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ANAL CANAL

The anal canal begins at the anorectal junction and ends at the anal verge (Figs 67.43–67.45). It is angulated in relation to the rectum because the pull of the sling-like puborectalis produces the anorectal angle. It lies 2–3 cm in front of and slightly below the tip of the coccyx, which is opposite the apex of the prostate in males. The anal verge is marked by a sharp turn where the squamous epithelium which lines the lower anal canal becomes continuous with the skin of the perineum. The pigmentation of skin around the anal verge approximately corresponds to the extent of the external sphincter. Identification of the anal verge may be difficult, particularly in males in whom the perineum may ‘funnel’ upwards into the lower anal canal, however, the characteristic puckering of the external epithelium caused by the penetrating fibres of the conjoint longitudinal layer makes a useful landmark. The functional anal canal is represented by a zone of high pressure which roughly equates to the anatomical canal. The anal canal consists of an inner epithelial lining, a vascular subepithelium, the internal and external anal sphincters and fibromuscular supporting tissue as well as dense neuronal networks of both autonomic and somatic origin. It is between 2.5 and 5 cm long in adults although the anterior wall is slightly shorter than the posterior. It is usually shorter in females. At rest it forms an oval slit in the anteroposterior plane rather than a circular canal: the arrangement of the external anal sphincter and its attachments to the perineal body and coccyx create sites of maximum pressure within the anal canal in the anterior and posterior midline.

  

Fig. 67.44  A, Mid-coronal MRI endocoil image of the anal canal. B, Anterior coronal MRI endocoil section in a woman showing the transverse perineii (TP) joining the external anal sphincter anteriorly (between arrows). EAS, external anal sphincter; IAS, internal anal sphincter; PR, puborectalis.

  

Fig. 67.45  MRI endocoil mid sagittal view of the anal canal in a man. Cs, corpus spongiosus; Tp, transverse perineii; Eas, external anal sphincter; Lm, longitudinal muscle; Ias, internal anal sphincter; PR, puborectalis; Bs, bulbospongiosus.

Anteriorly, the middle third of the anal canal is attached by dense connective tissue to the perineal body, which separates it from the membranous urethra and penile bulb in males or from the lower vagina in females. Laterally and posteriorly, the anal canal is surrounded by loose adipose tissue within the ischio-anal fossae; this offers a potential pathway for the spread of perianal sepsis from one side to the other (see Ch. 63). Posteriorly, the anal canal is attached to the coccyx by the anococcygeal ligament, a midline fibroelastic structure which may possess some skeletal muscle elements, and which runs between the posterior aspect of the middle portion of the external sphincter and the coccyx. Just above this is the raphe of the levator ani ‘plate’, the fusion of the two halves of iliococcygeus, which merges anteriorly with puborectalis. Between these two structures is a potential ‘postanal’ space.

The ischial spines may be palpated laterally by an examining finger in the upper anal canal. The pudendal nerves pass over the ischial spines at this point and pudendal nerve motor terminal latency may be measured digitally using a modified electrode worn on the examining glove.

LINING OF THE ANAL CANAL

The upper portion of the anal canal is lined by columnar epithelium similar to that of the rectum. It contains secretory and absorptive cells with numerous tubular glands or crypts. The subepithelial tissues are mobile and relatively distensible and possess profuse submucosal arterial and venous plexuses. Terminal branches of the superior rectal vessels pass downwards towards the anal columns. The submucosal veins drain into the submucosal rectal venous plexus and also through the fibres of the upper internal anal sphincter into an intermuscular venous plexus.

There are 6–10 vertical folds, the anal columns, in the mid anal canal. They tend to be obvious in children but less well-defined in adults. Each column contains a terminal radicle of the superior rectal artery and vein: the vessels are largest in the left-lateral, right-posterior and right-anterior quadrants of the wall of the canal where the subepithelial tissues expand into three ‘anal cushions’ (Fig. 67.46). The cushions help to seal the anal canal, to maintain continence to flatus and fluid, and are also important in the pathogenesis of haemorrhoids. The lower ends of the columns may form small crescentic folds, called the anal valves, between which lie small recesses referred to as anal sinuses. The anal valves and sinuses together form the dentate (or pectinate) line. About six anal glands open into small depressions, anal crypts, in the anal valves. The glands are branched and lined by stratified columnar epithelium. Cystic dilatations in the glands may extend through the internal sphincter and even into the external sphincter (Seow-Cheon & Ho 1994).

  

Fig. 67.46  Endoscopic appearance of the anorectal junction.

The mucosa below the dentate line is smooth, and termed the pectin. It is non-keratinized stratified squamous epithelium which lacks sweat and sebaceous glands and hair follicles, but contains numerous somatic nerve endings. It extends down to the intersphincteric groove, a depression at the lower border of the internal sphincter. The canal below the intersphincteric groove is lined by hair-bearing, keratinizing, stratified epithelium which is continuous with the perianal skin. The submucosa in this region contains profuse arterial and venous plexuses and has more connective tissue than the upper canal. It may be tethered to fibres of the conjoint longitudinal layer in the region of the intersphincteric groove.

The junction between the columnar and squamous epithelia is referred to as the anal transition zone (ATZ). It is variable in height and position and often contains islands of squamous epithelium extending up into the columnar mucosa; it is not the same as the dentate line which is an indistinct line observed in the anal canal. Nerve endings including thermoreceptors exist in the submucosa around the upper ATZ; they probably play a role in continence by providing a highly specialized ‘sampling’ mechanism to identify the contents of the lower rectum when the upper anal canal relaxes to allow rectal contents to come into contact with the upper anal canal epithelium (Duthie & Gairns 1960).

The well-defined muscularis mucosae of the rectum continues into the upper canal. Fibres from the longitudinal muscle pass through the internal sphincter and surround the submucosal venous plexus before turning upwards to merge with the muscularis mucosae and form the musculus submucosae ani.

Haemorrhoids

Haemorrhoids represent abnormal enlargement of the anal cushions. The partial drainage of the venous plexus into the intermuscular plane may play a role in this chronic engorgement as a result of obstruction of the venous flow during prolonged straining and defecation. Since the subepithelial vascular cushions of the upper, mid and lower anal canal form a continuous plexus, the differentiation of haemorrhoids into ‘internal’ and ‘external’ is somewhat arbitrary. The laxity of the upper anal canal submucosa is probably responsible for the fact that internal haemorrhoids may form more easily, although engorgement of the plexus deep to the lower anal epithelium may also occur. Since the epithelium in the lower canal is well supplied with sensory nerve endings, acute distension or invasive treatment to haemorrhoids in this area causes profound discomfort, whereas invasive or destructive therapy with relatively few symptoms is possible in the upper canal because the latter is lined with insensate columnar mucosa.

Anal fissures

Acute, transient, vertical breaks in the anal epithelium are not uncommon. The development of chronic, non-healing fissures is less common. They are more likely to develop in the anterior, and particularly the posterior, midline of the anus, possibly because stress within the anal canal is concentrated in these regions by the arrangement of the fibres of the external sphincter: the relative paucity of the arterial supply in these regions probably contributes to poor healing. Persistent hypertonicity of the anal sphincter is a primary pathogenic factor, possibly developing as a consequence of disordered local reflex mechanisms between the lower rectum and internal anal sphincter.

Cryptoglandular sepsis and fistula in ano

Infection of the anal glands is the main cause of anal sepsis and cryptogenic fistula formation (Parks et al 1976). Because many of these glands extend into the intersphincteric plane, so-called cryptoglandular sepsis may readily spread into the same plane. The commonest route of drainage of this sepsis is downwards between the internal and external anal sphincters, appearing beneath the skin at the anal verge. Rupture or drainage of the sepsis at this point will form a fistula in ano in the intersphincteric plane. Less commonly, the infection drains outwards from the intersphincteric plane and passes through the fibres of the external anal sphincter, possibly along the spreading septa of the conjoint longitudinal coat, and appears in the perianal skin outside the external anal sphincter as a transphincteric fistula. If the sepsis does not originate in the intersphincteric plane, drainage is likely to be beneath the anal mucosa alone and a submucous fistula forms. Sepsis within the ischio-anal fossa surrounding the lower anal canal can spread with relative ease because the connective tissue is mostly loose adipose tissue. Pus may spread posteriorly, behind the lower anal canal into the contralateral ischio-anal fossa, because the lower anal canal lacks any ligamentous attachments (Parkes 1961).

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