Anal musculature and innervation

The anal canal is 3–4 cm long in males and slightly shorter in females. It consists of two concentric muscle layers known as the internal and external sphincters (Fig. 17.1). The internal sphincter is a condensation of the circular smooth muscle of the rectum and is a continuation of the circular muscle of the gastrointestinal tract. It is controlled by the autonomic nervous system with fibres from the pelvic sympathetic nerves, the lower lumbar ganglia and the pre-aortic/inferior mesenteric plexus. Parasympathetic fibres arise from the sacral plexus. The smooth muscle of the internal sphincter maintains tone and contributes to resting pressure within the anal canal, so playing an important role in maintaining continence. The longitudinal muscle of the gut ends at the anus as a series of fibrous bands that radiate to the perianal skin, and is of little consequence to perianal disease. The striated muscle of the external sphincter is under voluntary control, being innervated bilaterally by the internal pudendal nerves and the fourth branch of the sacral plexus. The circular muscle tube of the external sphincter blends with the lower part of the levator ani, known as the puborectalis sling (Fig. 17.2). The puborectalis fibres of the levator ani originate from the posterior aspect of the pubic symphysis and pass posteriorly to join the external sphincter. The levator ani muscles themselves are also important in maintaining the relationship of the anus and rectum during defaecation.

Fig. 17.1 Musculature of the anorectum.

Fig. 17.2 The puborectalis sling establishing the anorectal angle.

Anal canal epithelium

The cell type of the anal canal epithelium determines why certain diseases, such as tumours and viral infections, affect only particular levels of the canal. The epithelium of the anal canal is specialized and contains three distinct zones. The external zone (from dentate line to anal verge) is keratinized, stratified squamous epithelium. There is a short, modified, anal transitional zone of non-keratinized squamous epithelium, which lies immediately proximal to the dentate line, separated from the columnar epithelial of the anal canal but continuous with the rectal epithelium. The anal valves are crescentic mucosal folds that form a serrated or dentate line on the luminal aspect of the mid-anal canal (Fig. 17.3). The dentate line represents the line of fusion between the endoderm of the embryonic hindgut and the ectoderm of the anal pit. Thus, the epithelium is innervated by the autonomic nervous system and is insensate with respect to somatic sensation. The canal lining below the dentate line is innervated by the peripheral nervous system and so conditions affecting this region, such as abscess, anal fissure or tumour, result in anal pain.

Fig. 17.3 The lining of the anorectal canal.

The composition of the epithelium of the anorectum determines the type of tumour that affects the region. Thus, squamous cell carcinoma of the anal canal arises from the epithelium below the dentate line or in the transitional zone of non-keratinized squamous epithelium. Because the canal above the anal transition zone contains columnar glandular epithelium, tumours of the upper anal canal are adenocarcinoma; they are best considered as a low rectal cancer and treated accordingly.

There are 4–8 specialized anal glands located within the substance of the internal sphincter or in the space between the internal and external sphincters at the level of the mid-anal canal; these glands have ducts that open directly on to the dentate line (Fig. 17.4). They are involved in the aetiology of perianal abscess and fistula-in-ano. The function of the anal glands is to secrete mucus, lubricating and protecting the delicate epithelium of the anal transition zone. The ducts from these glands open into the folds of mucosa at the dentate line. The relevance of these glands lies in the fact that they are the source of most perianal abscesses. When an anal gland duct becomes occluded, the obstructed gland may become infected with gut organisms such as coliforms, and anaerobic bacteria such as Bacteroides.

Fig. 17.4 Principal anorectal spaces in relation to the anal sphincters and rectum.

The anal (haemorrhoidal) cushions

Although the internal and external sphincters, the pubo- rectalis sling and the anorectal angle play important roles in maintaining anal continence, fine control is aided by the anal ‘cushions’ that lie in the submucosa within the anal canal, above the dentate line. The anal cushions are specialized vascular structures comprised of fibroconnective tissue containing arteriovenous communications, fed by the terminal branches of the superior rectal artery with inconstant anastomoses to the middle and inferior rectal arteries. There are usually three anal cushions because there are three terminal branches of the artery (left, right posterior and right anterior, corresponding with the 3, 7 and 11 o’clock positions when the patient is in the lithotomy position). These positions determine the position of haemorrhoids, which are caused by distension and prolapse of the anal cushions. Haemorrhoids are not ‘varicose veins’ of the anal canal, but prolapse of the specialized anal cushions; indeed, haemorrhoids are uncommon in patients with portal hypertension, despite the fact that the anal canal represents a potential portosystemic anastomosis.

Lymphatic drainage of the anal canal

Lymphatic drainage of the anus below the dentate line is to the inguinal lymph nodes. This contrasts with the lower rectum where lymphatic drainage passes superiorly through the mesorectum to follow the superior rectal artery and on to the inferior mesenteric and aortic chains. There are also some lymphatic channels that follow the course of the middle rectal arteries to drain to the nodes around the internal iliac arteries. This anatomical distinction between the lymphatic drainage of the anus and the rectum has important implications for the management of tumours of the rectum and anus. Anal cancer frequently metastasizes to inguinal lymph nodes, whereas rectal cancer metastasizes upwards to the mesorectum and onwards to the para-aortic chain. Thus, radiotherapy fields for anal squamous cancer normally incorporate the inguinal nodes.

Haemorrhoids

Despite haemorrhoids (colloquially known as piles) being very common, the aetiology remains obscure. Almost all haemorrhoids are primary, with only a tiny proportion due to other factors, such as a cancer in the distal rectum. Haemorrhoids are enlarged, prolapsed anal cushions and the pathophysiology involves degeneration of the supporting fibroelastic tissue and smooth muscle, with enlargement and protrusion of the cushions at the 3, 7 and 11 o’clock position. As the cushions prolapse, there is keratinization and hypertrophy of the overlying anal transitional zone and eventually prolapse of the columnar epithelial component in advanced stages. However, the underlying cause of the stretching of the fibroelastic support is unknown. Constipation and straining at stool are common features. These may be aggravated by a high anal sphincter pressure, with further entrapment of prolapsed piles. Haemorrhoids during pregnancy are very common and are probably due to hormonal effects inducing connective tissue laxity, combined with constipation and pressure from the baby’s head. Sitting on the toilet for long periods, such as when reading, is also held to be an associated aetiological factor. However, as with other putative aetiological factors, there is no real evidence for cause and effect.

Clinical features

Bleeding and prolapse are the cardinal features and may occur in isolation or together. The bleeding is typically intermittent ‘outlet-type’ bleeding, separate from the stool and evident in the pan or only on wiping. There may also be aching or dragging discomfort on defaecation, and patients may self-reduce their piles to obtain relief after each bowel motion. Severe constant pain is unusual and in such cases other pathology should be suspected. In the later stages, haemorrhoids remain prolapsed at all times and there is staining of the underwear with mucus and faecal fluid. However, it is very unusual for patients to present with incontinence of solid faeces and a sphincter defect should be suspected in such cases. In cases of constant prolapse, there is often pruritus due to the discharge, with irritation of the perianal skin.

Haemorrhoids can be staged according to the degree of prolapse, but it is important to note that this classification does not necessarily relate to the amount of trouble that symptoms cause the patient:

• First-degree piles are those that bleed, are visible on proctoscopy but do not prolapse

• Second-degree piles are those that prolapse during defaecation but reduce spontaneously

• Third-degree piles are prolapsed constantly but can be reduced manually (Fig. 17.5)

• Fourth-degree piles are irreducibly prolapsed.

Fig. 17.5 Third-degree haemorrhoids.

Patients may present as an emergency with a complication of haemorrhoids, such as thrombosed prolapsed piles or torrential haemorrhage. Prolapsing haemorrhoids may acutely thrombose and there is associated marked sphincter spasm. The thrombosed piles are large, swollen, irreducible haemorrhoids, which are dark blue or even black owing to necrosis and submucosal haemorrhage. They are acutely painful and tender and the diagnosis is easily made on inspection, but a rectal examination will be impossible because of pain. Major haemorrhage, resulting in significant hypovolaemia and anaemia, is unusual but should be excluded in any patient presenting with a major fresh rectal bleed.

Management

In many cases, reassurance after appropriate evaluation is all that many patients require. Specific treatment is not required for most cases, as symptoms are minor and intermittent. A high-fibre diet with plenty of vegetables is commonly recommended, although there is no good evidence that this actually provides any benefit at all. However, if constipation is a feature, it does seem reasonable advice; in some cases, bulk laxatives or stool softeners may be indicated. Patients often self-medicate with proprietary ointments and creams. There is no good evidence from controlled trials that these are effective, but if patients find that they help, then it seems reasonable to advise their intermittent use.

Non-operative approaches

There are many non-operative approaches to the treatment of haemorrhoids, the aim of which is to cause fibrosis and shrinkage of the protruding haemorrhoidal cushion in order to prevent bleeding and prolapse. Current outpatient clinic treatment approaches include application of small rubber bands to strangulate the pile (using a special Barron’s bander); submucosal injection of sclerosant; and the application of heat by infrared photocoagulation. There is no strong evidence that any of these approaches is much better than doing nothing at all. In the long term, the symptoms of untreated piles tend to wax and wane, and the recurrence of symptoms after any of these procedures is much the same as without any treatment. However, of all the non-operative treatments, rubber band ligation (Fig. 17.6) may be the most effective in the short term. Where there is a significant cutaneous component to the piles, any of the outpatient treatments is likely to be painful because of the cutaneous nerve supply, and is also unlikely to succeed. In these circumstances, the decision should be to do nothing but reassure the patient, or to offer an operation.

Fig. 17.6 Application of Barron’s rubber band to haemorrhoids.

Operative approaches

The principle of haemorrhoidectomy involves total removal of the haemorrhoidal mass and securing of haemostasis of the feeding vessel. The wound can be left open or can be closed, but there are rarely problems with healing or infection. In some cases, there are secondary haemorrhoids between the main right anterior, right posterior and left lateral positions, and these are also removed as part of the operation. Recently, a different surgical approach using a circular stapler has been developed, the stapled anopexy. This technique aims to divide the mucosa and haemorrhoidal cushions above the dentate line in order to transect the feeding vessels and hitch up the stretched supporting fibroelastic tissue, rather than removing the whole haemorrhoidal mass as in the standard haemorrhoidectomy. Stapled haemorrhoidectomy may have a place for the treatment of symptomatic first- and second-degree piles (EBM 17.1). With all surgical approaches to treating

17.1 Haemorrhoids

‘Non-operative treatment is preferable wherever possible but surgery may be required for a small proportion of cases. Open haemorrhoidectomy is superior to stapled haemorrhoidectomy both in terms of symptom control and recurrence; rubber band ligation has similar efficacy to haemorrhoidectomy.’

Acheson AG, Scholefield JH. Management of haemorrhoids. BMJ. 2008 Feb 16;336(7640):380-3. http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2244760

Lumb KJ, Colquhoun PH.D., Malthaner R, Jayaraman S. Stapled versus conventional surgery for hemorrhoids. Cochrane Database of Systematic Reviews 2006, Issue 4. Art. No.: CD005393. DOI: 10.1002/14651858.CD005393.pub2.

Shanmugam V, Hakeem A, Campbell KL, Rabindranath KS, Steele RJC, Thaha MA, Loudon MA.Rubber band ligation versus excisional haemorrhoidectomy for haemorrhoids. Cochrane Database of Systematic Reviews 2005, Issue 3. Art. No.: CD005034. DOI: 10.1002/14651858.CD005034.pub2.

Summary Box 17.3 Haemorrhoids

Haemorrhoids are common and best treated conservatively.

Classification:

• First-degree: visible in the lumen on proctoscopy but do not prolapse

• Second-degree: prolapse on defaecation but return spontaneously

• Third-degree: remain prolapsed but can be replaced digitally

• Fourth-degree: long-standing prolapse and cannot be replaced in the anal canal.

Symptoms:

• Outlet-type bleeding, prolapse, mucus discharge, discomfort and thrombosis.

Treatment:

• First-degree: advice on avoiding constipation and straining

• Second-degree: conservative management, banding, injection sclerotherapy, haemorrhoidectomy

• Third-degree: if symptomatic, haemorrhoidectomy

• Fourth-degree: thrombosed piles are usually treated conservatively in the first instance; interval haemorrhoidectomy may not be required.

piles, it is important to consider that the haemorrhoidal cushions contribute to fine control of continence. Hence, an element of anal incontinence can be one of the long-term sequelae of any haemorrhoidectomy. Surgery should not be considered lightly.

Fissure-in-ano

Fissure-in-ano is a common condition characterized by a linear anal ulcer, often with the internal sphincter visible in the base, affecting the anal canal below the dentate line from the anal transition zone to the anal verge (Fig. 17.7). There is often little in the way of granulation tissue in the ulcer base. Owing to failed attempts at healing, there may be a tag of skin at the lowermost extent of the fissure, known as a ‘sentinel pile’. At the proximal extent of the fissure there may be a hypertrophied anal papilla. Sometimes fissures will heal incompletely and mucosa will bridge the edges of the fissure. This results in a low perianal fistula and may present years later. Fissures are most frequently observed in the posterior midline of the anal canal, although anterior fissures may occur in women following childbirth; they are rarely seen in males.

Fig. 17.7 Anal fissure.

A The fissure (arrow) comprises a linear ulcer at the typical 6 o’clock position (arrow). B Explanatory diagram.

The condition most commonly affects people in their twenties and thirties, with a slight male preponderance. Most fissures are idiopathic, but it is clear that the pathophysiology involves ischaemia in the base of the ulcer, associated with marked anal spasm and a significantly raised resting anal pressure. Successive bowel motions provoke further trauma, pain and anal spasm, resulting in a vicious circle of anal pain and sphincter spasm that causes further trauma to the anal mucosa during defaecation. Fissures may be acute and settle spontaneously, but chronic anal fissure is defined as an ulcer that has been present for at least 6 weeks. Recurrent multiple or unusually extensive fissures affecting areas other than the midline should raise the suspicion of Crohn’s disease, which can occasionally present with anal fissure as the sole initial complaint. Occasionally, anal fissure may be associated with ulcerative colitis. Fissure is an uncommon complication of haemorrhoidectomy and results from a non-healing wound combined with anal spasm.

Fissure-in-ano is one of the most common causes of constipation in infants and children. The pain associated with the fissure leads to a pattern of behaviour in which the child tries to avoid defaecation. This results in stool retention and rectal stool bolus formation. The rectum becomes overdistended and the child becomes unaware of the need to pass stool. Overflow incontinence and soiling result.