Perineal tears are classified into four grades depending
on severity of extension. First- and second-degree tears
involve the vaginal epithelium and perineal muscles,
respectively, while third- and fourth-degree tears
involve OASIS. Third-degree perineal tears involve
injury to the anal sphincter and are subdivided into:
Grade 3a, 50%
of EAS thickness torn; and Grade 3c, both EAS and IAS
torn. Fourth-degree tears involve the anal sphincter as
well as the anorectal epithelium.10