Chapter 01 — Anatomy and Physiology in Three Minutes

Purpose: four landmarks explain almost every anorectal symptom — learn them once and every examination, referral, and operation in this book makes sense.

🎨 INFOGRAPHIC / VISUAL BLUEPRINT — CHAPTER 01

• Chapter colour palette: Deep Teal (#006D77) header bar, Mint-Teal (#E2F3F4) callout panels, Slate (#1E293B) body text.
• Hero graphic: Coronal and sagittal anorectal cross-sections with the dentate line drawn as a glowing horizontal divider (visceral innervation above, somatic below), the puborectalis sling shown pulling the anorectal junction forward, and a side gauge labelled “approximate contributions to resting anal pressure — estimates vary by method”.
• Icon set: Shield (internal sphincter), clenched hand (external sphincter), rubber washer (haemorrhoidal cushions), lightning bolt (somatic pain below the dentate line), feather (visceral sensation above it), red spine (cauda equina warning).
• Bedside aphorism banner: “Most resting anal pressure comes from ONE involuntary muscle — the internal sphincter. Recurrence can be re-treated; incontinence is often permanent.”

1. The Four Landmarks That Dictate Every Clinical Decision

You do not need a surgical textbook to understand the anal canal. You need four landmarks and the job each one does.

Table 1.1 — The four landmarks

Evidence note: the percentage contributions to resting pressure come from physiological studies using different methods; treat them as approximate [U]. The clinical message — protect the internal sphincter — is not in doubt.

Two nerve facts that save patients

• The sacral roots (S2–S4) serve both the anal sphincters and the bladder. Severe anorectal pain, spinal or general anaesthesia, and opioids commonly cause urinary retention — the first thing to check when a post-operative patient complains of “anal pain”.

• New faecal incontinence, reduced anal tone, saddle numbness, or urinary retention with back pain or bilateral leg symptoms is suspected cauda equina syndrome until proven otherwise — an emergency MRI indication, not an anorectal problem [C].

2. The Continence History in 60 Seconds

Ask these questions before any referral for fissure, fistula, haemorrhoid, or prolapse surgery. The answers decide which procedures are safe.

Table 1.2 — Continence questions and what they localise

Algorithm 1.1 — Localising pain and leakage

[ANORECTAL SYMPTOM] -> ask: WHERE is the pain?  WHAT kind of leakage?
|
+-- PAIN
|   +-- Sharp, localised, worse with stool or sitting
|   |     -> BELOW the dentate line (somatic anoderm)
|   |     -> think: fissure, abscess, thrombosed external
|   |        haemorrhoid, ulcer, anal cancer
|   +-- Dull ache, fullness, painless bleeding
|         -> ABOVE the dentate line (visceral mucosa)
|         -> think: internal haemorrhoids, prolapse,
|            proctitis, rectal tumour
|
+-- LEAKAGE
    +-- Passive, unnoticed (often at night) + LOW resting tone
    |     -> INTERNAL sphincter problem
    +-- Urgency, cannot defer + WEAK voluntary squeeze
    |     -> EXTERNAL sphincter / puborectalis problem
    +-- Watery leak + hard stool in the rectum
    |     -> OVERFLOW from impaction: NO loperamide
    +-- NEW leak + saddle numbness, retention, or back pain
          -> RED FLAG: suspected cauda equina syndrome
             EMERGENCY MRI + spinal team (local pathway)

3. Dangerous Mimics: When “Anal Pain” or “Leakage” Is Something Else

Table 1.3 — Dangerous mimics

4. Role-Targeted Action Boxes

🚑 ER DOCTOR ACTION BOX

• Warn before you touch. Anoderm below the dentate line is exquisitely sensitive. In a painful patient, apply 2% lidocaine gel to the anal verge and wait 5–10 minutes before a gentle examination.
• Record resting tone and voluntary squeeze separately on every DRE. High, gripping resting tone points to fissure or intersphincteric spasm; a lax ring suggests neurological weakness, prolapse, or prior surgical or obstetric injury.
• New incontinence or reduced anal tone with back pain, bilateral sciatica, saddle numbness, or urinary retention is suspected cauda equina syndrome: arrange emergency MRI through the local spinal pathway [C][L].

🩺 FAMILY MEDICINE / GP CLINIC BOX

• Explain the anatomy simply: “Everyone has these cushions — they are the seal that keeps you dry. We treat the symptoms, not normal anatomy.”
• Document baseline continence before referring for fissure or fistula surgery: gas, liquid, solid; passive or urge; pads; obstetric history. A validated score makes the referral far more useful.
• Always ask about bowel-habit change and bleeding, even when the presenting complaint is itching or pain.

🏥 ER & WARD NURSE CARE AND SBAR BOX

• Immediate severe pain after rubber band ligation suggests the band sits on or below the dentate line. Escalate promptly for surgical review and possible band removal. SBAR example: “Mr X had banding 30 minutes ago; pain is 9/10 despite paracetamol; please review within 30 minutes.”
• Post-operative “anal pain” with restlessness: scan the bladder before giving more opioid.

💊 CROSS-SPECIALTY PRESCRIBING TRAP

• Bulk-forming laxatives without enough fluid can cause obstruction. Psyllium (ispaghula) is usually started at about 3.5 g once or twice daily and titrated; each dose must be taken with a full glass of water (at least 150 mL). Avoid in faecal impaction, bowel obstruction, and immobile patients with poor fluid intake.
• Anticholinergics, opioids, and antidiarrhoeals can convert constipation into impaction with overflow. Review the medication list whenever continence changes.

⛔ NEVER DO THIS

• Never force a finger or anoscope past severe pain or spasm — stop, treat, or book an examination under anaesthesia (EUA).
• Never place bands, injections, or sutures below the dentate line in an awake patient.
• Never refer for a sphincter-dividing procedure without documenting baseline continence and risk factors.
• Never attribute new faecal incontinence to “age” before excluding cauda equina syndrome, impaction, and rectal tumour.

📝 COMMON DOCUMENTATION ERROR

• “DRE normal” tells the next clinician nothing. Write what you assessed: position, chaperone, inspection, resting tone, squeeze, masses, tenderness, prostate or cervix, and what was on the glove.
• Clock-face positions without the patient’s position and the convention used are uninterpretable — use anatomical quadrants (Chapter 17).
• “Incontinent” without type (gas/liquid/solid; passive/urge), frequency, and pad use.

📨 REFERRAL URGENCY BOX

The Patient in Front of You — Chapter 01 Mini-Case

🧑‍⚕️ THE PATIENT IN FRONT OF YOU

A 64-year-old woman who had a “haemorrhoid and fissure operation” abroad 3 years ago presents to your clinic embarrassed by daily staining of her underwear with liquid stool and mucus. There is no warning, especially at night. On examination the anal verge is scarred and resting tone is markedly lax, but her voluntary squeeze when asked to “hold on” is strong. She has no back pain, numbness, or urinary symptoms.

Step 1 Localise the defect. Passive, unnoticed nocturnal leakage with low resting tone points to the internal sphincter, which provides most resting closure — consistent with a previous sphincterotomy or anal stretch.

Step 2 Explain the preserved squeeze. A strong voluntary squeeze shows the external sphincter and puborectalis are working.

Step 3 Exclude the red flags. No neurological symptoms; check for impaction, proctitis, and a rectal mass on DRE and anoscopy.

Step 4 Act. Firm the stool (fibre titration; loperamide can be appropriate for loose stool once impaction and infection are excluded [S, ACG 2021]), protect the skin with a zinc-oxide barrier, refer to pelvic-floor physiotherapy/biofeedback, and refer to colorectal surgery for anal physiology and endoanal ultrasound. Avoid any further sphincter-dividing procedure.

Board-Style Question — Chapter 01

A 68-year-old woman with passive nocturnal staining undergoes anorectal manometry. Resting anal pressure is markedly reduced, while voluntary squeeze pressure is preserved. Dysfunction of which structure best explains the reduced resting pressure?

• A. External anal sphincter
• B. Internal anal sphincter
• C. Puborectalis muscle
• D. Haemorrhoidal cushions

Answer and explanation

• ❌ A The external sphincter is voluntary striated muscle; it generates squeeze pressure, which is preserved here.
• ✅ B Correct. The involuntary internal sphincter provides most of the resting anal pressure. Low resting pressure with preserved squeeze localises the problem to the internal sphincter; passive leakage is its typical symptom.
• ❌ C The puborectalis maintains the anorectal angle and contributes to voluntary continence, not baseline resting pressure.
• ❌ D The cushions contribute only a minority (roughly 15%) of resting pressure; their loss alone does not cause a marked fall.

Ten-Item Rapid Revision Checklist — Chapter 01

• 01 I can locate the dentate line and explain why banding above it is painless and below it is not.
• 02 I know the internal sphincter provides most resting tone and that its injury causes passive leakage.
• 03 I know external sphincter or puborectalis injury causes urge incontinence.
• 04 I record resting tone and voluntary squeeze separately on every DRE.
• 05 I document continence (gas/liquid/solid, passive/urge, pads) before any sphincter-procedure referral.
• 06 I ask about OASIS, prior anal surgery, pelvic radiation, diabetes, and neurological disease.
• 07 I recognise cauda equina red flags and know the emergency pathway.
• 08 I check the bladder first when a post-operative patient reports anal pain.
• 09 I prescribe bulk laxatives with fluid instructions and never in impaction.
• 10 I describe findings by anatomical quadrant and distance from the verge, not by clock face.

Five Most Dangerous Mistakes — Chapter 01

1. ⚠️ Labelling new faecal incontinence as “age-related” and missing cauda equina syndrome, impaction, or a rectal tumour.
2. ⚠️ Forcing a DRE through severe spasm — losing the patient’s trust and missing a deep abscess.
3. ⚠️ Referring for sphincterotomy or fistulotomy without documenting baseline continence and obstetric history.
4. ⚠️ Treating post-operative “anal pain” with more opioid when the bladder is full.
5. ⚠️ Using clock-face notation that the operating team reads in a different orientation.

Suggested Figure and Table Captions for the Medical Illustrator — Chapter 01

• Figure 1.1. Coronal section of the anal canal showing the internal and external sphincters, the intersphincteric plane, the dentate line, and anal crypts and glands.
• Figure 1.2. Sagittal view of the puborectalis sling and the anorectal angle at rest and during straining.
• Figure 1.3. Innervation map: visceral (stretch-sensitive) mucosa above the dentate line and somatic (pain-sensitive) anoderm below it, with the sites of painless office procedures marked.
• Figure 1.4. Approximate contributions to resting anal pressure (labelled “estimates vary by method”).
• Table 1.1. The four landmarks that dictate every clinical decision.
• Table 1.2. Continence questions and what they localise.
• Table 1.3. Dangerous mimics of anal pain and leakage.
• Algorithm 1.1. Localising pain and leakage, including the cauda equina red flag.