Anatomy of the Anal Canal

Anal Abscesses and Fistulae

• An anal abscess is a collection of pus in the tissues surrounding the anus, which can point into the anal canal, perianal skin, or ischiorectal space.
• Perianal sweat and sebaceous glands or an infected perianal haematoma can initiate a superficial perianal abscess.
• Pelvirectal abscess is not of anal origin but is due to an intra-abdominal pelvic abscess collection, e.g. due to appendicitis, diverticulitis, or gynaecological sepsis.
• A small proportion of anal sepsis cases are associated with Crohn's disease.

Clinical Features of Anal Abscesses

• Pain in the anus begins slowly and builds up over 24 to 48 hours, becoming severe, especially on defecation.
• Constitutional upset is not great in the case of superficial abscesses, but a mild fever may develop.
• With deeper abscesses, such as ischiorectal or pelvirectal, a high fever, malaise, tachycardia, and leucocytosis may be present.

Treatment of Anal Abscesses

• Simple abscesses require division of the membrane at the base of the anal pit.
• More complicated abnormalities with fistula formation require repair at a later stage.
• Absence of the rectum should be treated by defunctioning colostomy when this is discovered, and a definitive repair made when the child is older.

Injuries to the Rectum

• The rectum may be injured by objects introduced by accident or deliberately, during an enema or sigmoidoscopy, or by perineal tear during childbirth.
• High-speed trauma, as in road accidents, with multiple fractures of the pelvis may injure the rectum as well as the bladder and urethra.

Clinical Features of Rectal Injuries

• An intraperitoneal perforation will cause leakage of air and faeces into the peritoneal cavity, leading to lower abdominal pain and peritonitis.
• A lower perforation or laceration should be examined under anaesthetic, debrided, and the sphincter tested.
• The history of discharge followed by healing and then further discharge is typical and may have occurred for many months or years prior to presentation.

Treatment of Rectal Injuries

• A thorough examination including sigmoidoscopy and proctoscopy should be performed under anaesthetic prior to any treatment.
• An anaesthetic which preserves the tone of the anal muscles is preferred.
• The anal canal should be palpated and then inspected to find an internal opening, bearing in mind Goodsall's rule.

Anorectal Fistulae

• An anorectal fistula is a rare condition that communicates between the rectum and the skin through the levator ani and above the anorectal ring.
• Treatment of anorectal fistula is to core out the fistula track from below through the levator ani.
• The anorectal ring cannot be cut without incontinence resulting.
• Sometimes a colostomy is required while the healing takes place.

Other Conditions

• Prolapse of the rectum may be mucosal only or full-thickness rectal prolapse.
• Clinical features include bleeding, mucous discharge, a prolapsing swelling, and incontinence of faeces.
• Treatment of minor degrees of mucosal prolapse may involve injections of haemorrhoids or excision of the mucosa.

Anatomy of the Rectum and Anus

• The internal sphincter is a thickening of the circular smooth muscle of the bowel alongside the anal canal.
• The external sphincter lies outside the longitudinal muscle and is composed of striated muscle.
• The anal canal is lined in its upper half by columnar epithelium, and then by intermediate cuboidal epithelium and squamous epithelium with sebaceous and sweat glands.
• Anal glands are present at the pectinate line.
• The blood supply to the rectum is via the superior rectal artery, middle rectal arteries, and inferior rectal arteries.
• The mucosa of the lower rectum and upper anal canal contains a rich plexus of veins.

Test your knowledge on the anatomy of the anal canal including the internal and external sphincters, epithelial lining, and anal glands.