(OASIS) a spectrum of injuries that encompasses both third- and fourth-degree perineal tears. Injury to the anal sphincter mechanism during childbirth may arise secondarily to direct disruption of the sphincter muscles and/or traction of the pudendal nerves. Disruption of the anal sphincter muscles is best assessed by anal ultrasound examination. This is usually performed using a high-frequency (10 MHz) endoanal probe. In selected cases with complex injury and/or suspected rectovaginal fistula, magnetic resonance imaging (MRI) may also be employed.
Subjects: Medicine and Health.