Anorectal disorders range from simple to complex, may be varied
and multiple, and, at times, can manifest signs and symptoms of
underlying serious local or systemic disorders that may be life-threatening.
It may be difficult to determine a precise cause for anorectal disorders.
Pertinent history and a careful examination can narrow the differential
diagnosis and aid timely and appropriate management.
The rectum is an anatomic structure that begins at the S3 vertebral
body and descends for about 13 to 15 cm. This entodermal intestine
unites with and opens into an orifice of ectodermal origin, the anal
canal, which is about 4 cm long. (Figure 88-1).
Midsagittal section of the anorectum.
The junction of these two embryonic structures is the dentate
line, which marks the anatomic beginning of the anal canal and is
in continuity more distally with the perianal skin at the anal verge.
The mucosa of the anal canal consists of stratified squamous epithelium
but contains no hair follicles or sweat glands. At the anal verge,
the anoderm thickens and includes in its structure hair follicles
and other cutaneous appendages. Proximal to the dentate line, the
rectal ampulla narrows to conform to the opening of the anal canal.
In doing so its mucosa takes
on a pleated appearance, forming 8 to 14 convoluted longitudinal folds:
the columns of Morgagni. Each adjacent column is connected at the
dentate line by a flap of mucosa that forms a small anal crypt,
normally 1 to 3 mm deep. Anal sepsis, cryptitis, perianal abscesses,
and fistulas result from inflammation, obstruction, and infection
of the crypts and glands. The anal wall separates the internal from
the external sphincters and is a continuation of the usual layers
of the wall of the colon and rectum. The innermost lining, the mucosa,
continues to the anal verge. Just proximal to the dentate line,
the mucosa transitions from rectal columnar to cuboidal to squamous
epithelium. The submucosa, which normally contains the bulk of the
bowel’s blood vessels and autonomic nerves, thickens considerably
proximal to the dentate line. The hemorrhoidal arteries supply the
anorectum, whereas the venous network in this area is the internal
hemorrhoidal plexus. The superior (internal) hemorrhoidal veins
drain into the portal system, whereas the inferior (external) hemorrhoidal
veins drain into the inferior vena cava. The inner circular muscle
layer of the rectum thickens considerably as it terminates distally
in the anorectum to form the involuntary internal sphincter muscles.
The more attenuated longitudinal muscles of the rectum extend caudally,
blending with fibers of voluntary skeletal muscles from the levator
ani and external sphincter groups, to form the intersphincteric
space (Figure 88-2).
Coronal section of the anorectum.
The external sphincters, voluntary skeletal muscles, are actually
a caudal ...