PERINEUM

Tuesday, August 12, 2008

The perineum lies between the pubis, thighs, and buttocks and is limited superiorly by the levator ani. Viewed from below, the symphysis pubis, ischial tuberosities, and coccyx outline the diamond shape of the perineum; the inferior ischiopubic rami and sacrotuberous ligaments form its bony and ligamentous walls (Figs. 2-35 and 2-36 [35] [36]). A line drawn through the ischial tuberosities divides the perineum into an anal and a urogenital triangle.


Figure 2-35 Male perineum. (From Anson BJ, McVay CB: Surgical anatomy, 6th ed. Philadelphia, WB Saunders, 1984, p 893.)



Anal Triangle

At the apex of the prostate, the rectum turns approximately 90 degrees posteriorly and inferiorly to become the anus (see Figs. 2-10 and 2-30 [10] [30]). It traverses 4 cm to reach the skin near the center of the anal triangle. The subcutaneous fat that surrounds the anus is continuous with that of the urogenital triangle, buttocks, and medial thigh. Laterally, the fat fills the ischiorectal fossa, a space bounded by the levator ani medially, and obturator internus, and the sacrotuberous ligament laterally (see Fig. 2-14 ). Anteriorly, this space extends into a recess above the urogenital diaphragm; posteriorly, it is continuous with the intermediate stratum of the pelvis through the sciatic foramina. Through this continuity, infections may travel between the perineum and the pelvic cavity.

The anal sphincter is divided into internal and external components. The internal sphincter represents a thickening of the inner circular smooth muscle layer of the rectum. The outer longitudinal smooth muscle thins beyond the rectourethralis and blends with the external sphincter, although a few fibers insert in the skin around the anus (corrugator cutis ani) to give it a puckered appearance. The external sphincter surrounds the internal sphincter and is divided into subcutaneous, superficial, and deep portions. The subcutaneous part attaches to the perineal body by collagenous and muscular fibers that are thickest superficially and referred to as the central tendon of the perineum. The superficial sphincter attaches to the perineal body and coccyx. At the posterior inflection of the rectum, the deep sphincter blends with the puborectalis sling of levator ani. At this level, a firm band may be felt on rectal examination and corresponds to the internal and external sphincter. Division of this muscular band results in fecal incontinence. The prostate may be accessed anterior to the sphincter, by dividing the central tendon and sphincteric attachments to the perineum (Young's procedure) or by following the anterior rectal wall beneath the external anal sphincter (Belt's procedure).

Male Urogenital Triangle

The entire urogenital triangle is bridged by the urogenital diaphragm. The scrotum is dependent from the anterior aspect of the urogenital triangle; in the posterior aspect, skin and subcutaneous fat overlie Colles' fascia. The perineal membrane and the posterior and lateral attachments of Colles' fascia limit a potential space known as the superficial pouch (see Figs. 2-3, 2-14, and 2-36 [3] [14] [36]). In this space, the three erectile bodies of the penis have their bony and fascial attachments (the root of the penis). The paired corpora cavernosa attach to the inferior ischiopubic rami and perineal membrane and are surrounded by the ischiocavernosus muscles. The corpus spongiosum dilates as the bulb of the penis and is fixed to the center of the perineal membrane. It is encompassed by the bulbospongiosus muscles that arise from the perineal body and from a central tendinous raphe and pass around the bulb to attach to the perineal membrane and dorsum of the penis. Contraction of the ischiocavernosus and bulbospongiosus muscles compresses the erectile bodies and potentiates penile erection. The transversus perinei muscles (superficial and deep) run along the posterior edge of the perineal membrane and are thought to stabilize the perineal body. Deep to the perineal membrane rests the striated urethral sphincter (discussed earlier).

Blood supply to the anal and urogenital triangles is derived largely from the internal pudendal vessels ( Fig. 2-37 ). After entering the perineum through the lesser sciatic foramen, the artery runs in a fascial sheath on the medial aspect of obturator internus, the pudendal canal (of Alcock). Early in its course, it gives off three or four inferior rectal branches to the anus. Its perineal branch pierces Colles' fascia to supply the muscles of the superficial pouch and continues anteriorly to supply the back of the scrotum. The internal pudendal terminates as the common penile artery (to be discussed).


Figure 2-37 Male perineum, illustrating the internal pudendal artery and its branches on the left and the pudendal nerve and its branches on the right.


The internal pudendal veins communicate freely with the dorsal vein complex by piercing the levator ani. These communicating vessels enter the pelvic venous plexus on the lateral surface of the prostate and are a common, often unexpected, source of bleeding during apical dissection of the prostate. The inferior rectal veins anastomose with the middle and superior rectal veins and produce an important connection between the portal and the systemic circulation. Obstruction of the portal or systemic venous system may cause shunting of collateral venous drainage through the portal system, manifested by hemorrhoids.

The pudendal nerve follows the vessels in their course through the perineum (see Fig. 2-37 ). Its first branch, the dorsal nerve of the penis, travels ventral to the main pudendal trunk in Alcock's canal. Several inferior rectal branches supply the external sphincter muscle and provide sensation to perianal skin. The perineal branches follow the perineal artery into the superficial pouch to supply the ischiocavernosus, bulbospongiosus, and transversus perinei muscles. A few of these branches continue anteriorly to supply sensation to the posterior scrotum. Additional perineal branches pass deep to the perineal membrane to supply the levator ani and striated urethral sphincter.

Penis

As discussed, the root of the penis is fixed to the perineum within the superficial pouch. The corpora cavernosa join beneath the pubis (penile hilum) to form the major portion of the body of the penis. They are separated by a septum that becomes pectiniform distally, so that their vascular spaces freely communicate. They are enclosed by the tough tunica albuginea, which is predominantly collagenous ( Fig. 2-38 ). Its outer longitudinal and inner circular fibers form an undulating meshwork when the penis is flaccid and appear tightly stretched with erection ( Goldstein et al, 1982 ). Smooth muscle bundles traverse the erectile bodies to form the endothelium-lined cavernous sinuses. These sinuses give the erectile tissue a spongy appearance on gross examination.


Figure 2-38 Cross section of the penis, demonstrating the relationship between the corporal bodies, penile fascia, vessels, and nerves. (From Devine CJ Jr, Angermeier KW: Anatomy of the penis and male perineum. AUA Update Series 1994;13:10-23.)



Distal to the bulb, the corpus spongiosum tapers and runs on the underside (ventrum) of the corpora cavernosa and then expands to cap them as the glans penis. The corona separates the base of the glans from the shaft of the penis. The spongiosum is traversed throughout its length by the anterior urethra, which begins at the perineal membrane (see Fig. 2-27 ). The anterior urethra is dilated in its bulbar and glanular segments (fossa navicularis) and narrowest at the external meatus. Proximally, it is lined by stratified and pseudostratified columnar epithelium, distally by stratified squamous epithelium. The mucus-secreting glands (of Littre) may be seen as small outpouchings of the mucosa.

Buck's fascia surrounds both cavernosal bodies dorsally and splits to surround the spongiosum ventrally (see Fig. 2-38 ). Elastic and collagenous fibers from the rectus sheath blend with and surround Buck's fascia as the fundiform ligament of the penis. Deeper fibers from the pubis form the suspensory ligament of the penis. In the perineum, Buck's fascia fuses with the tunica albuginea deep to the muscles of the erectile bodies ( Uhlenhuth et al, 1949 ). Distally, it fuses with the base of the glans at the corona. Bleeding from a tear in the corporal bodies (e.g., penile fracture) is usually contained within Buck's fascia, and ecchymosis is limited to the penile shaft.

The skin of the penile shaft is highly elastic and without appendages (hair or glandular elements), except for the smegma-producing glands at the base of the corona. It is devoid of fat and quite mobile because of the loose attachment of its dartos backing to Buck's fascia. Distally, it folds over the glans as the foreskin and attaches firmly below the corona. Its blood supply is independent of the erectile bodies and is derived from the external pudendal branches of the femoral vessels (see Fig. 2-4 ). These vessels enter the base of the penis to run longitudinally in the dartos fascia as a richly anastomotic network. Thus, penile skin may be mobilized on a vascular pedicle as the ideal tissue for urethral reconstruction. The skin of the glans is immobile as a result of its direct attachment to the underlying thin tunica albuginea.

The common penile artery continues in Alcock's canal, above the perineal membrane, and terminates in three branches to supply the erectile bodies ( Fig. 2-39 ). The bulbourethral artery penetrates the perineal membrane to enter the spongiosum from above at its posterolateral border. This large, short artery can be difficult to isolate and control during urethrectomy. It supplies the urethra, spongiosum, and glans. The cavernosal artery pierces the corporal body in the penile hilum to near the center of its erectile tissue. It gives off straight and helicine arteries that ramify to supply the cavernous sinuses. The dorsal artery of the penis passes between the crus penis and the pubis to reach the dorsal surface of the corporal bodies. It runs between the dorsal vein and the dorsal penile nerve and with them attaches to the underside of Buck's fascia (see Fig. 2-41 ). As it courses to the glans, it gives off cavernous branches and circumferential branches to the spongiosum and urethra. The rich blood supply to the spongiosum allows safe division of the urethra during stricture repair ( Devine and Angermeier, 1994 ).


Figure 2-39 Arterial supply of the penis.


The surgeon contemplating penile revascularization must be aware that the penile arteries are highly variable in their branching, courses, and anastomoses ( Bare et al, 1994 ). It is not uncommon for a single cavernosal artery to supply both corporal bodies or to be absent altogether. Alternatively, an accessory pudendal artery may supplement or completely replace branches of the common penile artery ( Fig. 2-40 ). This artery usually arises from the obturator or inferior vesical arteries and runs anterolateral to or within the prostate to reach the penis in the company of the dorsal vein. This artery has been identified in 7 of 10 cadaveric specimens ( Breza et al, 1989 ) and noted at 4% of radical prostatectomies ( Polascik and Walsh, 1995 ); its resection at prostatectomy may adversely affect postoperative potency ( Droupy et al, 1999 ).

At the base of the glans, several venous channels coalesce to form the dorsal vein of the penis, which runs in a groove between the corporal bodies and drains into the preprostatic plexus ( Fig. 2-41 ). The circumflex veins originate in the spongiosum and pass around the cavernosa to meet the deep dorsal vein perpendicularly. They are present only in the distal two thirds of the penile shaft and number 3 to 10. Intermediary venules form from the cavernous sinuses to drain into a subtunical capillary plexus. These plexuses give rise to emissary veins, which commonly follow an oblique path between the layers of the tunica and drain into the circumflex veins dorsolaterally. Emissary veins in the proximal third of the penis join on the dorsomedial surface of the cavernous bodies to form two to five cavernous veins. At the hilum of the penis, these vessels pass between the crura and the bulb, receiving branches from each, and join the internal pudendal veins. Valves are found in the emissary, cavernosal, and deep dorsal veins and may thwart attempts to revascularize the penis by arteriovenous anastomosis ( Sohn, 1994 ).

The dorsal nerves provide sensory innervation to the penis. These nerves follow the course of the dorsal arteries and richly supply the glans (see Fig. 2-41 ). Small branches from the perineal nerve supply the ventrum of the penis near the urethra as far as the glans distally ( Uchio et al, 1999 ). These nerves must be anesthetized when performing a penile block to numb the ventrum of the penis. The route of the cavernous nerves has been described. After piercing the corporal bodies, they ramify in the erectile tissue to supply sympathetic and parasympathetic innervation from the pelvic plexus. Tonic sympathetic tone inhibits erection. Parasympathetic nerves release acetylcholine, nitric oxide, and vasoactive intestinal polypeptide, which cause the cavernosal smooth muscle and arterial relaxation necessary for erection ( Burnett, 1995 ). It is thought that during erection, the subtunical venules are occluded by being compressed against the nondistensible tunica albuginea. Insufficient venous occlusion, particularly in vessels draining into the deep dorsal and cavernosal veins, is thought to cause vasculogenic impotence.

Scrotum

The scrotal skin is pigmented, hair bearing, devoid of fat, and rich in sebaceous and sweat glands. It varies from loose and shiny to highly folded with transverse rugae, depending on the tone of its underlying smooth muscle. A midline raphe runs from the urethral meatus to the anus and represents the line of fusion of the genital tubercles. Deep to this raphe, the scrotum is separated into two compartments by a septum.

The dartos layer of smooth muscle is continuous with Colles', Scarpa's, and the dartos fascia of the penis (see Figs. 2-3 and 2-36 [3] [36]). The testes are suspended by their cords in the scrotal compartments. As the testes descend, they acquire coverings from the layers of the abdominal wall, known as the spermatic fascia, that form part of the scrotal wall ( Fig. 2-42 ). The external spermatic fascia derives from the external oblique fascia and remains firmly attached to the borders of the external ring. The cremasteric muscle and fascia arise from the internal oblique muscle and attach laterally to the inguinal ligament and iliopsoas fascia and medially to the pubic tubercle. The internal spermatic fascia is a continuation of the transversalis fascia. The parietal and visceral tunica vaginalis surround the testis with a mesothelium-lined pouch and are derived from the peritoneum. They are continuous at the posterolateral border of the testis at its mesentery, where it is fixed to the scrotal wall. The testis is also fixed at its lower pole by the gubernaculum. Occasionally, the mesentery and gubernaculum may be deficient, leaving the testis unfixed (bell-clapper deformity) and predisposing to torsion of the cord.


Figure 2-42 Scrotum and its layers. (From Pansky B: Review of Gross Anatomy, 6th ed. New York, McGraw-Hill, 1987, p 483.)



The anterior wall of the scrotum is supplied by the external pudendal vessels and the ilioinguinal and genitofemoral nerves (see Fig. 2-4 ). The anterior vessels and nerves typically run parallel to the rugae and do not cross the raphe; thus, transverse or midline raphe scrotal incisions are most appropriate. The back of the scrotum is supplied by the posterior scrotal branches of the perineal vessels and nerves (see Fig. 2-37 ). In addition, the posterior femoral cutaneous nerve (S3) gives a perineal branch to supply the scrotum and perineum (see Fig. 2-8 ). In accordance with their origin, the spermatic fasciae have a blood supply (cremasteric, vasal, testicular) separate from that of the scrotal wall. Fournier's gangrene usually does not involve these structures, and they may be spared during d├ębridement.

Perineal Lymphatics

The penis, scrotum, and perineum drain into the inguinal lymph nodes. These nodes may be divided into superficial and deep groups, which are separated by the deep fascia of the thigh (fascia lata). In relation to the external pudendal, superficial inferior epigastric, and superficial circumflex iliac vessels, the superficial nodes lie at the saphenofemoral junction. At the saphenous opening (fossa ovalis) in the fascia lata, the greater saphenous vein joins the femoral vein, and the superficial nodes communicate with the deep group. Most of the deep inguinal nodes lie medial to the femoral vein and send their efferents through the femoral ring (beneath the inguinal ligament) to the external iliac and obturator nodes. Just outside the femoral ring, a large node (Cloquet's or Rosenmuller's node) is consistently present.

The scrotal lymphatics do not cross the median raphe and drain into the ipsilateral superficial inguinal lymph nodes. Lymphatics from the shaft of the penis converge on the dorsum and then ramify to both sides of the groin. Those of the glans pass deep to Buck's fascia dorsally and drain to superficial and deep groups in both sides of the groin. Direct lymphatic channels from the glans to the pelvic nodes, which bypass the inguinal nodes, have been proposed by anatomists; however, clinical studies have not confirmed their existence. Other studies have suggested that all penile lymphatic drainage passes through “sentinel nodes,” which lie medial to the superficial inferior epigastric veins. Clinical studies have also called this speculation into question ( Catalona, 1988 ). The perineal skin and fasciae drain into superficial nodes; the structures of the superficial pouch likely drain into the superficial and deep groups.

Testes

The testes are 4 to 5 cm long, 3 cm wide, and 2.5 cm deep and have a volume of 30 mL. They are enclosed in a tough capsule comprising (1) the visceral tunica vaginalis; (2) tunica albuginea, with collagenous and smooth muscle elements; and (3) the tunica vasculosa. The epididymis attaches to the posterolateral aspect of the testis. Beneath it, the tunica albuginea projects inward to form the mediastinum testis, the point at which vessels and ducts traverse the testicular capsule ( Fig. 2-43 ). Septa radiate from the mediastinum to attach to the inner surface of the tunica albuginea to form 200 to 300 cone-shaped lobules, each of which contains one or more convoluted seminiferous tubules. Each tubule is U-shaped and has a stretched length of nearly 1 m. Interstitial (Leydig) cells lie in the loose tissue surrounding the tubules and are responsible for testosterone production. Toward the apices of the lobules, the seminiferous tubules become straight (tubuli recti) and enter the mediastinum testis to form an anastomosing network of tubules lined by flattened epithelium. This network, known as the rete testis, forms 12 to 20 efferent ductules and passes into the largest portion of the epididymis, the caput. Here, the efferent ductules enlarge, become more convoluted, and form conical lobules. The duct from each lobule drains into a single epididymal duct, which winds approximately 6 m within the fibrous sheath of the epididymis to form its body and tail. As the duct approaches the tail, it thickens and straightens to become the vas deferens.

The spermatic cord is composed of the vas deferens, testicular vessels, and spermatic fasciae. As discussed in Chapter 1 , Surgical Anatomy of the Retroperitoneum, Kidneys, and Ureters, the testicular arteries arise from the aorta and travel in the intermediate stratum of the retroperitoneum to reach the internal inguinal ring. Lateral to the internal inguinal ring, the attachments of the intermediate stratum form the lateral spermatic fascia. These attachments may be taken down at orchidopexy to gain cord length. At the internal ring, the vessels are joined by the genital branch of the genitofemoral nerve, the ilioinguinal nerve, the cremasteric artery, and the vas deferens and its artery.

In its course to the testis, the testicular artery branches into an internal artery and an inferior testicular artery and into a capital artery to the head of the epididymis ( Fig. 2-44 ). The level of this branching varies and has been noted to occur within the inguinal canal in 31% to 88% of cases ( Beck et al, 1992 ; Jarow et al, 1992 ). When performing an inguinal varicocelectomy, the surgeon must remember that there may be two or three arterial branches at this level ( Hopps et al, 2003 ). A rich arterial anastomosis occurs at the head of the epididymis, between the testicular and the capital arteries, and at the tail between the testicular, the epididymal, the cremasteric, and the vasal arteries (see Fig. 2-44 ). The testicular arteries enter the mediastinum and ramify in the tunica vasculosa, principally in the anterior, medial, and lateral portions of the lower pole and the anterior segment of the upper pole ( Fig. 2-45 ). Thus, placement of a traction suture through the lower pole tunica albuginea risks damaging these important superficial vessels and devascularizing the testis ( Jarow, 1991 ). Testicular biopsy should be carried out in the medial or lateral surface of the upper pole, where the risk of vascular injury is minimal.


Figure 2-45 Distribution of subtunical testicular arteries compiled from 27 right and 26 left vascular casts. The highest density of subtunical arteries is found at the anterior upper pole and the entire lower pole. Lateral (L) and medial (M) sides of the upper pole are relatively free of arterial branches. (From Jarow JP: Clinical significance of intratesticular anatomy. J Urol 1991;145:777-779.)



The testicular veins form several highly anastomotic channels that surround the testicular artery as the pampiniform plexus. This arrangement allows countercurrent heat exchange, which cools the blood in the testicular artery. At the level of the inguinal canal, the veins join to form two or three channels and then a single vein that drains into the inferior vena cava on the right and the renal vein on the left. The testicular veins may anastomose with the external pudendal, cremasteric, and vasal veins ( Fig. 2-46 ). These connections can allow varicoceles to recur after ablative procedures. Testicular lymphatic vessels drain to the para-aortic and interaortocaval nodes as detailed in Chapter 1 , Surgical Anatomy of the Retroperitoneum, Kidneys, and Ureters.


Figure 2-46 Venous drainage of the testis and epididymis. Note connections between the pampiniform plexus and the saphenous, internal iliac, and external iliac veins.


Visceral innervation to the testis and epididymis travels by two routes. A portion arises in the renal and aortic plexuses and travels with the gonadal vessels. Additional gonadal afferent and efferent nerves course from the pelvic plexus in association with the vas deferens ( Rauchenwald et al, 1995 ). Intractable orchialgia may respond to anesthesia of the pelvic plexus ( Zorn et al, 1994 ). Intriguingly, some afferent and efferent nerves cross over to the contralateral pelvic plexus ( Taguchi et al, 1999 ). This neural cross-communication may explain how pathologic processes in one testis (e.g., tumor or varicocele) may affect the function of the contralateral testis. The genital branch of the genitofemoral nerve supplies sensation to the parietal and visceral tunica vaginalis and the overlying scrotum.

KEY POINTS: LOWER URINARY TRACT AND MALE GENITALIA

The pelvic cavity is divided into the false pelvis superiorly, and the true pelvis, inferiorly, wherein lie all of the pelvic organs.
The bony prominences and ligaments of the pelvis and lower abdomen will orient the surgeon during physical examination and in the operating room.
The pelvic floor is closed off by the levator ani and urogenital diaphragm, and the muscles and fasciae of the pelvic floor provide critical support for the pelvic organs.
The rectum, bladder, prostate, seminal vesicles, uterus, vagina, penis, and clitoris receive blood supply from the anterior trunk of the internal iliac artery and innervation from the pelvic autonomic plexus.
The urethra, vagina, and anus exit through the perineum in association with the external genitalia.
Detailed knowledge of the relationships of the pelvic organs to each other and the bones and muscles of the pelvis, as well as the locations of the blood supply and innervation of all pelvic and perineal structures, is critical for safely performing all pelvic operations.

Female Urogenital Triangle

The vestibule of the vagina runs vertically throughout the length of the urogenital triangle. The labia majora form its lateral sides and fuse anteriorly as the hood of the clitoris. The subcutaneous fat pad of the mons pubis continues posteriorly in the labia majora to frame the vestibule. The labial fat pads receive blood supply from the external pudendal vessels and may be raised on these vessels as a rotational flap for repair of vesicovaginal or urethrovaginal fistulas ( Fig. 2-47 ). The urethra enters the vestibule between the clitoris and the vagina.


Figure 2-47 Arteries and nerves of the female perineum. (From Doherty MG: Clinical anatomy of the pelvis. In Copeland LJ [ed]: Textbook of Gynecology. Philadelphia, WB Saunders, 1993, p 51.)



The structure of the superficial pouch is similar to that of the male ( Fig. 2-48 ). The crura of the clitoris attach to the inferior ischiopubic rami, surrounded by the ischiocavernosus muscles, and converge to form the body of the clitoris. The vestibular bulbs lie to either side of the vaginal vestibule, covered by the bulbospongiosus muscles. As homologues of the penile bulb, they are composed of erectile tissue and meet anteriorly to form the glans of the clitoris. The vestibular glands are deep to the vestibular bulbs but, unlike the bulbourethral glands in the male, are superficial to the perineal membrane. Their ducts travel 2 cm to open in the vaginal vestibule on the posteromedial sides of the labia minora. The perineal membrane, pierced in its center by the vagina, is less well developed than that of the male. The innervation, blood supply, and lymphatic drainage of the external genitalia and superficial pouch are similar to those described in the male.


Figure 2-48 Female superficial perineal pouch. On the left side, the muscles have been removed to show the vestibular bulb and Bartholin's gland. (From Williams PL, Warwick R: Gray's Anatomy, 35th British ed. Philadelphia, WB Saunders, 1973, p 1364.)



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