Introductory Chapter: A Quick Glance at Female Pelvis

1. Introduction

Female pelvis plays a critical role in providing protection and support for the bladder, urethra, productive organs, and rectum. Anatomically, three different systems including bone, muscle, and connective tissue act as the guard and sling functions [1]. Of those, the female bony pelvis consists of the sacrum and two hip bones which are formed by the fusion of the ilium, ischium, and pubis. The bony pelvis provides not only protection for pelvic organs but also an essential framework for the muscular and fascial supports. The levator ani muscles, which are composed of the pubococcygeus, puborectalis, and iliococcygeus, play an important part in slinging the pelvic organs, since the urethra, vagina, and rectum pass through the levator ani hiatus. The normal levator ani muscle tone is sufficient to maintain the hiatus closed, which serves as a support to pelvic organs. A voluntary contraction of levator ani muscles can make the hiatus further closed by compassing mid-urethra, vagina, and rectum against the pubic bone, which contributes to resisting excess load. The pelvic connective tissue composed of endopelvic fasciae and ligaments is also responsible for holding the pelvic organs in place. In addition, the coordination between levator ani muscles and pelvic floor fasciae may further strengthen the pelvic floor support. As is known, the connective tissue in human beings has adoptive extensibility which means it may be stretched by subjecting to continuous stress. However, the adoptive extension is not observed in the fasciae and ligaments within pelvis because the resting tone of levator ani muscles bear the weight of abdominal and pelvic organs [2]. The normal function of these three systems guarantees female pelvic health.

In general, the female pelvis is more delicate than the male pelvis since both pregnancy and vaginal delivery may lead to a significant increase in the possibility of pelvic floor dysfunction (PFD). According to the terminology released by International Urogynecological Association (IUGA) and International Continence Society (ICS), PFD is an umbrella diagnosis covering a number of disorders including bladder and bowel dysfunction, pelvic organ prolapse (POP), sexual dysfunction, and pelvic pain [3]. Based on two epidemiological studies, as high as 46.5% to 67.5% of women suffered from at least one type of PFDs [4, 5]. Of those women, approximately one-third experienced two or more types of PFDs and one-fifth failed conservative management and required surgical intervention [4, 6, 7]. Not only does PFD diminish women’s quality of life significantly, but its related costs also pose a huge burden for the healthcare system [8, 9]. In terms of specific disorders, urinary incontinence and POP are the most common types of PFDs.

2. Urinary incontinence

Urinary incontinence is one of the most common PFDs, with prevalence ranging from 9.3 to 69%, depending on the study population and methodological difference [10]. Based on different manifestations, urinary incontinence can be classified into three subtypes: stress urinary incontinence (SUI), urgency urinary incontinence (UUI), and mixed urinary incontinence (MUI). ICS defines SUI as involuntary urine leakage with effort or physical exertion, UUI as involuntary urine leakage accompanied by urgency, and MUI as the coexistence of both SUI and UUI [11].

Urodynamic study provides theoretical insights in normal continence mechanism and the pathogenesis of urinary incontinence subtypes. In order to maintain continence, the urethral pressure must be higher than the bladder pressure, both at rest and during activities. It is traditionally believed that urethral striated muscles (urethral sphincter), smooth muscle, and vascular bed within the submucosa contribute equally to urethral pressure at rest [12]. However, more recent studies demonstrated that the spontaneous activity of circular and longitudinal urethral smooth muscles provided resting urethral pressure, while urethral striated muscles were not involved [13, 14]. By contrast, the urethral striated muscles play an important role in providing additional urethral pressure under conditions of physical stress. During physical activity, the contraction of the diaphragm and abdominal muscles can result in a sudden increase in abdominal pressure, which is transmitted to bladder, causing a corresponding increase in bladder pressure. It is observed that a hard cough can lead to an increase in bladder pressure by more than 200 cm H₂O [2]. In response to the stress, urethral striated muscles contract reflexively, causing an elevation in urethral pressure. Besides, the reflex contraction of the levator ani muscles can also enhance urethral pressure by tensing the sub-urethral fasciae, which decreases vesical neck motion [15, 16]. The damage or dysfunction of levator ani muscles and/or urethral striated muscles can result in SUI. It is widely recognized that urethral hypermobility and intrinsic sphincter deficiency (ISD) are the exact etiology of SUI [17]. Impaired anatomical support of urethra due to levator ani muscles dysfunction is the main reason of urethral hypermobility [18]. Some studies, using translabial ultrasound or magnetic resonance imaging (MRI), showed that the area and circumference of urethral striated muscles were shorter and smaller in women with SUI in comparison with the controllers [19, 20]. Compared to SUI, only limited studies reported the association between pelvic anatomical abnormalities and UUI. Wang et al. found that women with overactive bladder (OAB) or UUI had weaker urethral sphincter and levator ani muscles, causing an over-relaxed levator ani hiatus compared to health controllers [21]. The more understanding of the pathogenesis of urinary incontinence we have, the more precise treatment strategy we can develop.

In terms of the management of urinary incontinence, lifestyle modification and behavioral therapy have been considered as the foundation of treatment, no matter what subtype the patient has [22]. Pelvic floor muscle training (PFMT), as the first-line treatment of SUI, can strengthen levator ani muscles and consequently enhance urethral support [23]. The effectiveness of PFMT for SUI has been confirmed by moderate to high certainty evidence [24]. However, adherence to PFMT has been problematic. It is reported that the short-term and long-term adherence to PFMT is only 64% and 23%, respectively [25]. With the increasing use of social media, a number of mobile applications have been developed as an assisted tool of PFMT. A study showed that mobile applications may improve the potential adherence to PFMT and enhance the effect of treatment [26]. Initially, PFMT was designed for SUI treatment, and some studies further revealed it could be an effective approach for OAB and UUI. Evidence showed that the contraction of pelvic floor muscles could suppress the detrusor overactivity and relieve the urgency symptoms [27]. For patients with urinary incontinence failed to behavioral therapy, pharmacotherapy can be considered. Although a Cochrane systematic review demonstrated that duloxetine, a serotonin-noradrenaline reuptake inhibitor, could improve SUI symptoms significantly [28], it has not been approved by U.S. Food and Drug Administration (FDA) for the treatment of SUI. Antimuscarinics and beta-agonists are the two different kinds of medications used for the treatment of OAB and UUI. The former can suppress detrusor overactivity by inhibiting parasympathetic nervous system, while the latter can relax detrusor by activating sympathetic nervous system. For patients who do not respond to pharmacotherapy, minimally invasive and surgical intervention can be the treatment options. The mid-urethral sling, pubovaginal sling, and urethral bulking agents are the common procedures for SUI [22], whereas sacral neuromodulation, percutaneous tibial nerve stimulation, and intradetrusor injection of onabotulinumtoxin A are used for refractory OAB or UUI [29]. In addition, growing evidence demonstrates acupuncture may act as an optional treatment for SUI [30].

3. Pop

Pelvic organ support depends on the normal structure and function of pelvic floor muscles and connective tissues. The impairment of levator ani muscles and/or connective tissues may result in POP. A number of studies presented a strong association between levator ani defects and POP [31]. It is reported that 55% of women with POP had major levator ani defects, and 83% of women with levator ani defects presented POP [32, 33]. Furthermore, two important theories of pelvic organ support demonstrated the role of pelvic connective tissues in the development of POP.

In 1990, Petros et al. proposed their “Integral Theory” [34]. According to the theory, the pelvic organs are supported in their normal locations by a group of pelvic floor fasciae and ligaments. Functionally, these fasciae and ligaments act like suspension bridges, and their over-laxity can lead to POP and related symptoms. The integral theory further divided the female pelvis into three areas: anterior, middle, and posterior, which are used to indicate the impaired area and implement precision treatment [35].

DeLancey described his anatomic “hammock theory” in 1992, which divided the support of the pelvic organs into three levels [36]. Level 1 support refers to uterosacral ligament’s suspension to the cervix and upper vagina. Failure of level I support results in the uterine or vaginal vault prolapse (apical prolapse). Level 2 support consists of the vagina’s lateral connections to the arcus tendineus fasciae pelvis, which can hold up the mid-vagina. Failure of level II support leads to anterior and posterior vaginal wall prolapse, as well as SUI. Level 3 support is comprised of the connections between the perineal membrane and perineal body fasciae, which can support the lower one-third of vagina. Failure of level II support results in SUI or distal rectocele [37, 38].

The reported prevalence of POP varies from 3 to 50% depending on the different study population and investigative methodology [39]. The confirmed risk factors for POP mainly include age, obesity, increasing parity, vaginal delivery, connective tissue disorders, menopausal status, and chronic constipation [40]. The management of POP is based on patients’ symptoms, degree of prolapse, and influence on daily life. In general, observation is recommended for women with asymptomatic POP [40]. The conservative therapy including pelvic floor physical therapy, biofeedback, and electrogalvanic stimulation can be considered as the initial treatment for symptomatic POP. Moreover, pessary is also an optional therapy with a reported success rate of 93% [41]. For women with POP who failed conservative treatment, surgical intervention can be an effective treatment option. Surgery can be divided into reconstructive and obliterative procedures. The former focuses on maintaining the vaginal canal, whereas the latter centers on closing the vaginal canal.

4. Summary

Female pelvis is a complex anatomical region composed of bony pelvis, pelvic muscle, connective tissue, and pelvic organs. The levator ani muscles as well as pelvic floor fasciae and ligaments play a critical role in maintaining pelvic organs in normal location. The injury and defects of these muscles and connective tissues can result in PFDs. With a deep understanding of functional anatomy of female pelvis, precision treatment can be achieved for the management of PDFs.

Acknowledgments

This work was supported by the National Natural Science Foundation of China (81804069), the Science and Technology Innovation Project of China Academy of Chinese Medical Sciences (CI2021A01502), and grant 2019S443 from the Guang’anmen Hospital, China Academy of Chinese Medical Sciences.

Conflict of interest

None.