Examination of the anal canal is important to evaluate several
common patient complaints relating to the anus including itching,
pain, and bleeding. While it is possible to examine parts of this area
with flexible instruments or a rigid rectosigmoidoscope, the only
method that will give a consistent clear view of the anal canal
is anoscopy.1 To properly perform this examination it is
necessary to thoroughly understand the anatomy, be aware of the
possible causes of the symptoms you are evaluating, use the appropriate
equipment, and position the patient correctly.
It is necessary to understand the anatomy of the anal canal in
order to evaluate the patient’s signs and symptoms properly.
The anatomy can be divided into topical anatomy and major supporting structures.2 The
topical anatomy is depicted in Figure 59-1.
The topical anatomy of the anal canal.
Perineal skin covers the perineum, is fully innervated, and includes
both hair follicles and apocrine glands. It can be grossly distinguished
from the anoderm surrounding the anal canal by the visible hair.
The anoderm is specialized squamous epithelium that lines the majority
of the anal canal. It is fully innervated but does not have apocrine
glands or hair follicles. This epithelium is very thin, elastic,
and if destroyed by surgery or infection may relate to stricture
formation during healing.
Looking into the anal canal, the anoderm can be seen to end in
an irregular line called the dentate line. This is a demarcation
of anoderm to transition zone mucosa. Proximal to the dentate line there
is no longer cutaneous sensation. This allows minor therapeutic
procedures like banding or suture ligation to be done without an
anesthetic agent. It is also the reason internal hemorrhoids do not
routinely cause pain. The transition zone continues proximally for
a variable length of 6 to 12 mm before it becomes the rectal mucosa.
The junction of the transitional zone with the rectal mucosa is
not visible to the naked eye. The rectal mucosa decreases in diameter
in the area of the transitional zone. The mucosa appears to be bunched
together in columns called the Columns of Morgagni at the level
of the dentate line. Crypts are formed between the columns as the
transitional zone becomes the dentate line. Under the anoderm in
the crypts are multiple anal glands. Blockage of the anal glands
by foreign material leads to infection. This or primary infection
of the glands causes the majority of abscesses that arise around
the anus. The crypts also are areas to look for foreign bodies such
as fish or chicken bones.
External hemorrhoids are located in the left-lateral, right-posterior,
and right-anterior portions of the distal anal canal and are covered
with anoderm. Their normal position is outside the anal canal and
they can be examined by gently spreading the buttocks. ...