The urogenital diaphragm (perineal membrane) is composed of two regions: one dorsal and one ventral. The dorsal portion consists of bilateral transverse fibrous sheets that attach the lateral wall of the vagina and perineal body to the ischiopubic ramus. This portion is devoid of striated muscle. The ventral portion is part of a solid three-dimensional tissue mass in which several structures are embedded. It is intimately associated with the compressor urethrae and the urethrovaginal sphincter muscle of the distal urethra with the urethra and its surrounding connective tissue. In this region the perineal membrane is continuous with the insertion of the arcus tendineus fascia pelvis. The levator ani muscles are connected with the cranial surface of the perineal membrane. The vestibular bulb and clitoral crus are fused with the membrane’s caudal surface (37).
Figure 5.22 The complete urogenital sphincter musculature, bladder, and vagina. (Redrawn from Haderer JM, Pannu HK, Genadry R, et al. Controversies in female urethral anatomy and their significance for understanding urinary continence: observations and literature review. Int Urogynecol J Pelvic Floor Dysfunct 2002;13:236–252, with permission.)
The sphincter urethrae (Fig. 5.22) is a continuous muscle fanning out as it develops proximally and distally, including the following: 1.
External urethral sphincter, which surrounds the middle third of the urethra.
2.
Compressor urethrae, arcing across the ventral side of the urethra.
3.
Urethrovaginal sphincter, which surrounds the ventral aspect of the urethra and terminates in the lateral vaginal wall.
The deep transverse perineal muscle originates at the internal aspect of the ischial bone, parallels the muscle compressor urethrae, and attaches to the lateral vaginal wall along the perineal membrane. Studies show that a smaller striated urogenital sphincter is associated with stress incontinence and poorer pelvic ﬂoor muscle function (38).
The urinary and genital tracts have a common reliance on several interdependent structures for support. The cardinal and uterosacral ligaments are condensations of endopelvic fascia that support the cervix and upper vagina over the levator plate. Laterally, endopelvic fascial condensations attach the midvagina to the pelvic walls at the arcus tendineus fascia pelvis anteriorly and the arcus tendineus levator ani posteriorly. The distal anterior vagina and urethra are anchored to the urogenital diaphragm and the distal posterior vagina to the perineal body.
Anteriorly, the pubourethral ligaments and pubovesical fascia and ligaments provide fixation and stabilization for the urethra and bladder. Posteriorly, they rely on the vagina and lower uterus for support. Partial resection or relaxation of the uterosacral ligaments often leads to relaxation of the genitourinary complex, resulting in the formation of a cystocele. Studies indicate that half of the observed variation in anterior compartment support may be explained by apical support (39). Various types and degrees of genital tract prolapse or relaxation are almost always associated with similar findings in the bladder, urethra, or both.