Anal Fissure

Introduction

An anal fissure is a longitudinal tear in the squamous epithelium of the anal canal, distal to the dentate line. Because that epithelium is somatically innervated, fissures are exquisitely painful, and the pain is out of all proportion to the size of the lesion - many patients describe it as the worst pain they have experienced, and it is a common cause of considerable distress and avoidance of defaecation.

It is a common condition, with an estimated lifetime incidence around 11%, affecting men and women roughly equally and peaking between the ages of 15 and 40. Despite being so common, it is frequently misdiagnosed as haemorrhoids, and the wrong treatment - a steroid suppository for a painful anus - does nothing for the underlying problem.

Fissures are classified as acute if present for less than six weeks and chronic if they persist beyond that. The distinction determines treatment: acute fissures usually heal with laxatives and topical measures, while chronic fissures need something that breaks the ischaemic cycle.

Pathophysiology

The reason fissures fail to heal, and the reason almost all of them are posterior, comes down to blood supply. It is one of the more satisfying pieces of applied anatomy in surgery.

1. A tear occurs, usually from the passage of a hard stool, from straining, or from childbirth or anal trauma
2. Pain triggers reflex spasm of the internal anal sphincter, which is smooth muscle in a state of continuous tonic contraction and is not under voluntary control. Anal canal resting pressures in patients with fissures are measurably higher than in controls.
3. Spasm compresses the branches of the inferior rectal artery as they run through the sphincter to supply the anoderm
4. The posterior commissure is already the least perfused part of the anal canal. Anatomical studies show that the arterial supply there is sparsest, and perfusion falls further as sphincter pressure rises. This is why 90% of fissures sit in the posterior midline.
5. Ischaemia prevents healing, the fissure persists, defaecation re-traumatises it, and the cycle repeats
6. Chronic changes develop - fibrosis of the fissure edges, a sentinel skin tag at the distal end from oedema and chronic inflammation, a hypertrophied anal papilla at the proximal end, and exposure of the white transverse fibres of the internal sphincter in the base

Every effective treatment for a chronic fissure works by interrupting this cycle at the sphincter step. Topical nitrates and calcium channel blockers relax the internal sphincter pharmacologically, botulinum toxin paralyses it temporarily, and sphincterotomy divides it permanently. Understanding this makes the treatment ladder logical rather than a list to memorise.

Risk factors

• Constipation and passage of hard stool - the dominant cause, and the main target of treatment
• Straining at defaecation, including from a low-fibre diet or ignoring the call to stool
• Childbirth - particularly instrumental delivery and perineal tearing, and the usual explanation for an anterior fissure in a woman
• Chronic diarrhoea, which is a less obvious but genuine cause
• Anal trauma, including receptive anal intercourse and instrumentation
• Inflammatory bowel disease, especially Crohn disease with perianal involvement
• Opioid analgesia and other constipating drugs
• Previous anal surgery with resulting stenosis or scarring

Clinical features

• Severe, sharp, tearing pain during defaecation, classically described as passing broken glass or razor blades
• Pain persisting after defaecation for minutes to several hours, from continuing sphincter spasm. This lingering pain is the feature that distinguishes a fissure from haemorrhoids most reliably.
• Bright red blood, small in volume, on the paper or streaking the surface of the stool
• Fear of defaecation, leading the patient to delay going, which hardens the stool and worsens the fissure - a behavioural loop that mirrors the vascular one
• Pruritus ani and discharge in chronic fissures
• A palpable skin tag at the anal margin, which patients often mistake for a haemorrhoid

Examination

1. Position the patient in the left lateral position and gently part the buttocks. In most cases the fissure is visible at the posterior midline without any instrument.
2. Look for the features of chronicity - a sentinel tag distally, a hypertrophied papilla proximally, indurated edges, and white internal sphincter fibres in the base
3. Note the position - midline is typical, lateral is not
4. Do not force a digital rectal examination. It is often intolerable and adds nothing when the diagnosis is visible. Document that it was deferred because of pain and arrange to repeat it once the fissure has healed, or perform an examination under anaesthetic if the diagnosis is uncertain.
5. Proctoscopy is similarly deferred in an acute painful fissure
6. Examine the abdomen and look for perianal Crohn disease - tags, fistulae, abscesses and induration

Differential diagnosis

Investigations

A typical fissure is a clinical diagnosis and needs no investigation. Testing is directed at atypical features or at an alternative diagnosis.

• FBC if bleeding has been significant or prolonged, or if anaemia is suspected
• Inflammatory markers, faecal calprotectin and gastroenterology referral where inflammatory bowel disease is suspected
• Sexual health screening including HIV and syphilis serology for atypical, multiple or non-healing fissures
• Examination under anaesthetic with biopsy for any fissure with an irregular, indurated or ulcerated appearance
• Flexible sigmoidoscopy or colonoscopy where there are red flags for colorectal cancer, where bleeding is atypical, or where the fissure fails to heal
• Anorectal manometry is not needed routinely, but may be considered before sphincterotomy in patients at higher risk of incontinence, such as multiparous women and those with previous anal surgery

Management

First-line: soften the stool and relieve the pain

Around half of acute fissures heal with these measures alone, and they should continue alongside every other treatment.¹

• Increase dietary fibre to around 30 g daily and increase fluid intake
• A bulk-forming laxative such as ispaghula husk, with an osmotic laxative such as macrogol if needed. The aim is a soft, formed stool passed without straining.
• Warm baths or sitz baths, which relax the internal sphincter and give genuine symptomatic relief
• Topical local anaesthetic such as lidocaine 5% ointment applied before defaecation, for short-term use
• Simple oral analgesia, avoiding codeine and other opioids because of constipation
• Explain the vicious cycle to the patient. Understanding why delaying defaecation makes things worse improves adherence more than any prescription.

Second-line: relax the internal sphincter

For chronic fissures, or acute fissures that have not healed after one to two weeks, add a topical agent that reduces sphincter tone and restores perfusion to the anoderm.

A Cochrane review found that medical therapy is only marginally better than placebo for healing chronic fissures and is substantially less effective than surgery, but it avoids the risk of permanent incontinence, which is why it remains first-line.² Adherence matters: a full six to eight week course applied correctly gives a considerably better chance than a fortnight of intermittent use.

Surgery

• Lateral internal sphincterotomy is the surgical gold standard, dividing the internal sphincter laterally - away from the fissure, at the 3 or 9 o'clock position - to permanently reduce resting pressure. Healing rates exceed 90%, and it is markedly more effective than any medical treatment.³
• The trade-off is continence. Rates of some degree of incontinence, most often to flatus, are reported between 1 and 10% depending on the study and the length of muscle divided. It is a small but permanent risk, and it must be discussed explicitly before consent.
• Anal advancement flap - a flap of healthy perianal skin is mobilised to cover the fissure, avoiding division of the sphincter. It is preferred where the resting pressure is not raised, in women with obstetric sphincter injury, and in patients who cannot accept the incontinence risk.
• Fissurectomy with botulinum toxin is an alternative sphincter-preserving option
• In Crohn disease, avoid sphincterotomy. Healing is poor, the risk of a non-healing wound and fistula is high, and treatment should be directed at the underlying disease with medical therapy and specialist colorectal input.

Complications

• Chronicity and recurrence - the commonest outcome without effective treatment, with symptoms recurring in a substantial minority even after healing
• Anal stenosis, from fibrosis of a longstanding fissure or over-aggressive surgery
• Perianal abscess and fistula formation, where the fissure base becomes infected
• Faecal or flatus incontinence after sphincterotomy - the principal iatrogenic complication, and the reason the operation is not offered first
• Keyhole deformity after posterior midline sphincterotomy, which is why the lateral approach is used
• Chronic pain and avoidance of defaecation, with secondary faecal loading and overflow, and a significant effect on quality of life and mood

Red flags

Prognosis

Roughly half of acute fissures heal within a few weeks with laxatives, fibre and topical measures alone. Of those that become chronic, medical therapy with GTN or diltiazem heals around 50 to 70%, and a meaningful proportion of those recur, particularly if the underlying constipation is not addressed.

Lateral internal sphincterotomy heals over 90% and has a low recurrence rate, which makes the decision to operate a straightforward trade of a high chance of cure against a small chance of permanent minor incontinence. For a young man with normal continence and a high resting pressure, that trade usually favours surgery. For a multiparous woman with an occult obstetric sphincter defect, it often does not, and a flap procedure or botulinum toxin is the better route.

The long-term outcome depends on something less dramatic than any of the above. Fissures recur because stools are hard and patients strain, so continuing the laxative and the fibre after the fissure has healed does more to prevent recurrence than the choice of procedure that healed it.