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Obstetric Anal Sphincter InjurieS (OASIS):

Impact and prevention

Jittima Manonai Bartlett, MD, MHM

Female Pelvic Medicine & Reconstructive Surgery Division

Department of Obstetrics & Gynaecology
Faculty of Medicine Ramathibodi Hospital

Mahidol University

Introduction

(1)
The classification of perineal tears was described by Dr Abdul Sultan in 1999 and
adopted by the World Health Organization, the International Consultation on Incontinence (ICI),

and many national and international societies worldwide. Obstetric anal sphincter injuries
(OASIS) are severe perineal tears that extend into or through the anal sphincter complex (both

third- and fourth-degree tears).

A third-degree tear refers to 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.

A fourth-degree tear is diagnosed when injury to perineum involving the anal sphincter

complex (EAS and IAS) and anorectal mucosa occurs.
OASIS are diagnosed clinically by performing a vaginal and rectal examination immediately

after delivery. In addition, endoanal ultrasound is a useful tool to identify anal sphincter tears

and determine the extent of trauma. OASIS rates are increasing in many countries. Overall,
a quarter of women who delivered vaginally were diagnosed to have an anal sphincter defect on

(2)
ultrasound (26 %, 95% CI 21-30).

Obstetric Anal Sphincter InjurieS (OASIS): Impact and prevention 69