Anatomical supports and sphincters (outlet)
External (voluntary) sphincter (rhabdosphincter)
Striated muscle fibres under voluntary control – innervated by somatic nerves originating from Onuf’s nucleus in S2 – S4, travelling via pudendal nerve.
Combination of slow and fast twitch fibres – fast twitch fibres more useful for rapid increases in abdominal pressure, slow twitch more tonic contraction during normal slow filling.
Females:
- Horseshoe shaped, running ventrally and intermixed with connective tissue of anterior vaginal wall. Thickest mid urethra.
- Predominantly slow twitch fibres of urethral sphincter
Internal (involuntary) sphincter (bladder neck)
- Concentric circular smooth muscle – supplied by alpha-adrenergic nerves from T11 – L2.
- Remains closed during filling, not under voluntary control.
- Generally less well developed in women cf. men.
Other mechanisms of female continence
Continence in women is largely a result of proximal and mid urethral forces due to absence of a functional internal or bladder neck sphincter.
External sphincter
- As above – thickest mid urethrally, with tonic pudendal stimulation
- Distally – striated muscle from perineal membrane and pelvic floor (urethra compressae)
Urethral attributes
- Spongy vascular submucosal layer (oestrogen sensitive) enhances mucosal apposition
- Mucosa itself tends to adhere to itself and aid in further compression
Surrounding supports
- Strong posterior support and compression from anterior vaginal wall
- Pubourethral ligaments fix mid urethra anteriorly and avoid excess force transmission from abdomen
- Extra support from fascial attachments anterolaterally to ATFP / white line
- Periurethral striated muscle of the pelvic floor – can be strengthened with pelvic floor exercises
Additional factors for continence
- Viscoelastic properties of the bladder – a compliant structure without significant increase in pressure as the bladder fills.
- Conscious control and cerebral modulation
Pelvic supports
The primary support is the pelvic floor musculature. These are damaged by childbirth, atrophy with age, and cannot be restored surgically.
The secondary support is the visceral fascia or fibromuscular connective tissue – and this can be used surgically to re-support.
The uterine and vaginal levels of support were defined by DeLancey and correspond to differing areas of support.
- Level 1 (suspension) – parametrium and paracolpium support vagina and uterus, with support from cardinal ligament and uterosacral ligaments
- Supports uterus and vaginal apex
- Broad ligament and round ligament do not have a significant role in pelvic organ support
- Posterior vaginal wall supported by paracolpium attaching to rectovaginal fascia – sometimes referred to as rectal pillars
- Upper portion of paracolpium responsible for suspending apex of vagina after hysterectomy
- Loss of level 1 supports leads to uterine and vault prolapse (apical prolapse)
- Level 2 (attachment) – paravaginal attachments to ATFP and to arcus tendineus rectovaginalis
- i.e. pubocervical fascia anteriorly, rectovaginal fascia posteriorly
- Loss of level 2 support leads to anterior and posterior prolapse
- Level 3 (fusion) – distal vagina is directly attached to surrounding structures fusing with urethra and perineal membrane, and laterally attached to levator ani and perineal body
- Level 3 support relates to urethra, and disruption results in urethral hypermobility
Obstetrics, Gynaecology & Reproductive Medicine Volume 21, Issue 7, July 2011, Pages 190-197