Obstetric Anal Sphincter Injuries (OASIS)

What is OASIS?

Perineal trauma can sometimes extend into the anal sphincter muscles and is known as Obstetric Anal Sphincter Injuries (OASIS). OASIS are further classified as third or fourth degree tears whereby a third degree tear involves a partial or complete disruption of the anal sphincter complex (external and internal anal sphincters), and a fourth degree tear which involves complete disruption of the anal sphincter complex and the anal epithelium.

Incidence of OASIS

The reported worldwide incidence of OASIS is between 4% and 6.6% of vaginal births (supporting evidence - 16-19). A national survey has indicated that the overall incidence in the UK is 2.9% for all vaginal births, with incidences of 6.1% and 1.7% for nulliparous (first time mothers) and multiparous (had a previous baby) women respectively (supporting evidence - 20).

Method of OASIS repair

To optimize the outcome from OASIS repair it is important that the most appropriate repair technique is used. There are two techniques for repair of the external anal sphincter (EAS), either an ‘end-to-end’ technique by which the damaged ends are sutured by approximation, or an ‘overlap’ technique whereby the damaged ends are placed one on top of the other and sutured to create an overlapping of the muscle. For a full thickness EAS tear either an ‘end-to-end’ or ‘overlap’ technique can be used (supporting evidence - 21). However, for a partial thickness EAS tear, such as 3A OASIS or a 3B that does not extend through 100% of the EAS, an ‘end-to-end’ repair technique should be used (supporting evidence - 22).

The internal anal sphincter (IAS) is smooth muscle which has less fibrous tissue than striated muscle and it is more likely to tear when placed under tension. Consequently, to minimize this potential risk tears to the IAS should be repaired separately by using an ‘end-to-end’ technique (supporting evidence).

A torn anorectal mucosa should be repaired by bringing the torn edges together, using either a continuous or interrupted technique.

Postnatal follow-up for women with OASIS

For women who have sustained an OASIS the recommendation in the Royal College of Obstetrics & Gynaecology Green-top guideline is for review at a convenient time (usually 6–12 weeks postpartum), where possible, by clinicians with a special interest in OASIS (supporting evidence - 31).

Possible complications of OASIS

OASIS is recognised as the most common cause of anal incontinence (AI) in childbearing age women (supporting evidence - 23), encompassing symptoms of flatus incontinence, passive soiling, incontinence of liquid or solid stool and faecal urgency. These symptoms can have severe social and psychological implications for the women and their families. AI is a distressing and disabling condition and the symptoms can cause social and hygienic problems that lead to isolation, limiting occupational and social activity, negative effect on sexual function and consequent impact on relationships, reduced self-esteem and reduced quality of life (QoL) (supporting evidence - 24-27). Incidence of AI is often under reported by women due to feelings of embarrassment or regarded as an expected consequence of a vaginal birth (supporting evidence - 24, 26-27). In fact AI has been called the ‘unvoiced symptom’ due to the embarrassment experienced by women who suffer from it (supporting evidence - 26). The reported incidence of AI in women with OASIS ranges from 41% to 61% and is two to three times higher than for women who do not sustain an OASIS during childbirth (supporting evidence - 28-30).