First degree Injury to perineal skin
Second degree Injury to perineum involving perineal muscles but not involving the anal sphincters
Third degree Injury to perineum involving the anal sphincter complex: 3a: <50% of ext anal sphincter(EAS) thickness 3b: >50% EAS thickness 3c: both EAS and internal anal sphincter (IAS) involved
Fourth degree Involves anal sphincter complex (EAS and IAS) and anorectal mucosa.