Perineum and pudendal nerve
Perineum: Pudendal Nerve, Ischioanal Fossa and Perineal Pouches
Perineum for FCPS Part 1: triangles, superficial and deep pouches, ischioanal fossa, pudendal nerve and vessels, perineal body and lymph drainage. Explains urine extravasation, abscess spread, pudendal block, episiotomy and obstetric tears.
Boundaries and triangles
- The perineum is the diamond-shaped region below the pelvic diaphragm: pubic symphysis in front, ischiopubic rami and ischial tuberosities at the sides, sacrotuberous ligaments and coccyx behind.
- A line between the ischial tuberosities divides it into the urogenital triangle (front) and the anal triangle (back).
- The perineal body is the wedge-shaped fibromuscular mass at the centre, between the two triangles.
Perineal pouches
| Space | Boundaries | Contents |
|---|---|---|
| Superficial perineal pouch | Between Colles' fascia (membranous layer of superficial fascia) and the perineal membrane | Crura and bulb of penis or clitoris, ischiocavernosus, bulbospongiosus, superficial transverse perinei, greater vestibular (Bartholin) glands, bulbar spongy urethra, perineal vessels and nerves |
| Deep perineal pouch | Above the perineal membrane, below the pelvic diaphragm | External urethral sphincter, deep transverse perinei, membranous urethra, bulbourethral (Cowper) glands in men, dorsal nerve and artery of penis or clitoris |
- The muscles of the deep pouch (the older "urogenital diaphragm") are supplied by the perineal branch of the pudendal nerve.
Spread of urine, blood and pus
- Bulbar urethral rupture (straddle injury, below the perineal membrane): urine and blood fill the superficial pouch and spread deep to Colles' fascia into the scrotum, penis and anterior abdominal wall under Scarpa's fascia. They do not reach the thighs because the membranous fascia fuses with the fascia lata just below the inguinal ligament, nor the anal triangle because Colles' fascia fuses with the posterior edge of the perineal membrane.
- Membranous urethral rupture (pelvic fracture, above the perineal membrane): extravasation into the extraperitoneal pelvic space around the bladder and prostate.
- Infection in the superficial pouch follows the same route to the scrotum and lower abdominal wall.
- A perianal boil or anal gland abscess eroding laterally through the external sphincter enters the ischioanal fossa.
Ischioanal (ischiorectal) fossa
| Boundary | Structure |
|---|---|
| Medial | Levator ani and external anal sphincter |
| Lateral | Obturator internus with its fascia, containing the pudendal canal |
| Floor | Perianal skin |
| Apex | Where levator ani meets the obturator fascia |
- Filled with poorly vascular fat that lets the anal canal distend; abscesses here can grow large.
- The two fossae communicate behind the anal canal through the deep postanal space, so pus can spread from one side to the other (horseshoe abscess).
Pudendal nerve (S2–S4)
- Leaves the pelvis through the greater sciatic foramen below piriformis, crosses the sacrospinous ligament just medial to the ischial spine, re-enters through the lesser sciatic foramen and runs in the pudendal (Alcock) canal.
| Branch | Supplies |
|---|---|
| Inferior rectal | External anal sphincter, perianal skin, anal canal below the pectinate line |
| Perineal | Muscles of the urogenital triangle including the external urethral sphincter; posterior scrotal or labial skin |
| Dorsal nerve of penis or clitoris | Sensation of the shaft and glans |
- Pudendal block: injected transvaginally at the ischial spine for instrumental delivery; it can briefly weaken the external urethral sphincter in the deep pouch.
- The pudendal nerve does not supply the front of the vulva or scrotum. After a good pudendal block, pain remains over the mons and anterior labia majora, supplied by the ilioinguinal nerve and the genital branch of the genitofemoral nerve. The lateral perineum also receives the perineal branch of the posterior femoral cutaneous nerve.
- Stretching in a prolonged second stage causes pudendal neuropathy with faecal and stress urinary incontinence.
Vessels and lymph
- Internal pudendal artery (anterior division of internal iliac) follows the nerve: inferior rectal, perineal, and arteries of the bulb, deep and dorsal arteries of the penis or clitoris.
- The superficial and deep external pudendal arteries come from the femoral artery and supply the anterior scrotal or labial skin; they are spared if the internal pudendal artery is cut.
| Region | Lymph nodes |
|---|---|
| Perineal skin, scrotum, vulva, anal canal below pectinate line, lower third of vagina | Superficial inguinal |
| Glans penis and glans clitoridis | Deep inguinal nodes (and nodes in the femoral canal) |
| Testis | Para-aortic |
Perineal body, episiotomy and tears
- Muscles attached to the perineal body: bulbospongiosus, superficial and deep transverse perinei, external anal sphincter, fibres of levator ani (pubococcygeus). A median episiotomy cuts through it and can extend straight into the sphincter.
- Mediolateral episiotomy (at least 45–60° from the midline) is preferred because it is less likely to extend into the anal sphincter; it cuts skin, vaginal wall, bulbospongiosus and superficial transverse perineal muscle.
| Tear degree | Involves |
|---|---|
| First | Perineal or vaginal skin only |
| Second | Perineal muscles, sphincters intact |
| Third | External anal sphincter (3a < 50%, 3b > 50%, 3c internal sphincter too) |
| Fourth | Sphincter complex and anorectal mucosa |
- After delivery, endoanal ultrasound shows where the sphincter is torn; anorectal manometry shows how weak it is.
Muscles and sexual function
- Ischiocavernosus compresses the crura to sustain erection; bulbospongiosus empties the urethra of urine and semen.
- Erection: parasympathetic pelvic splanchnics (S2–S4), nitric oxide. Emission: sympathetic. Ejaculation: pudendal nerve.
- Hypospadias: failure of fusion of the urethral (urogenital) folds; opening on the ventral penis.
Remember Superficial pouch: Colles to membrane, Bartholin and bulb. Deep pouch: sphincter, membranous urethra, Cowper.
Exam trap Bulbar urethral rupture spreads to scrotum, penis and abdominal wall but never the thighs.
Exam trap A woman still feeling the front of the labia after pudendal block has an intact ilioinguinal and genitofemoral supply, not a failed block.
How FCPS asks about this
- Swelling of scrotum and abdominal wall but not thigh after perineal trauma: urethra ruptured below the perineal membrane (bulbar).
- Landmark for pudendal block: ischial spine; root value S2–S4.
- Glands in each pouch: Bartholin in the superficial pouch; Cowper in the deep pouch.
- Why mediolateral episiotomy: less likely to extend into the anal sphincter.
- Tear through perineal muscles with sphincters intact: second-degree tear.
- Best test for a torn sphincter after delivery: endoanal ultrasound.
- Lymph from glans and lower vagina: deep inguinal and superficial inguinal nodes respectively.
Test yourself on this topic
5 of the 51 questions in our bank on this topic. Choose an answer to see why every option is right or wrong.
In a man with a rupture of the membranous urethra, which small glands lying beside it in the deep perineal pouch may also be damaged?
Answer: Bulbourethral (Cowper) glands
The deep perineal pouch lies above the perineal membrane. In men it contains the membranous urethra, the external urethral sphincter, deep transverse perineal muscles and the pea-sized bulbourethral (Cowper) glands, whose ducts pierce the perineal membrane to open into the bulbar urethra. Their mucus lubricates the urethra before ejaculation. The greater vestibular (Bartholin) glands are the female equivalent but lie in the superficial pouch. Seminal vesicles and prostate lie in the pelvis above the pouch.
Deep perineal pouch (male)
- Membranous urethra
- External urethral sphincter
- Bulbourethral glands
- Deep transverse perineal muscles
Remember Cowper glands sit deep; Bartholin glands sit superficial.
Exam trap Bulbourethral glands lie in the deep pouch, but their ducts open into the bulbar urethra in the superficial pouch.
Why the other options are wrong
- Greater vestibular glands Female glands in the superficial pouch.
- Seminal vesicles Lie behind the bladder in the pelvis.
- Urethral glands of Littré Scattered along the penile urethra.
- Prostatic glands Lie above the pouch around the prostatic urethra.
After forcible bending of the erect penis during intercourse, a man has a swollen, bruised penile shaft and blood at the meatus. The scrotum and perineum are not swollen. Which layer has confined the blood and urine to the shaft?
Answer: Deep (Buck's) fascia of the penis
This is a penile fracture: rupture of the tunica albuginea of the corpus cavernosum, sometimes with urethral injury. The deep fascia of the penis (Buck's fascia) encloses the corpora cavernosa and corpus spongiosum. While it remains intact, blood and extravasated urine stay within it, so swelling is limited to the shaft, giving an 'aubergine' penis. If Buck's fascia is also torn, fluid spreads under the membranous (Colles) fascia into the scrotum, perineum and lower abdominal wall, as in bulbar urethral injury.
Penile fascial layers
- Buck's fascia surrounds all three corpora
- Intact: swelling limited to the shaft
- Torn: spreads under Colles fascia
- To scrotum, perineum, abdominal wall
Remember Shaft-only swelling: Buck's fascia is holding.
Exam trap Blood at the meatus with a penile fracture suggests urethral injury and needs urethral assessment.
Why the other options are wrong
- Membranous (Colles) fascia Limits spread once Buck's fascia is breached, giving scrotal and perineal swelling.
- Tunica albuginea of the testis Covers the testis, not the penis.
- Dartos layer of the scrotum Lies in the scrotal wall; would allow scrotal spread.
- Perineal membrane Limits spread upwards from the superficial pouch, not along the shaft.
Why is a mediolateral rather than a midline episiotomy usually preferred during an instrumental delivery?
Answer: It is less likely to extend into the anal sphincter
A midline episiotomy cuts straight back through the perineal body towards the anus, so if the tear extends it easily runs into the external anal sphincter and rectal mucosa (third- or fourth-degree tear). A mediolateral incision starts at the posterior fourchette and is directed at about 60° from the midline, away from the anus, so extensions run into the ischioanal fat rather than the sphincter. It still divides bulbospongiosus and the superficial transverse perineal muscle. Midline cuts actually heal with less pain, which is why some countries still use them.
Episiotomy
- Midline: through the perineal body
- Midline extends into the anal sphincter
- Mediolateral: 60° from midline
- Extension runs into ischioanal fat
Remember Angle the cut away from the anus.
Exam trap Cutting too close to the midline (under 40°) loses the protective benefit of a mediolateral cut.
Why the other options are wrong
- It avoids cutting any perineal muscle It divides bulbospongiosus and the transverse perinei.
- It heals faster with less pain Midline cuts tend to heal with less pain.
- It spares the pudendal nerve trunk completely Pudendal branches are not the main reason for the choice.
- It avoids the vestibular bulbs and their bleeding Bleeding from the bulbs is not the main reason.
A 60-year-old para 5 had poorly repaired perineal tears in her deliveries. She has a bulge in the lower posterior vaginal wall and must press on it to empty her bowel. The perineum is short with a gaping introitus. Which of the following structures is most likely deficient?
Answer: Perineal body
The perineal body is a fibromuscular node between the lower vagina and the anal canal. The bulbospongiosus, superficial and deep transverse perineal muscles, external anal sphincter and parts of levator ani attach to it, and it supports the lower posterior vaginal wall. If it is torn and not properly repaired, the lower rectum bulges into the vagina (a low rectocele), the perineum shortens and the introitus gapes. The cardinal and uterosacral ligaments support the cervix and upper vagina, and pubocervical fascia supports the bladder.
Perineal body
- Node between vagina and anus
- Many perineal muscles attach to it
- Torn: low rectocele, gaping introitus
- Repair it carefully after tears
Remember Central point of the perineum holds the back wall up.
Exam trap A high rectocele or enterocele reflects failure of upper vaginal supports, not just the perineal body.
Why the other options are wrong
- Cardinal ligament Supports the cervix; weakness causes uterine prolapse.
- Pubocervical fascia Supports the bladder; weakness causes a cystocele.
- Round ligament Holds the uterus forward; not a true support.
- Uterosacral ligament Supports the cervix from behind; weakness gives enterocele or prolapse.
A Bartholin gland abscess lies in which of the following perineal spaces?
Answer: Superficial perineal pouch
The superficial perineal pouch lies between the perineal membrane and the membranous layer of superficial fascia (Colles fascia). In women it contains the bulbs of the vestibule, the crura of the clitoris, the bulbospongiosus, ischiocavernosus and superficial transverse perineal muscles, and the greater vestibular (Bartholin) glands, which lie at the posterior end of each bulb. Their ducts open into the vestibule beside the vaginal orifice. The deep pouch lies above the membrane.
Superficial perineal pouch (female)
- Between perineal membrane and Colles fascia
- Bulbs of vestibule and clitoral crura
- Perineal muscles
- Greater vestibular glands
Remember Bartholin glands sit superficial, behind the bulbs.
Exam trap The male equivalent glands (Cowper) lie in the deep, not the superficial, pouch.
Why the other options are wrong
- Deep perineal pouch Contains the urethral sphincter, not Bartholin glands.
- Ischioanal fossa Lies beside the anal canal, further back.
- Rectouterine pouch (Douglas) A peritoneal space within the pelvis.
- Retropubic space of Retzius Lies in front of the bladder in the pelvis.
Where this sits in the CPSP syllabus
- Paper 1: Pelvis and perineum, Perineum, pudendal nerve, ischioanal fossa, perineal pouches
- Surgery & Allied, Paper 2: Pelvis and perineum, Perineum, pudendal nerve, ischioanal fossa, perineal pouches