Anal sphincters
Anal canal and anal sphincters; obstetric anal sphincter injury
The anal canal is where hindgut endoderm meets skin ectoderm, so its upper and lower halves differ in lining, blood, lymph and nerves. Continence depends on two sphincters with different muscle, nerves and jobs, and childbirth is the commonest cause of their injury in women. CPSP asks the pectinate line differences, sphincter supply and function, and how tears and incontinence are classified.
Extent and relations
- About 4 cm long, from the anorectal ring (puborectalis sling at the anorectal junction) to the anal verge; it runs downwards and backwards.
- In front: perineal body and lower vagina (separated by a thin rectovaginal septum). Behind: anococcygeal ligament and coccyx. Sides: ischioanal fossae.
- Landmarks from above down: anorectal ring, anal columns, pectinate (dentate) line with anal valves and sinuses, pecten (non-keratinised squamous, no hair or glands; fissures lie here), intersphincteric groove (lower edge of the internal sphincter), anal verge and perianal skin.
- Anal glands open into the sinuses at the pectinate line; their infection starts most anorectal abscesses, which spread in the intersphincteric plane. The commonest fistula is intersphincteric.
Above and below the pectinate line
| Feature | Above | Below |
|---|---|---|
| Embryology | Hindgut endoderm | Proctodeum ectoderm |
| Epithelium | Columnar (transitional near the line) | Stratified squamous |
| Artery | Superior rectal (inferior mesenteric) | Inferior rectal (internal pudendal) |
| Vein | Superior rectal to portal system (liver metastases) | Inferior rectal to systemic veins (lung) |
| Lymph | Internal iliac (and inferior mesenteric) | Superficial inguinal (horizontal group) |
| Nerve | Visceral (autonomic): stretch only, painless | Somatic (inferior rectal, pudendal): painful |
| Typical disease | Internal haemorrhoids bleed painlessly; adenocarcinoma | Fissure, thrombosed external pile; squamous carcinoma |
- The middle rectal artery (internal iliac) supplies the muscle wall. The line marks the site of the embryonic anal membrane.
The two sphincters
| Feature | Internal anal sphincter (IAS) | External anal sphincter (EAS) |
|---|---|---|
| Muscle | Smooth, thickened circular muscle of the rectum | Striated, voluntary |
| Nerve | Sympathetic L1–L2 contracts; parasympathetic S2–S4 relaxes; myogenic tone | Inferior rectal nerve (pudendal, S2–S4) |
| Function | Most of the resting pressure | Squeeze pressure, voluntary hold |
| Damage gives | Passive soiling, flatus leak | Urgency incontinence |
| Endoanal ultrasound | Hypoechoic (dark) ring | Hyperechoic (bright) ring |
- EAS parts: subcutaneous (around the orifice), superficial (perineal body to coccyx via the anococcygeal ligament), deep (blends with puborectalis).
- Lateral sphincterotomy for chronic fissure divides the internal sphincter.
- Anal cushions (submucosal vascular pads at 3, 7 and 11 o'clock) seal the canal and give about 15–20% of resting pressure; enlarged, they are haemorrhoids. In late pregnancy the uterus raises pelvic venous pressure and progesterone relaxes veins, so piles appear and usually improve after delivery.
Continence reflexes
- Rectoanal inhibitory reflex: rectal distension relaxes the IAS through the myenteric plexus; absent in Hirschsprung disease.
- Contents then reach the richly innervated upper anal canal, which tells gas, liquid and solid apart (sampling), while the EAS and puborectalis hold until it is convenient.
- Puborectalis keeps the anorectal angle; straining at defaecation relaxes it and straightens the angle.
- Parasympathetic supply to rectum and anal canal is from S2–S4 pelvic splanchnic nerves.
Obstetric anal sphincter injury
| Grade | Injury |
|---|---|
| 3a | Under 50% of EAS thickness |
| 3b | Over 50% of EAS thickness |
| 3c | EAS and IAS torn |
| Fourth degree | Both sphincters plus anal epithelium |
- Risks: forceps, birth weight over 4 kg, occipitoposterior delivery, primigravida, prolonged second stage, midline episiotomy.
- Every perineal tear needs a rectal examination; a missed fourth-degree tear can break down into a rectovaginal fistula (flatus or faeces per vagina weeks later).
- Incontinence with intact sphincters on ultrasound points to pudendal neuropathy: the inferior rectal nerve stretched at the ischial spine in a long second stage.
- Investigations: endoanal ultrasound shows the anatomy of a defect; anorectal manometry shows function (low resting pressure, IAS; low squeeze pressure, EAS).
- After repair stools are kept soft: lactulose (osmotic, non-absorbed disaccharide) and avoidance of codeine, which constipates through mu receptors in the myenteric plexus.
Remember Pectinate line: above is gut (painless, portal, internal iliac nodes); below is skin (painful, systemic, superficial inguinal nodes).
Exam trap The internal sphincter is smooth muscle tightened by sympathetic fibres; the external sphincter is striated and supplied by the pudendal nerve.
Exam trap Grade 3c means the internal sphincter is torn; fourth degree needs the anal epithelium torn as well.
Test yourself on this topic
5 of the 28 questions in our bank on this topic. Choose an answer to see why every option is right or wrong.
A 55-year-old woman has a tumour of the anal canal arising just above the pectinate line. To which lymph nodes does this part of the canal mainly drain?
Answer: Internal iliac (hypogastric) nodes
Lymph drainage of the anal canal follows the blood supply and the embryological division at the pectinate line. Above the line, lymph passes with the middle and inferior rectal vessels to the internal iliac nodes, and from the upper canal with the superior rectal vessels to the inferior mesenteric nodes. Below the line, lymph drains to the superficial inguinal nodes. Inguinal node examination is therefore important for tumours below, but not above, the line.
Anal canal lymph
- Above line: internal iliac
- Upper canal also: inferior mesenteric
- Below line: superficial inguinal
- Follows the blood supply
Remember Above the line, inside the pelvis; below, the groin.
Exam trap A tumour above the line may spread to pelvic nodes with normal groin nodes.
Why the other options are wrong
- Superficial inguinal (horizontal) nodes Receive lymph from below the pectinate line.
- Deep inguinal (Cloquet) nodes Receive lymph mainly from the superficial nodes and clitoris.
- Para-aortic nodes at L2 Receive lymph from the ovaries and testes.
- External iliac nodes Receive lymph from the bladder and upper vagina, not this part of the canal.
After a forceps delivery, rectal examination shows a perineal tear through the full thickness of the external anal sphincter; the pale circular fibres of the internal anal sphincter beneath are also torn, but the anal epithelium is intact. How is this tear classified?
Answer: Third-degree, grade 3c
Obstetric anal sphincter injuries are third- and fourth-degree tears. Grade 3a involves less than 50% of the external anal sphincter thickness, 3b more than 50%, and 3c both the external and the internal anal sphincter. A fourth-degree tear also involves the anal epithelium. Tearing of the internal sphincter matters because it maintains most of the resting anal pressure, and if left unrepaired it causes passive soiling and flatus incontinence.
Third-degree tears
- 3a: < 50% external sphincter
- 3b: > 50% external sphincter
- 3c: internal sphincter also torn
- 4th: plus anal epithelium
Remember Internal sphincter torn means 3c.
Exam trap If unsure between 3a and 3b, choose the higher grade.
Why the other options are wrong
- Third-degree, grade 3a Would involve less than half of the external sphincter only.
- Third-degree, grade 3b Would involve more than half of the external sphincter but spare the internal sphincter.
- Fourth-degree tear Would involve the anal epithelium, which is intact.
- Second-degree tear Would spare the anal sphincter completely.
After a vaginal delivery of a 4.2 kg baby, examination shows a tear through about two-thirds of the thickness of the external anal sphincter. The internal anal sphincter and anal epithelium are intact. How is the tear graded?
Answer: Third-degree, grade 3b
A tear involving more than 50% of the external anal sphincter thickness, with an intact internal sphincter, is a grade 3b tear. Grade 3a involves less than 50%, and 3c also involves the internal sphincter. A fourth-degree tear involves the anal epithelium as well. Accurate grading matters because it guides the repair method (overlapping or end-to-end) and follow-up, and macrosomia is a recognised risk factor.
Grading the external sphincter
- < 50%: grade 3a
- > 50%: grade 3b
- Internal sphincter too: 3c
- Risks: forceps, big baby, OP
Remember More than half of the EAS: 3b.
Exam trap Grade using the thickness torn, not the length of the tear.
Why the other options are wrong
- Third-degree, grade 3a Less than half of the external sphincter is torn in 3a.
- Third-degree, grade 3c Needs the internal sphincter to be torn.
- Fourth-degree tear Needs the anal epithelium to be torn.
- Second-degree tear Spares the anal sphincter completely.
Which additional structure must be torn for a perineal tear involving both anal sphincters to be called fourth-degree?
Answer: Anal epithelium
A fourth-degree tear involves the external and internal anal sphincters and the anal epithelium (anorectal mucosa), so the anal lumen is opened. It needs careful layered repair in theatre: the anal epithelium, then the internal sphincter, then the external sphincter, followed by the perineal muscles and skin. Missed or poorly healed fourth-degree tears can lead to rectovaginal fistula and faecal incontinence.
Fourth-degree tear
- Both sphincters torn
- Plus anal epithelium
- Anal lumen opened
- Repair in theatre
Remember Fourth-degree: you can see into the anal canal.
Exam trap An isolated buttonhole of rectal mucosa with intact sphincters is not a fourth-degree tear.
Why the other options are wrong
- Perineal skin Torn in every grade, from first-degree up.
- Levator ani Its tearing does not change the grade.
- Bulbospongiosus Torn in second-degree and higher tears.
- Rectovaginal fascia Its tearing is not part of the grading.
Healthy people can tell whether rectal contents are gas, liquid or solid, and so can release flatus selectively. Which structure provides this discrimination?
Answer: Sensory endings of the upper anal canal
When the rectum fills, the internal anal sphincter relaxes briefly (rectoanal inhibitory reflex) and a little rectal content moves down into the upper anal canal. This transitional zone around the pectinate line is richly supplied with somatic sensory endings for touch, pain, temperature and friction, which identify the content as gas, liquid or solid. The external sphincter is then contracted or relaxed accordingly. Rectal wall receptors sense only fullness.
Sampling reflex
- Rectum fills, IAS relaxes
- Contents reach upper anal canal
- Rich sensory endings there
- Gas, liquid or solid identified
Remember The anal canal samples; the rectum only feels fullness.
Exam trap Scarring of the anal canal lining after surgery can impair discrimination.
Why the other options are wrong
- Stretch receptors of the sigmoid colon Sense distension, not the type of content.
- Chemoreceptors in the rectal mucosa The rectal mucosa does not discriminate gas from solid this way.
- Proprioceptors of the external sphincter Sense muscle stretch, not the nature of contents.
- Pacinian corpuscles in the ischioanal fat The fat is not involved in sensing contents.
Where this sits in the CPSP syllabus
- Obstetrics & Gynaecology, Paper 2: Pelvis, pelvic floor and perineum, Anal canal and anal sphincters; obstetric sphincter injury
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