An anal fissure, or fissure-in-ano, is one of the most common
anal disorders seen by physicians. It is a linear tear or crack
that extends into the anoderm from the mucocutaneous junction to
the dentate line (Figure 56-1). An anal fissure usually results
from the passage of hard stool that traumatizes and tears the anoderm.
Frequent bowel movements with diarrhea can cause similar “cracks” that
eventually result in fissures. A fissure may be acute or chronic,
occur at any age, and affect both genders equally. It is the most
common cause of rectal bleeding in infants. Fissures occur primarily
(90 percent) in the posterior midline. The remaining 10 percent
are found in the anterior area. There is a slight gender difference
with 1 to 7 percent of anal fissures found anteriorly in men and
up to 12 percent anteriorly in women.1 Atypical locations
(e.g., lateral) suggest the presence of an underlying disease such
as Crohn’s disease, anal cancer, previous anal surgery,
leukemia, syphilis, tuberculosis, and other infections.
The anal canal begins at the level of the anorectal ring and
extends distally for 4 cm to the anal verge. The internal anal sphincter
and external anal sphincter muscles surround the anal canal. The
internal anal sphincter muscle is a continuation of the involuntary
layer of circular smooth muscle of the rectum that begins at the
anorectal ring. It is contracted at rest so that the lower margin
can be palpated 1 to 2 cm below the dentate line in the intersphincteric
groove. The internal anal sphincter muscle supplies up to 60 percent
of the resting tone of the anus.2 The external anal sphincter
muscle is an elliptical cylinder of voluntary striated muscle tethered to
the coccyx and surrounding the anal canal. Columnar epithelium lines
the upper anal canal while the lower anal canal is lined by squamous
epithelium. The transitional zone lies between the two different
types of mucosa. The anoderm is a thin layer of stratified squamous
epithelium around the anus, distal to the dentate or pectinate line,
that lacks sweat glands and hair follicles. This area is richly
endowed with cutaneous sensory nerve endings.
It is hypothesized that anal fissures usually occur in the posterior
midline secondary to a decreased vascular supply causing ischemia
or a decreased number of external sphincter muscle fibers predisposing
the posterior area to a weakness.3 Constipation or a hard
bowel movement causes a tear in the anoderm that causes pain. Spasm
of the internal anal sphincter muscle occurs, which results in a
tighter anal canal that causes more pain with subsequent bowel movements.
A vicious cycle results and may lead to a chronic anal fissure.
A tight anal sphincter is another hypothesis for the etiology
of a fissure. Anal manometry in patients with fissures reveals an ...