Describe the pelvic diaphragm with its parts and functions. Add a note on the perineum, the perineal pouches and the perineal body.

The Pelvic Diaphragm

• Nerve supply — S3 and S4 directly, and the perineal branch of the pudendal nerve
• Structures piercing it — the urethra, anal canal, and in the female the vagina
• Supports the pelvic viscera against the intra-abdominal pressure
• Sphincteric — puborectalis maintains the anorectal angle; the pubococcygeus supports the urethra and vagina
• Resists rises in intra-abdominal pressure during coughing, straining and lifting
• Guides the fetal head in its rotation during labour

The Perineum

• A line joining the two ischial tuberosities divides it into:The urogenital triangle in front, containing the external genitalia and urethra
• The anal triangle behind, containing the anal canal and the ischiorectal fossae

The Urogenital Diaphragm and Perineal Pouches

• Colles fascia is continuous with Scarpa fascia of the abdominal wall, and is attached laterally to the ischiopubic rami and posteriorly to the perineal membrane

The Perineal Body

• Muscles converging on it — external anal sphincter, bulbospongiosus, superficial and deep transverse perinei, levator ani (puborectalis and pubococcygeus), and the longitudinal muscle of the rectum
• It is the central tendon of the perineum — the keystone of the pelvic floor
• Far larger and more important in the female, where it separates the vagina from the anal canal

Contents of the Male and Female Perineum

• The female perineal membrane is pierced twice, which is one reason the female pelvic floor is inherently weaker and more prone to prolapse

Applied Aspects

• Damage to the perineal body during childbirth is the commonest cause of prolapse of the uterus, cystocele and rectocele in later life, because the central support of the pelvic floor is lost
• Episiotomy is made mediolaterally to avoid the perineal body and the anal sphincter; a midline incision heals better but risks extending into the anal canal
• Perineal tears are graded: first degree involves skin only; second, the perineal body; third, the anal sphincter; fourth, the anal mucosa. Third and fourth degree tears must be recognised and repaired at once, or incontinence follows
• Extravasation of urine after rupture of the bulbar urethra spreads into the superficial perineal pouch, scrotum, penis and anterior abdominal wall, but not into the thigh, because of the fascial attachments
• Pelvic floor exercises strengthen levator ani and are effective first-line treatment for stress incontinence and mild prolapse
• Bartholin abscess — the gland lies in the superficial perineal pouch and its duct is easily blocked; treated by marsupialisation
• Perineal descent from chronic straining stretches the pudendal nerve and contributes to incontinence, which is why constipation must be treated
• Fournier gangrene — necrotising fasciitis of the perineum, spreading rapidly along the fascial planes of Colles and Scarpa; a surgical emergency, commoner in diabetics
• Levator ani has a dual nerve supply, from above by S3 and S4 and from below by the pudendal nerve, so it may partly escape in a unilateral lesion
• The perineum is a common site of obstetric injury in India, where unattended deliveries can cause vesicovaginal and rectovaginal fistulae
• Squatting straightens the anorectal angle, which is one reason the traditional Indian lavatory eases defaecation