The anatomy of the anus and rectum dictates the clinical evaluation and treatment of patients with anorectal disorders (Figure 33–1).
Anatomy of the anorectal canal.
From external to internal, the surface anatomy of the anorectum is composed of gluteal skin, anoderm, the anal transitional zone, and proximally the rectal mucosa. The gluteal skin includes hair, sebaceous glands, and sweat glands. This area, particularly 3-5 cm of anal margin skin circumferentially around the anus, can commonly become infected with the human papillomavirus, resulting in anal condyloma. Anal condyloma can also affect more proximal tissue in the anoderm and lower rectal mucosa. Perianal hidradenitis is also relatively common and develops in the apocrine sweat glands of the perianal skin. Unlike anal condyloma, perianal hidradenitis can only occur in the gluteal skin because there are no sweat glands in the anoderm.
The anoderm begins at the anal verge and ends at the dentate line. Unlike the gluteal skin, the anoderm is devoid of hair and sweat glands. Anal fissures occur in the anoderm and can be associated with a sentinel tag externally and a hypertrophied anal papilla internally (Figure 33-2). Surgical excision of too much anoderm during hemorrhoidectomy or other anorectal surgery can result in anal stenosis.
Diagram of the anorectum showing the fissure or ulcer triad.
The anal transitional zone lies between the squamous anoderm and the rectal mucosa. In this zone, squamous, cuboidal, transitional, and columnar epithelium exist with longitudinal ridges called the columns of Morgagni. Between the columns of Morgagni are anal crypts with associated anal glands that open into their bases. Clinically, the anal transitional zone is important for two main reasons. First, the anal transitional zone is the crossover from somatic to visceral innervation and for lymphatic drainage from the inguinal to the pelvic nodes. Lymphatics from the anal canal above the dentate line drain via the superior rectal lymphatics to the inferior mesenteric lymph nodes and laterally to the internal iliac nodes. Below the dentate line, drainage occurs to the inguinal lymph nodes but can occur to the inferior or superior rectal lymph nodes. Second, the anal glands in the crypts are the site of anorectal abscesses and anal fistulas. Anatomically the anal glands in the crypts extend to a variable depth, resulting in perianal, intersphincteric, or ischiorectal abscesses when these glands become blocked.
Proximal to the anal transitional zone is the rectal mucosa. Above the dentate line and underlying the rectal mucosa are the vessels that, when abnormally engorged, manifest as internal hemorrhoids. Hemorrhoids are not veins but arteriovenous connections that have pulsatile flow. Patients with bleeding from hemorrhoids can have significant blood loss. Further, internal hemorrhoids ...