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Definition of Obstetric Anal Sphincter Injuries (OASIS)

Obstetric Anal Sphincter Injuries (OASIS) encompass both third – and fourth – degree perineal tears.

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Classification of Perineal Tears

Classification by Sultan

First – degree tear : Injury to perineal skin and/or vaginal mucosa.

Second – degree tear : Injury to perineum involving perineal muscles but not involving the anal sphincter.

Third – degree tear : Injury to perineum involving the anal sphincter complex.

Grade 3a tear : Less than 50% of external anal sphincter (EAS) thickness torn.

Grade 3b tear : More than 50% of EAS thickness torn.

Grade 3c tear: Both EAS and internal anal sphincter (IAS) torn

Fourth-degree tear : Injury to perineum involving the anal sphincter complex (EAS and IAS) and anorectal mucosa.

If there is any doubt about the degree of third-degree tear, it is advisable to classify it to the higher degree rather than the lower degree.


Prevention of OASIS

Role of episiotomy in prevention of third and fourth degree perineal tears is conflicting.

Role of prophylactic episiotomy in subsequent pregnancies in women who had OASIS in previous pregnancies is also not known.

Mediolateral episiotomies with the angle 60 degrees away from the midline when the perineum is distended, should be considered in instrumental deliveries.

Perineal protection at crowning can be protective.

Warm compression during the second stage of labour reduces the risk of OASIS.

All women who have sustained OASIS in a previous pregnancy and who are symptomatic or have abnormal endoanal ultrasonography and/or manometry should be counselled regarding elective caesarean section

Identification of OASIS

All women having vaginal delivery should be examined systematically, including digital rectal examination to rule out obstetric anal sphincter injuries.

Repair of OASIS

Repair should take place in an Operating Theatre, under regional or general anaesthesia.

What should NOT be done? – Figure of eight sutures avoided as they are hemostatic in nature and may cause tissue ischemia.

A rectal examination should be performed after the repair to ensure that sutures have not been inadvertently inserted through the anorectal mucosa.  If a suture is identified, it should be removed.

Internal anal sphincter – repair with interrupted or mattress sutures without any attempt to overlap the IAS.

Full thickness external anal sphincter – Either an overlapping or an end – to – end (approximation) method.

Partial thickness external anal sphincter (all 3a and some 3b tears) – end – to – end technique.

Mucosa should be sutured with 3 – 0 polyglactin, as it causes less irritation and discomfort than polydioxanone (PDS).

EAS and/or IAS should be sutured with 3 – 0 PDS or 2 -0 polyglactin.

After repair, the knot should be buried beneath the superficial perineal muscles to minimise the risk of knot and suture migration to the skin.

Rectal buttonhole tear

A tear that involves the rectal mucosa with an intact anal sphincter complex is known as rectal buttonhole tear.  It is by definition, not a fourth – degree tear as the anal sphincter mechanism is intact.  It is important because if not recognised and repaired, this type of tear may lead to rectovaginal fistula.

The torn anorectal mucosa should be repaired with sutures using either the continuous or interrupted technique.

Postoperative Care after Repair of Perineal Tears

Broad – spectrum antibiotics – to reduce the risk of postoperative infections and wound dehiscence.

Laxatives – to reduce the risk of dehiscence.

Bulking agents – NOT to be given routinely with laxatives.

Physiotherapy – should be advised.

Review – after 6 – 12 weeks of delivery.

If woman is experiencing incontinence or pain at follow – up, referral to a specialist gynaecologist or colorectal surgeon should be considered.

Prognosis following surgical repair of third - and fourth- degree perineal tears

Percentage of women asymptomatic at 12 months following delivery and EAS repair – 60 – 80%.

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Revise Numbers and Percentages in this article.

Overall incidence of OASIS – 2.9%.

Incidence of OASIS in primiparae – 6.1% and in multiparae – 1.7%.

Percentage of women asymptomatic at 12 months following delivery and EAS repair – 60 – 80%.

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