Anatomical Spaces of the Pelvic Cavity
The pelvic cavity is formed by three bilateral pairs of bones (pubis, ilium and ischium) and two posteriorly located bones (sacrum and coccyx). The cavity is home to the some of the urinary organs, most of the reproductive organs and the distal part of the digestive tract in humans. The pelvic cavity is described as having two parts:
- There is a false pelvis that spans the region below the iliac crest and above the arcuate line (pectinate line, anteriorly)
- And there is a true pelvis that begins below the arcuate line.
The caudal end of the peritoneal cavity rests in the pelvic cavity. As a result, spaces associated with this region are also considered as being part of the pelvic cavity spaces. Additionally, there are other anatomical spaces that exist inferior to the peritoneum. Since there are differences between the male and female internal and external reproductive organs, there are also some differences in the spaces of the pelvic cavity in that regards. This article aims to highlight the spaces and point out any gender-related differences.
Rectovesical Pouch (Male)
The superior and upper posterior parts of the urinary bladder as well as the upper and middle part of the rectum are covered by a single layer of peritoneum. The peritoneal covering creates a continuous surface coursing over the superior part of the urinary bladder to the middle and upper rectum. A depression exists at the junction of the two surfaces that is referred to as the rectovesical pouch.
Rectouterine Pouch of Douglas (Female)
A similar situation to that previously described also exists in the female pelvis. However, the space has an additional structure. In the female pelvic cavity, the uterus and vagina occupy the space between the urinary bladder and the rectum. The peritoneum is continuous over all three structures and consequently, an additional space is created. The space between the middle and superior rectum and the uterus is known as the rectouterine pouch of Douglas.
In both males and females, the rectovesical pouch and the rectouterine pouch of Douglas (respectively) not only communicate superiorly with the peritoneal cavity, but also bilaterally with the pararectal fossae. As suggested by the nomenclature, the spaces surround the intraperitoneal portion of the rectum. They are limited laterally (on either side) by the vesicosacral (uterosacral) folds in females and the sacrogenital folds in males. The pararectal fossa, rectovesical pouch and rectouterine pouch of Douglas are commonly occupied by loops of small intestines.
Vesicouterine Pouch (Female)
The second pelvic space formed by peritoneal covering in females is the vesicouterine pouch. The space exists between the posterior surface of the urinary bladder and the anterior surface of the uterus. This space is also continuous superiorly with the peritoneal cavity. The broad ligament and the round ligament of the uterus are lateral and posterior to the space. Due to the fact that the uterus is most commonly found in an anteverted (bent forward) position, its anterior surface is more closely related to the posterosuperior surface of the urinary bladder than its posterior surface is to the intraperitoneal rectum.
Rectovesical Space (Male)
Inferior to the rectovesical pouch, is a region known as the rectovesical space. The space is so named as it is located posterior to the urinary bladder and prostate, and anterior to the distal third of the rectum (extraperitoneal part). The space is further divided into three compartments by a septum, known as the rectovesical or rectoprostatic fascia of Denonvillier, which extends from the apex of the rectovesical pouch to the deep and superficial transverse perineal muscles. The three spaces are:
- The retrovesical part, which is the most superior of the three subdivisions. It is anterior to Denonvillier’s fascia and posterior to the urinary bladder. It also contains the seminal vesicles.
- The retroprostatic part is the most inferior part of the space. The posteroinferior part of the prostate gland and the posterior fibers of the sphincter urethrae are anterior to the space and Denonvillier’s fascia is posterior to it.
- Finally, the prerectal part of the space spans the full length of Denonvillier’s fascia. However, it is situated posterior to the septum and anterior to the distal third of the rectum; with the anterior rectal fascia being its closest posterior relation. The deep fibers of the eternal anal sphincter muscles and its fascia are immediately related to the inferior part of the space.
In females, the homologue to Denonvillier’s fascia is the rectovaginal fascia. A congenital abnormality, known as rectovaginal fistula would traverse the rectovaginal space, pierce the fascia and permit faecal transmission into the vagina. This defect can be corrected surgically.
Ischiorectal (Ischioanal) Fossa
Bilateral to the inferior aspect of the rectum and the anus are a pair of triangular spaces known as the ischiorectal (ischioanal) fossae. The apex of the space is located laterally, where its roof and lateral walls meet. The former is formed by the obliquely coursing levator ani muscle along with the inferior fascia of the pelvic diaphragm. The latter is formed by two structures: superiorly by the obturator internus muscle and its fascia and inferiorly by the medial part of the ischial tuberosity. Medially, the ischioanal fossa is limited by the medial part of levator ani (superiorly) and the external anal sphincter muscles (inferiorly). Finally, the perineal skin forms the floor of the region. The anterobasal and posterobasal constraints of the fossa are the posterior aspect of the perineal body along with the muscles of the urogenital diaphragm, and the gluteus maximus and the posterior sacrotuberous ligament, respectively.
The ischiorectal fossa houses several neurovascular and connective tissue structures. There are adipose lobules that gradually become smaller superoinferiorly. In the inferolateral wall, a neurovascular bundle courses through a tunnel formed in the fascia of obturator internus known as Alcock’s (pudendal) canal. Through this conduit, the internal pudendal vessels and the pudendal and perineal nerves course to their points of supply. Each pudendal neurovascular bundle enters the canal at the lesser sciatic notch and journey to the deep perineal pouch. The pudendal neurovascular bundle also gives off inferior rectal branches that cross the ischiorectal fossa. Additionally, the posterior labial (scrotal) neurovascular structures (branches of the pudendal neurovascular bundle) transverse the front of the fossa while the perineal branch of the sacral plexus (S4) and the perforating cutaneous nerve cross the back of the fossa to access their target sites.
Infected abscesses located near the midline of one fossa cannot contaminate the opposite fossa because communication between the two regions is limited at this point. On the other hand, cross contamination of the fossae is possible via the retrosphincteric space.
Other Pelvic Spaces
In addition to the larger spaces (potential spaces) mentioned above, there are smaller spaces found within the pelvic cavity. These include:
- The retropubic (prevesical) space of Retzius, which is located between the symphysis pubis and the anterior border of the bladder (and prostate gland in males),
- Between the rectal fascia and sacral fascia, and superior to the levator ani muscle and superior fascia of the pelvic diaphragm, is the presacral space,
- There are two postanal spaces: a deep postanal space lies inferior to the levator ani muscle, superior to the anococcygeal ligament and anterior to the tip of the coccyx; the superficial postanal space lies superior to the skin of the buttocks, inferior to the anococcygeal ligament and posterior to the anal canal. The superficial postanal space also overlaps with part of the perianal space,
- Inferior and posterior to the bulbospongiosus muscle is the superficial perineal pouch. It is limited posteriorly (and simultaneously separated from the perianal space) by Colle’s fascia. Anteriorly, the superficial perineal compartment limited by Gallaudet’s fascia.