The pelvic cavity, like most spaces in the body has an inlet and an outlet. For the most part, the pelvic outlet is closed off by the muscles of the pelvic floor (levator ani & coccygeus muscles). The region of the body superficial (caudal in a standing individual) to these muscles and medial to the thighs is known as the perineal region. It is a diamond shaped area that is best visualized (with the patient’s consent) when the individual is in a lithotomy position; that is, the upper body is supine while the knees are bent, the legs are elevated in stirrups, and the hips are flexed and abducted.
The anterior and posterior apices of the perineum are situated at the inferior aspect of the arcuate ligament and pubic symphysis and the tip of the coccyx respectively. Anterolateral boundaries are ischiopubic rami and ischial tuberosities on both sides while posterolaterally are the sacrotuberous ligaments. The external limitation of the perineal region is the skin of the area. It continues as the skin of the thigh and lower abdomen. The perineum is cranially (internally) limited by the pelvic floor muscles and its overlying fascia. The region is further subdivided by a horizontal line between each ischial tuberosity (interischial line) into a urogenital (anterior to the interischial line) and an anal triangle (posterior to the interischial line).
This article will address the contents of the perineum in addition to looking at anatomical disparities between the sexes. Neurovascular supply and lymphatic drainage, as well as come clinically relevant points will also be addressed.
Boundaries of the Perineal Region
The anal orifice can be appreciated in the anal triangle, while the respective external genitalia occupy the urogenital triangle of either sex. The skin of the perineum is extremely sensitive as a result of its rich innervation. The surface marking of the perineal body lies at the midpoint of the interischial line, posterior to the posterior commissure of the vagina (posterior base of the scrotum) and the anus. This region is also referred to by gynaecologists as the gynaecological perineum. The space is occupied by muscles, erectile and cavernous tissues that facilitate excretion, egestion and reproduction. Numerous blood vessels, lymphatics and nerves (discussed later) traverse this area to supply the urogenital and terminal digestive systems.
The urogenital triangle is the anterior portion of the perineal region bounded posteriorly by the interischial line. The general plan of this region is region is similar in males and females but a significant amount of sexual dimorphism exists with respect to the contents of the area. In both sexes, the urogenital triangle is divided into a superficial and a deep perineal space by the perineal membrane. The perineal membrane is a sheet of fibrous tissue and has thickened attachments along the borders of the urogenital triangle. It is also known as inferior fascia of urogenital diaphragm. At the apical point – where the membrane attaches to the arcuate ligament of the pubic symphysis – it is referred to as the transverse perineal ligament (pubourethral ligament in females). The posterior border of perineal membrane is attached to the perineal body.
The perineal membrane provides a caudal (superficial) boundary for the deep perineal pouch, which is also limited rostrally (deeply) by the endopelvic fascia of the pelvic floor. The deep perineal pouch is occupied by the pubourethralis and deep transverse perinei muscles. Superficial (caudal) to the perineal membrane is the superficial perineal pouch. This space is limited inferiorly by the deep perineal fascia (sheet of fascia that covers the superficial perineal muscles, viz. superficial transverse perinei, bulbospongiosus and ischiocavernosus). Its contents include the superficial transverse perineal muscles along with the pudendal neurovasculature, corpus spongiosum and corpora cavernosa.
Another important landmark of the urogenital triangle is the perineal body. This ill-defined, fibromuscular mass is located in the middle of the interischial line, between the two triangles. Many muscles get attachment at the perineal body. It has posterior communications with the external anal sphincter and anterior relations with bulbospongiosus and the deep and superficial transverse perinei. The perineal body also extends superiorly into the rectoprostatic (rectovaginal) septum of the pelvis.
Muscles of the Urogenital Triangle
There are several muscles found in the urogenital triangle that are important for reproduction and urinary excretion. Some of these muscles include:
- The superficial transverse perinei, as the name suggests, courses transversely across the superficial perineal space with the anus posterior to it. It has attachments on the anteromedial surface of the ischial tuberosity and perineal body.
- The deep transverse perinei has several attachments in the deep perineal pouch. As it spans the space in a transverse manner (albeit, incompletely), the muscle attaches to the perineal body posteriorly and the ischiopubic rami laterally. The muscle has an anterior deficiency.
- Bulbospongiosus differs between males and females. In the latter, the muscle is superficial to the bulbs of the vestibule and the associated glands. They subsequently travel lateral to the vagina and insert into the corpora cavernosa of the clitoris along with the body of the glans clitoris itself. However, in the former, the muscle starts as identical halves and fibers decussate in the perineal body and are attached to transverse superficial perinei muscles. The muscle assists with voiding the urethra after micturition and also in expelling semen or vaginal secretions during the ejaculatory process.
- Ischiocavernosus is found in both males and females, although it is significantly larger in males. It is attached medially to the ischial tuberosity and ischial rami. The aponeurotic ending of the muscle merges into the crus of the penis (clitoris). The primary action in males is to aid in stabilizing the erect penis; and in females it assists in promoting clitoral erection.
Female Urogenital Triangle
The female urogenital triangle contains the mons pubis, labia majora and labia minora, the clitoris, and the vaginal and urethral orifices. The perineal body is attached to the vaginal introitus, as well as the posterior commissure labia majora. In addition to the other muscles of the deep perineal pouch, females also have the urethrovaginalis muscle that surrounds both the urethral and vaginal orifices. The urogenital diaphragm is almost completely divided by the urethra and vagina into two triangular halves. The membrane is held in place by the pubourethral ligament, which is traversed by the vagina, urethra, Bartholin’s ducts, the labial neurovasculature and the deep dorsal neurovasculature of the clitoris.
Male Urogenital Triangle
The male urogenital triangle houses the bulb of the penis, the scrotum and serves as a point of attachment for the penis. The perineal body is continuous with the perineal raphe in the scrotal skin. In males, the bulbourethral ducts, urethra, deep dorsal neurovasculature of the penis and the nerves and vessels of the scrotum all pass over the perineal membrane.
The anorectal triangle is the posterior part of the perineal region and is generally identical in both males and females. The triangle is limited anteriorly by the posterior boundary of the urogenital triangle (the interischial line and perineal body) and its apex (located posteriorly) is at the tip of the coccyx. They differ only in size, where the female anal triangle is significantly wider than that of the male. This disparity is a direct result of the structural differences between the male and female pelvis. Not only is the transverse diameter (distance between the ischial tuberosities) of the female pelvis wider, but the plane of the pelvic outlet (the anteroposterior distance between the pubic arch and the tip of the coccyx) is also longer. This sexual dimorphism accommodates the passage of a baby through the pelvic cavity during childbirth.
The anal canal is present in the middle of this triangle and on each side is ischioanal (ischiorectal) fossa. The ischioanal fossa occupies most of the anal triangle. In a transverse section through the pelvis (in a lithotomy position), it has a horse shoe appearance, while in a coronal section (through a vertically erect individual) each fossa appears roughly pyramidal. Medially, the ischioanal fossa is limited by the external anal sphincters and the inferomedial part of the sloping levator ani muscle and limited laterally by the obturator internus muscle and its fascia. The two muscles of the walls abut and form the apex at the rostrolateral margin. The base of the pyramid is formed by skin. The space is filled with loose adipose tissue.
The fascia of obturator internus continues towards the superficial surface of the levator ani and covers the deep region of the ischiorectal fossa. At this point it is called the deep fascia of the anal triangle. Its counterpart, the superficial fascia of the anal triangle, is a continuation of the subcutaneous fascia of the peritoneal skin and the skin of the buttocks and thigh.
The release of end products of digestion from the gastrointestinal tract is regulated by the external anal sphincters. This oblong cluster of skeletal muscles circumscribes the anal canal and can be voluntarily constricted in order to restrict the process of defecation or the passage of flatus (gaseous by-product of bacterial activity in the intestines) in situations where it is not appropriate. Cranially, fibers of the muscle integrate with the inferior portion of puborectalis. Other fibers cross anteriorly and blend with the superficial transverse perineal muscles, while others attach to the anococcygeal raphe posteriorly. Caudally, the muscle has anteroposterior attachment from the perineal body to the anococcygeal ligament (and by extension, the coccyx).
The internal pudendal artery is the chief arterial supplier of the perineum. This branch of the internal iliac artery (along with its corresponding vein and nerve) enters the perineum by travelling through Alcock’s (pudendal) canal, which is located in the lateral wall of the anorectal fossa. It gives the cavernosal and dorsal arteries to the penis in males as well as branches to the vestibular bulb and vagina in females. The inferior rectal artery is a branch of the internal pudendal artery that supplies the external anal sphincter. The perineal arteries also arise from the internal pudendal artery. It forms anastomoses with the same vessel from the opposite side as well as with the posterior scrotal (labial) and inferior rectal arteries. They supply the scrotum (labia), perineal body and the transverse perinei muscles. The venous return is similar to the arterial homolog.
Like the blood supply to the perineum, the innervation is derived from the pudendal nerves. The pudendal nerves give branches known as the inferior rectal nerve, dorsal nerve of the clitoris (penis) and perineal nerves. The perineal nerve is the largest branch of the pudendal nerve. It gives rise to muscular supply to the transverse perinei, ischiocavernosus and bulbospongiosus as well as branches cutaneous innervation to the labia (scrotum). The inferior rectal nerve innervates the external anal sphincter, the skin around the anus and the distal anal canal. Finally, the dorsal nerve of the clitoris (penis) provides neuronal stimulation to the corpus cavernosa (sponge-like erectile tissue of the penis).