Curriculum · Gynecology / Women's Health / Infertility / Perinatal Care / Birth Spacing
Obstetric perineal tear and anal sphincter injury
What it is
A perineal tear is named by the depth of the injury. A first degree tear takes the perineal skin and the lining of the vagina. A second degree tear reaches the perineal muscles as well but not the anal sphincter, and it is the most common type of perineal laceration; the perineal muscles named for it are the bulbospongiosus, the superficial transverse perineal muscles and the levator ani. A third degree tear involves the anal sphincter complex, and a fourth degree tear runs on into the anal or rectal mucosa. The third degree group is divided by how much of the anal sphincter, the EAS, is torn: 3a is less than 50% of its thickness, 3b is more of it, and 3c has both the EAS and the IAS torn. A buttonhole tear stands apart: the EAS is whole while the anal or rectal mucosa is torn, with or without an IAS tear.
Causes and risk
Perineal lacerations are listed among the complications of an episiotomy, so the cut itself may end in the tear. In one described example a primigravida delivered a 3.8 kg boy by spontaneous vaginal delivery, and the wound extended to the muscle of the perineum, reaching the anal sphincter, which is a third degree perineal tear.