Perineal tears are classified into four grades dependingon severity of extension. First- and second-degree tearsinvolve the vaginal epithelium and perineal muscles,respectively, while third- and fourth-degree tearsinvolve OASIS. Third-degree perineal tears involveinjury to the anal sphincter and are subdivided into:Grade 3a, <50% of EAS thickness torn; Grade 3b, >50%of EAS thickness torn; and Grade 3c, both EAS and IAStorn. Fourth-degree tears involve the anal sphincter aswell as the anorectal epithelium.10
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