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Anatomy of anal sphincter complex

The anal sphincter complex is a multilayered cylindrical structure with the innermost layer
being the anal lining, with the subsequent layers: internal sphincter, the fat-containing

intersphincteric space with the longitudinal layer, and subsequently the outer striated muscle

layer. The internal anal sphincter is a smooth muscle in a state of continuous maximal contraction

and represents a natural barrier to the involuntary loss of stool and gas. The external anal
sphincter is subdivided into two parts, deep (deep sphincter and puborectalis) and superficial

(subcutaneous and superficial sphincter). (3) Endosonographically, the IAS is a 2 to 3 mm-thick

circular band and shows a uniform hypoechogenicity. The average thickness of EAS in females is

(4)
7.7 + 1.1 mm.

Impact of OASIS

Severe perineal tears after vaginal birth including anal sphincter complex injuries can have

serious short and long-term consequences for the health and well-being of women. OASIS are
commonly associated with increased risk of pelvic floor injury, urinary and anal incontinence, pain

and sexual dysfunction. These symptoms may persist or be present many years after giving birth.

In the immediate postpartum period, complications include bleeding from the laceration site,

increased pain, wound infection, wound breakdown, and healing abnormalities that may require
(5)
subsequent surgical intervention. Bowel control issues, such as fecal urgency, anal
incontinence, and perineal-rectal and rectovaginal fistula formation are also significant long-term

consequences. (6-8) Based on a recent systematic review and meta-analysis, 19% of women who

delivered vaginally experienced anal incontinence (95% CI, 14-25). Following primary repair of
OASIS, 55% (46-63) had persistent sphincter defect with 38% experiencing anal incontinence

(95% CI, 33-43). In addition, women who had sphincter defect diagnosed on ultrasound following

a vaginal delivery were 4-times more likely to have anal incontinence than those with intact

sphincter (RR 3.74, 95 % CI 2.17-6.45). (2) Given their significant consequences; OASIS prediction,
prevention, diagnosis and management are critical.

Obstetric Anal Sphincter InjurieS (OASIS): Impact and prevention 70