INTRAPERITONEAL PELVIC SPACES

• Rectovesical pouch: the peritoneum lines the posterior aspect of the bladder dome, is reflected inferiorly and then ascends in relation to the anterior portion of the mesorectum.
• Rectouterine pouch: the female equivalent of the rectovesical pouch. The uterus and the vagina are located between the bladder and the rectum. The peritoneum lines these structures, is reflected posteriorly and then ascends towards the mesorectum.
• Pararectal fossae: lateral extensions of the rectouterine and rectovesical fossae. The pararectal fossae flank the lateral slopes of the mid-superior rectum and are limited laterally by the uterosacral (women) or sacrogenital (men) folds.
• Vesicouterine pouch (women): the space between the posterior surface of the bladder dome and the anterior uterine surface. The utero-sacral ligaments delimit its lateral margins.

Fig 1: Sagittal view of the female pelvis. Artwork by Bruno Gutiérrez Dorta and Eduardo Gutiérrez Dorta.

Fig 2: Sagittal view of the male pelvis. Artwork by Bruno Gutiérrez Dorta and Eduardo Gutiérrez Dorta.

Fig 4: View from above of the pelvic portion of the peritoneal cavity. Artwork by Bruno Gutiérrez Dorta and Eduardo Gutiérrez Dorta.

EXTRAPERITONEAL PELVIC SPACES

• Rectovesical space (men): extraperitoneal space located caudally to the rectovesical pouch. The Denonvillier fascia divides it into 3 compartments:

1. -Rectovesical compartment: upper compartment. It is located posterior to the bladder and anterior to the Denonvillier fascia. It contains the seminal vesicles.
2. -Rectoprostatic compartment: lower compartment. It is located between the posterioinferior prostatic contour and the Denonvillier fascia.
3. -Prerectal compartment: this lies posterior to the Denonvillier fascia and anterior to the mesorectal fascia. It is the dissection plane for surgical excision of the mesorectum in patients with free circumferential resection margins.

Fig 3: Sagittal view of the rectovesical space. Artwork by Bruno Gutiérrez Dorta and Eduardo Gutiérrez Dorta.

The rectovaginal fascia is the female anatomical equivalent of the Denonvillier fascia.

• Ischioanal/ischiorectal fossae: pair of triangular spaces bilateral to the lower rectal portions and the anus. The apex of the space is located laterally, where its roof and lateral walls meet. The roof is formed by the levator ani muscle and the inferior fascia of the pelvic diaphragm. The lateral walls are formed superiorly by the obturator internus muscle and its fascia, and inferiorly by the medial part of the ischial tuberosity. Medially, it is limited by the medial part of the levator ani (superiorly) and the external anal sphincter (inferiorly). The perineal skin forms the floor of the region.
• Retzius' retropubic space: extraperitoneal space between the pubic symphysis and the bladder. Its anatomical limits are the transverse fascia (anterior wall), umbilical-vesical fascia (posterior wall), urachal attachment (ceiling) and pubo-vesical ligament (floor). Laterally, it communicates with the peritoneal abdominal fat.
• Presacral and retrorectal spaces: the presacral fascia divides the space between the mesorectal fascia and the sacrum into two: an anterior or retrorectal space and a posterior or presacral space. Caudally, the mesorectal and presacral fascias fuse to form Waldeyer's fascia, which inserts into the posterior wall of the rectum.
• Post-anal spaces: the deep post-anal space is located below the levator ani muscle and above the coccygeal ligament. The superficial post-anal space is located posterior to the anal canal, below the coccygeal ligament.

PELVIC DIAPHRAGM

The pelvic diaphragm is a muscular structure that provides active support to the pelvic organs. It is made up of the ischio-coccygeal and levator ani muscles. The levator ani muscle is in turn made up of the ilio-coccygeal, puborectalis and pubo-coccygeal muscles.

• The iliococcygeal muscle extends horizontally from the external anal sphincter towards the tendinous arch. Its posterior condensation forms the levator plate.
• The puborectalis is a U-shaped muscle that inserts into the parasymphysial area and extends posteriorly embracing the anorectal junction. It helps maintaining urinary continence by raising the neck of the bladder and compressing it against the pubic symphysis. It also controls the anorectal angle, maintaining anal continence by contracting and allowing bowel movement by relaxing.
• The pubococcygeal muscle originates from the pubic bone and extends laterally to insert into the tendon arch.

Fig 5: View from above of the female pelvis. Artwork by Bruno Gutiérrez Dorta and Eduardo Gutiérrez Dorta.

Fig 6: View from above of the male pelvis. Artwork by Bruno Gutiérrez Dorta and Eduardo Gutiérrez Dorta.

DYNAMIC PELVIC FLOOR STUDIES

The pelvis is divided into three functional compartments, connected by structures responsible for pelvic support: endopelvic fascia, the pelvic diaphragm and the urogenital diaphragm. The urogenital diaphragm is restricted to the anterior and middle compartments.

By observing the position and movement of the pelvic organs during different manoeuvres (rest, Valsalva and defecation) and by measuring the reference lines, we can diagnose and evaluate different pathologies and dysfunctions of the pelvic floor. These studies are useful for evaluating pelvic organ prolapse and other pelvic floor dysfunctions.

Fig 7: Pelvic functional compartments and reference lines.

MR sequences used include high-resolution T2 multiplanar sequences to evaluate static anatomy and dynamic TrueFISP sequences (or similar steady-state free precession sequences) during rest, Valsalva and defecation.

Valsalva manoeuvres are useful for detecting pelvic organ prolapse. However, the evacuation phase is more sensitive for identifying and grading compartmental prolapses and descents.

• Anterior compartment:
• Cystocele and urethral hypermobility: Bladder prolapse. It is evaluated by measuring the distance between the lowest point of the bladder and the LPC. The severity of cystocele is also correlated with the increase in the length of the H and M lines, due to the stretching of the levator ani muscle. The mobility of the urethra is also evaluated, looking for a descent or hypermobility of the same.
• Stress urinary incontinence and other functional disorders.
• Middle compartment:
• Uterine prolapse: descent of the uterus. This is evaluated by measuring the distance between the lowest point of the uterus and the PCL.
• Vaginal prolapse: descent of the vagina. The position of the vaginal walls and their relationship with the PCL can be evaluated.
• Posterior compartment:
• Rectocele: Anterior prolapse of the rectum towards the vagina. It can be assessed by measuring the distance between the theoretically correct location of the anterior rectal wall and the most anterior point of the rectum.
• Enterocele: Herniation of the small intestine into the rectovaginal space. Dilation of the rectovaginal space may be associated with enterocele.
• Rectal intussusception: Invagination of the rectum into the anal canal.
• Perineal descent: Descent of the pelvic floor and its organs. A descent of the anorectal junction and perineum is observed, with elongation of the M line and caudal angulation of the levator plate.
• Pelvic floor dyssynergia: Inability to coordinate the function of the abdominal and pelvic floor muscles to evacuate faeces. A paradoxical contraction of the puborectalis muscle and elevation of the levator plate is observed, resulting in a narrowing of the anorectal angle during attempts at defecation. The integrity and structure of the internal and external anal sphincters can also be evaluated.

Fig 8: Pelvic floor case 1.

Fig 9: Pelvic floor case 2.

It is important to emphasise that the pelvic floor functions as a unit, and dysfunctions tend to affect several compartments at the same time. Therefore, the evaluation of all compartments is crucial for an adequate diagnosis and treatment.

ANORECTAL SPHINCTER COMPLEX

The anorectal sphincter complex is a group of muscles that control continence and defecation. It includes the internal anal sphincter (IAS), the external anal sphincter (EAS) and the puborectalis muscle.

• The internal anal sphincter (IAS) is the continuation of the circular smooth muscle of the distal rectum. It constitutes the muscular wall of the anal canal and is separated from the external sphincter by the intersphincteric space.
• The external anal sphincter (EAS) is a skeletal muscle that surrounds the anus circumferentially and is continuous superiorly with the puborectalis muscle. It is divided into three portions: deep, superficial and subcutaneous.
• The subcutaneous portion wraps around the inferior border of the internal anal sphincter.
• The superficial portion is attached posteriorly to the levator ani muscle and anteriorly to the perineal body.
• The deep portion of the EAS form part of the surgical sphincter complex.
• The puborectalis is a U-shaped muscle that wraps around the posterior and lateral aspects of the external anal sphincter. It attaches to the posterior aspect of the pubic rami. It is part of the levator ani muscle.
• The levator ani muscle is formed by the puborectalis, pubococcygeus and iliococcygeus muscles. It forms the levator plate.
• The intersphincteric space contains fat and connective tissue, separates the IAS from the EAS and contains the anal glands. The fat of the intersphincteric space is continuous superiorly with the fat of the mesorectal fascia.

Fig 10: Anorectal sphincter complex. Axial view.

Fig 11: Anorectal sphincter complex. Coronal view.

Sphincteric complex’s anatomy is fundamental for the classification of fistulising perianal disease. Perianal fistulas are defined as abnormal connections between the anal canal and the perineal skin or the pelvic spaces. The Parks classification describes the path of the primary fistula in relation to the sphincter complex.

Fig 12: Parks Classification. Perianal Fistulizing Disease.