urethral meatus at the tip of the glans and an epispadiac urethra and dorsal chordee. The dorsal urethra may be blind-ended or associated with bladder exstrophy.
Ventral duplication and duplication in the same horizontal plane are other variants of urethral duplication. Treatment is necessary if incontinence or infections are an issue. Fulguration or excision of the abnormal urethra is the standard therapy.
Urethrorectal fistulae
Urethrorectal fistulae are rare and are usually associated with imperforate anus. The signs of this are passage of stool and air through the meatus. Alternatively, in cases with a patent anus, urine may pass via the anus. Management of urethrorectal fistula in the presence of an imperforate anus involves either opening the anus and closing the fistula or fecal diversion if the distance between the blind-ended rectum and the perineum is too far for immediate reconstruction.
Congenital strictures
Congenital urethral strictures are rare but most commonly occur at the membranous urethra and fossa navicularis. They can be diagnosed with excretory urography, retrograde urethrography, or urethroscopy. Treatment involves direct-vision internal urethrotomy for membranous or fossa navicularis strictures or dilation for membranous strictures. Failure of endoscopic therapy warrants reconstruction.