Female perineum

Boundaries and vulva

• Boundaries: symphysis in front, ischiopubic rami and ischial tuberosities laterally, sacrotuberous ligaments and coccyx behind. A line between the ischial tuberosities divides the urogenital triangle (front) from the anal triangle (back); the perineal body sits at its midpoint.
• Labia majora: skin with hair follicles, fat, sebaceous and apocrine glands; scrotal homologue (both come from the labioscrotal swellings, so the fused empty folds of a virilised CAH girl resemble a scrotum).
• Labia minora: no hair follicles and no fat, but many sebaceous glands. Low oestrogen in small girls allows labial adhesions.
• Hidradenitis suppurativa starts with occlusion of hair follicles, with secondary rupture, inflammation, sinus tracts and scarring; it affects the hair-bearing labia majora, groins and axillae and spares the labia minora.
• Fourchette: fold joining the back ends of the labia minora, the first part to tear at crowning.
• Vestibule: between the labia minora; holds the urethral meatus, vaginal orifice and Skene and Bartholin ducts (urogenital sinus).
• Clitoris: two corpora cavernosa (crura on the ischiopubic rami) and a glans; there is no corpus spongiosum because no urethra passes through it. The vestibular bulbs are the homologue of the bulb of the penis.

Perineal pouches

• The perineal membrane spans the ischiopubic rami and is pierced by urethra and vagina.
• A pudendal block paralyses the deep pouch sphincters (sphincter urethrae), so brief stress urinary leakage can follow.
• Colles fascia (membranous layer of superficial fascia) is fixed to the fascia lata just below the inguinal ligament and to the back edge of the perineal membrane, but is continuous in front with Scarpa fascia of the abdomen.
• So blood, urine or pus in the superficial pouch spreads into the labia (scrotum and penis in men) and up the anterior abdominal wall, but not into the thighs or the anal triangle. In a man this is the picture of a ruptured bulbous (spongy) urethra below the perineal membrane; rupture of the membranous urethra above it leaks into the extraperitoneal pelvis instead.

The perineal body

• Fibromuscular node between the vagina and anal canal where these converge: bulbospongiosus, superficial and deep transverse perineal muscles, external anal sphincter, and fibres of levator ani (puboperinealis).
• It supports the posterior vaginal wall and the pelvic floor. Cutting or tearing it (midline episiotomy, second-degree tear) divides these muscles; poorly repaired it leads to rectocele and prolapse.

Pudendal nerve

• S2, S3, S4. Leaves the pelvis through the greater sciatic foramen below piriformis, hooks round the ischial spine and sacrospinous ligament, re-enters through the lesser sciatic foramen and runs forward in the pudendal (Alcock) canal, a split in the obturator internus fascia on the lateral wall of the ischioanal fossa, with the internal pudendal vessels.
• Branches: inferior rectal (external anal sphincter, perianal skin), perineal (posterior labial skin, superficial and deep perineal muscles), dorsal nerve of the clitoris (glans).
• It is somatic: motor to the external anal and urethral sphincters and perineal muscles (bulbospongiosus contraction gives rhythmic ejaculation in men), sensory to most of the perineum. It does not supply the levator ani's pelvic surface (direct S3–S4 branches) or any viscera.
• Pudendal block: needle through the vagina aimed just below and behind the ischial spine, the palpable landmark. Areas still felt: anterior labia and mons (ilioinguinal and genital branch of genitofemoral, L1) and lateral perineum (perineal branch of posterior femoral cutaneous), so local infiltration is added.
• Entrapment in Alcock's canal (cyclists, sacrospinous fixation): burning perineal pain, worse sitting.
• Dermatomes: perineum S3, perianal S4–S5, mons L1. The anal wink and clitoroanal reflexes test S2–S4.

Vessels and lymph drainage

• Internal pudendal artery (anterior division of internal iliac): inferior rectal, perineal (bleeds in episiotomy), artery of the bulb, and deep and dorsal arteries of the clitoris.
• Superficial and deep external pudendal arteries are branches of the femoral artery and supply the front of the vulva and mons, so these areas survive if the internal pudendal is tied. Vulval varices drain through the external pudendal veins to the great saphenous vein.

• A boil on the labium majus therefore enlarges the superficial inguinal nodes. Vulval lymphatics cross the midline, so tumours spread to both groins.

Ischioanal fossa

• Lateral wall obturator internus fascia with the pudendal canal; medial wall levator ani and external anal sphincter; filled with soft septate fat that lets the anal canal distend.
• An abscess from the anal glands that breaks through the lateral side of the sphincters enters the ischioanal fossa. The two fossae communicate behind the anal canal, giving a horseshoe abscess.
• The inferior rectal nerve and vessels cross it, so deep incision of an abscess can weaken the sphincter. An anterior recess extends forward above the perineal membrane.

Bartholin (greater vestibular) gland

• Pea-sized, in the superficial pouch at the back of the vestibular bulb, under bulbospongiosus, in the posterior labium majus at 5 and 7 o'clock.
• Duct opens in the groove between the hymen and the labium minus. Secretes mucus during sexual arousal (parasympathetic). Homologue of the bulbourethral gland.
• Cyst: the duct blocks and secreted mucus is retained, giving a painless, soft, fluctuant, slow-growing swelling.
• Abscess: hot, very tender; often polymicrobial, but with a purulent urethral discharge and Gram-negative intracellular diplococci it is Neisseria gonorrhoeae. Chlamydia cannot be seen on Gram stain and needs NAAT. Treated by marsupialisation or a Word catheter.
• Excision is reserved for recurrence; its most troublesome complication is haemorrhage from the vestibular bulb venous plexus, with haematoma.
• The gland atrophies after menopause, so a new solid mass after 40 needs biopsy for carcinoma.
• Skene glands open beside the urethral meatus (prostate homologue).

Episiotomy and perineal tears

• Mediolateral is cut at 60 degrees at crowning; the stretched perineum recoils, so the scar lies at about 45 degrees. Doctors avoid the midline cut for fear of extension into the external anal sphincter.

• Faecal incontinence after delivery: endoanal ultrasound shows where the sphincter is torn; anorectal manometry shows only that it is weak.
• A vulval haematoma below the levator is visible; a supralevator haematoma is hidden, bulges high in the vaginal wall, tracks into the broad ligament and causes shock out of proportion to visible loss.

Remember The pudendal nerve leaves through the greater foramen, hooks round the ischial spine and enters through the lesser foramen: block it at the spine.

Exam trap Bartholin glands lie in the superficial pouch; the deep pouch holds the urethral sphincters (and, in men, Cowper glands).

Exam trap A mediolateral episiotomy cuts more tissue and bleeds more, but it protects the anal sphincter; the midline cut extends into it.

How FCPS asks about this

• Landmark for pudendal block: ischial spine felt through the vagina; nerve roots S2–S4.
• Incontinence after pudendal block: sphincter urethrae in the deep perineal pouch is paralysed.
• Muscles cut in mediolateral episiotomy: bulbospongiosus and superficial transverse perineal, sometimes levator ani; midline cut risks the anal sphincter.
• Swelling between labium majus and minus at 5 or 7 o'clock: Bartholin cyst from duct blockage with retained mucus.
• Lymph drainage of a labial boil: superficial inguinal nodes.
• Superficial pouch extravasation: spreads to labia or scrotum and anterior abdominal wall, not the thigh, because Colles fascia is fixed to the fascia lata.
• Superficial external pudendal artery is a branch of: the femoral artery.