Chapter 18 — Anal Stenosis

Anal stenosis means narrowing of the anal canal. It restricts the passage of stool, and often causes pain during defecation. This chapter explains anal stenosis in full. It covers the author's own grading system in complete detail. It also covers presentation, examination, differential diagnosis, investigations, and management. This includes the author's own surgical technique for severe cases. This is the same pattern used for every disease chapter in this manual.

18.1 What Is Anal Stenosis

Anal stenosis is a narrowing of the anal canal. It can be mild, causing only slight difficulty. It can be severe, causing near-complete obstruction to the passage of stool.

Anal stenosis is not one single disease. It is the end result of many different underlying processes, all of which cause fibrosis and narrowing of the anal canal. Chapter 11 introduced the fissure-specific link to stenosis, seen after repeated surgery for chronic fissure. This chapter covers stenosis from every cause, not only fissure-related stenosis.

18.2 Why It Happens

Previous anorectal surgery is the most common cause. Aggressive excision of tissue during haemorrhoidectomy, particularly when too little healthy anoderm is left behind, is a well- recognised cause. Repeated surgery for chronic fissure, described in Chapter 11, can also lead to scarring and stenosis over time.

Inflammatory bowel disease, particularly Crohn's disease, can cause anal stenosis through chronic inflammation and fibrosis of the anal canal, independent of any prior surgery.

Radiotherapy to the pelvis can cause progressive fibrosis and narrowing of the anal canal. It is usually given for a nearby cancer. Symptoms can appear months or years after treatment.

Trauma, including obstetric injury or accidental injury, can also lead to scarring and stenosis during healing.

A congenital anal stenosis, present from birth, is rare, and is usually identified and treated in infancy rather than in adult practice.

18.3 The OPB Grading of Anal Canal Stenosis

Chapter 11 introduced this grading system briefly, in the specific context of fissure-related stenosis. This section gives its full, definitive description, covering stenosis of any cause.

Anal stenosis has an established classification already in the surgical literature. Milsom and Mazier described this system in 1986. It grades stenosis as mild, moderate, or severe, based on what can be passed through the anal canal in the awake patient. Mild stenosis still admits a well-lubricated index finger, or a medium Hill-Ferguson retractor. Moderate stenosis needs forceful dilatation to admit either of these. Severe stenosis will not admit even the little finger,

without forceful dilatation. Milsom and Mazier also classified stenosis by its level in the anal canal, as low, middle, or high, and by its shape, as annular or tubular.

The OPB Grading of Anal Canal Stenosis is the author's own refinement of this established system, developed for three specific reasons.

First, assessment is performed under spinal anaesthesia, rather than in the awake patient. This removes pain and guarding as confounding factors, and gives a more accurate assessment of the true resting calibre of the canal.

Second, assessment uses a full set of numbered, calibrated anal dilators, rather than a single Hill-Ferguson retractor. This gives an objective, reproducible measurement, rather than a single pass-or-fail threshold.

Third, the OPB Grading divides stenosis into four grades rather than three, giving a finer distinction within what Milsom and Mazier would classify as a single severe category.

[AUTHOR TO CONFIRM: the precise calibre or dilator size defining each grade below has been drafted to a clinically reasonable structure, building on the Milsom-Mazier framework above, pending the author's own exact thresholds, before this section is treated as final.]

Grade 1 — mild stenosis. The anal canal admits the examining finger, and the largest calibrated dilators, with only mild resistance.

Grade 2 — moderate stenosis. The anal canal admits the examining finger with noticeable resistance, and admits only the mid-range calibrated dilators.

Grade 3 — severe stenosis. The anal canal admits the little finger only, with firm resistance, and admits only the smallest calibrated dilators.

Grade 4 — very severe stenosis. The anal canal barely admits the smallest dilator, or admits nothing at all, without further widening first.

This grading directly guides treatment, described in Section 18.8 below. Grade 1 and many Grade 2 cases can often be managed conservatively. Grade 3 and Grade 4 cases usually need surgical correction.

18.4 Presentation

Follow the history-taking method described in Chapter 2.

Difficulty passing stool, often described as a narrow or ribbon-like stool.

Straining, sometimes severe, to pass even a small amount of stool.

Pain during defecation, particularly if a fissure coexists within the narrowed segment.

A history of previous anorectal surgery, in most patients. Ask specifically about this, since patients do not always volunteer it unprompted.

In a severe case, a need for laxatives, enemas, or even dilators used at home, simply to pass stool at all.

18.5 Examination

Follow the sequence described in Chapter 3.

Inspection, described in Chapter 4, may show a visibly narrowed or puckered anal opening, particularly in a severe case.

Digital rectal examination, described in Chapter 5, should be attempted gently. In a mild case, the finger passes with some resistance. In a severe case, the finger may not pass at all in the awake patient, and further assessment under anaesthesia is needed.

Do not force the examination in an awake patient with suspected severe stenosis. This causes unnecessary pain, and gives an inaccurate assessment because of guarding. Proceed instead to examination under anaesthesia, as described in Section 18.3.

Formal grading should be performed under spinal anaesthesia in any patient with suspected Grade 3 or Grade 4 stenosis. Use the OPB Grading system described above.

18.6 Differential Diagnosis

Chronic anal fissure — described in Chapter 11. A fissure alone can cause spasm that mimics stenosis, without true fibrous narrowing. Assessment under anaesthesia distinguishes spasm from true stenosis.

Anorectal malignancy — a narrowing anal canal in an older patient should always raise suspicion of malignancy, especially without a clear history of prior surgery or inflammatory disease. Proper investigation is needed before assuming the stenosis is benign.

Crohn's disease — described in Section 18.2 above, as a cause of stenosis in its own right. It should also be considered as a coexisting diagnosis in any patient with stenosis and other bowel symptoms.

18.7 Investigations

Anal stenosis is primarily a clinical diagnosis, made on examination. Formal grading, described in Section 18.3, is itself the key diagnostic and planning step in a moderate or severe case.

A biopsy of the stenotic segment is advisable if malignancy is suspected, or if the cause is not clear from the history.

If Crohn's disease is suspected, appropriate investigation of the rest of the bowel, coordinated with a gastroenterologist, is needed.

18.8 Principles of Management

Grade 1, and many Grade 2, cases can be managed conservatively. Stool softeners, dietary fibre, and gentle self-dilatation, taught carefully to the patient, are often sufficient.

Grade 3 and Grade 4 cases usually need surgical correction. Simple dilatation alone is rarely a lasting solution in these more severe grades, and repeated forceful dilatation can itself cause further scarring.

Surgical options range widely. A simple lateral internal sphincterotomy suits a case where sphincter spasm contributes significantly. An anoplasty using a skin flap suits a more severe case. The author's own staged bilateral technique, described in Section 18.9, suits the most severe cases.

18.9 Surgical Techniques for Severe Stenosis

Flap anoplasty is a standard approach for severe stenosis. A flap of skin is mobilised and advanced into the anal canal. Common types include a Y-V advancement flap, a house flap, or a diamond flap. This widens the narrowed segment with well-vascularised tissue.

For severe stenosis, the author's own approach is a staged bilateral modification of the MOLIS technique, described fully in the companion techniques section of this manual.

The procedure is performed in a deliberate sequence, not as two simultaneous divisions. A left lateral, lower-third open sphincterotomy is performed first. This uses the same MOLIS principle described in the companion techniques section. The internal sphincter is delivered to the wound surface, and the lower one-third is divided under direct vision.

Trial dilatation follows immediately, on the operating table. A 23 millimetre dilator is passed to check the canal. This dilator typically passes without difficulty after the first, single-sided division alone.

A 27 millimetre dilator is then tried. In severe stenosis, this larger dilator typically does not pass after the first side alone, even when the 23 millimetre dilator has passed easily.

When the 27 millimetre dilator fails to pass, a second, opposite-side lateral sphincterotomy is performed, again restricted to the lower one-third of the internal sphincter. This right-sided, or contralateral, division follows the same principle as the first.

The 27 millimetre dilator is tried again after this second division. It typically passes easily at this stage. A 30 millimetre dilator can then be tried, and is typically seated, sometimes with some residual tightness.

This staged, calibration-driven approach avoids performing a second division unless the first division alone has genuinely proved insufficient, checked directly rather than assumed.

Both lateral sites, at the 3 o'clock and 9 o'clock positions, deliberately avoid the 6 o'clock posterior midline and the 12 o'clock anterior midline. Both midline sites are relatively poorly vascularised, and prone to poor healing if divided.

For the most severe stenosis, staged bilateral division alone may remain insufficient on calibration. Further sites can then be added at the same sitting, up to four sites in total, at the 10, 2, 8, and 4 o'clock positions. Each additional site follows the same lower one-third depth.

A representative case treated with this staged bilateral technique, Case 314, is discussed further in the companion techniques section of this manual.

Operative video: staged bilateral MOLIS for severe anal canal stenosis.

18.10 Special Situations

Crohn's-related stenosis — surgery carries a higher risk of poor healing in active Crohn's disease. Medical control of the underlying disease, coordinated with a gastroenterologist, should be optimised first, wherever possible, before elective surgery is considered.

Recurrent stenosis after previous surgery needs careful reassessment. Find the cause of the recurrence first. Do not simply repeat the same procedure that has already failed once.

Stenosis with a coexisting fissure — the fissure itself, described in Chapter 11, may need specific treatment alongside correction of the stenosis.

18.11 Complications of Treatment

Bleeding or infection at the operative site.

Recurrence of the stenosis, particularly if the underlying cause, such as active Crohn's disease, has not been addressed.

Incontinence, if sphincter division is more extensive than necessary. This is precisely why the calibrated, staged approach described in Section 18.9 is used, rather than a single, uncontrolled, deeper division.

Flap failure, in a case treated with anoplasty, usually related to poor blood supply to the flap or to tension at the suture line.

18.12 Common Mistakes to Avoid

Forcing a digital examination in an awake patient with suspected severe stenosis, rather than proceeding to assessment under anaesthesia.

Relying on repeated forceful dilatation alone in a Grade 3 or Grade 4 stenosis, rather than proceeding to definitive surgical correction.

Dividing the sphincter at a single, deep site, rather than distributing the release across multiple, shallower sites in severe stenosis.

Missing an underlying malignancy or Crohn's disease, by assuming every stenosis is a simple consequence of previous benign surgery.

18.13 Recording and Follow-Up

Record the grade, the suspected cause, and the treatment plan clearly. For example: "Anal stenosis, Grade 3 on examination under spinal anaesthesia, secondary to previous haemorrhoidectomy. Plan: staged bilateral MOLIS, left side first, calibrated with dilators intraoperatively. Review in 2 weeks." Arrange follow-up to confirm the stenosis has not recurred, and to check continence has been preserved.

18.14 Why This Chapter Matters

Anal stenosis restricts one of the most basic functions of daily life. Correctly grading its severity, and matching treatment to that grade, protects the patient. It avoids undertreatment, which leaves the obstruction unresolved. It also avoids overtreatment, which risks unnecessary loss of continence. The next chapter turns to a different anorectal condition: pilonidal sinus disease.