Mediolateral episiotomy: which muscles are cut
A standard mediolateral episiotomy is a surgical cut, angled to one side, in the perineum (the area between the vaginal opening and the anus) that divides superficial muscles.
Remember
A standard mediolateral episiotomy divides bulbospongiosus (the muscle that lies beside the vaginal opening) and may divide fibres of the transverse perineal muscles (which run across the perineum). The cut is angled away from the anal sphincter (the muscle ring that closes the anus).
An episiotomy is made to widen the vaginal outlet during childbirth. In a standard mediolateral episiotomy the cut starts in the midline and is angled to one side, away from the anal sphincter.
The muscles of the perineum are in two layers. The deep layer is the pelvic floor, which holds the pelvic organs up, and it includes the levator ani, a deep sling of muscle. The superficial layer has these thin muscles: bulbospongiosus (beside the vaginal opening), ischiocavernosus (along the line of the ischium, the lower part of the pelvic bone) and the superficial transverse perineal muscle (running across the perineum).
Bulbospongiosus lies directly in the path of the cut, so it is divided. The superficial transverse perineal muscle runs across the perineum to the perineal body (a small knot of fibrous and muscular tissue between the vagina and anus where several muscles meet). Fibres of it may be divided too, but the muscle is not always involved. Ischiocavernosus runs along the ischium, at the side of the perineum, and is not described as divided in an episiotomy.
The levator ani and the anal sphincter are not meant to be cut. Dividing bulbospongiosus and, in some episiotomies, fibres of the superficial transverse perineal muscle relieves pressure on the perineum and is intended to reduce the risk of a severe tear towards the sphincter.
Episiotomy does not reliably prevent perineal tears. NICE advises restrictive rather than routine use, meaning it is used only when clinically indicated.
Key facts
- Direction of the cut
- it starts in the midline and is angled to one side, away from the anal sphincter.
- Bulbospongiosus
- lies beside the vaginal opening and is divided by the standard cut.
- Superficial transverse perineal muscle
- runs across the perineum to the perineal body, and fibres of it may be divided.
- Ischiocavernosus
- runs along the ischium and is not described as divided in an episiotomy.
- Spared structures
- the levator ani and the anal sphincter are not meant to be cut.
- Policy
- NICE advises restrictive rather than routine use, and episiotomy does not reliably prevent tears.
Exam traps
- The levator ani muscle is not the one cut, because it forms the deep sling that supports the pelvic organs and lies above the muscles divided in a standard episiotomy.
- Pairing bulbospongiosus with ischiocavernosus is half right, because bulbospongiosus is divided but ischiocavernosus runs along the ischium rather than across the perineum.
- The external urethral sphincter is not cut, because it is the ring of muscle around the urethra that helps hold urine in, and it is a deep muscle rather than a superficial perineal one.
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