Anal Fissure
Medical Author: Thomas Sokol, MD, FACS, FASCRS
Medical Editor: Jay W. Marks, MD
What are anal fissures?
An anal fissure is a cut or tear occuring in the anus (the opening through
which stool passes out of
the body) that extends upwards into the anal canal. Fissures are a common condition
of the anus and anal canal and are responsible
for 6-15% of the visits to a colonic and rectal
(colorectal) surgeon. They affect men and women equally and both the
young and the old. Fissures
usually cause pain during bowel movements that often is severe. Anal fissure is the most
common cause of rectal bleeding
in
infancy.
Anal fissures occur in the specialized tissue that lines
the anus and anal canal, called anoderm. At a line just inside the
anus--referred to as the anal verge or intersphincteric groove--the skin
(dermis) of the inner buttocks changes to anoderm. Unlike skin, anoderm has no
hairs, sweat glands, or
sebaceous (oil) glands and contains a larger number of somatic sensory nerves
that sense light touch and pain. (The abundance of nerves explains why anal
fissures are so painful.) The hairless, gland-less, extremely sensitive anoderm
continues for the entire length of the anal canal until it meets the demarcating
line for the rectum, called the dentate line. (The rectum is the distal 15 cm of
the colon that lies just above the anus and rectum and just below the sigmoid colon.)
What causes anal fissures?
Anal fissures are caused by trauma to the anus and anal
canal. The cause of the trauma usually is a bowel movement, and many patients
can remember the exact bowel movement during which their pain began. The fissure
may be caused by a hard stool or repeated episodes of diarrhea. Occasionally,
the insertion of a rectal thermometer, enema tip, endoscope, or
ultrasound probe (for examining the
prostate gland) can result in sufficient trauma to produce a fissure. During
childbirth, trauma to the perineum
(the skin between the posterior vagina and the anus) may cause a tear that extends into the anoderm.
The most common location for an anal fissure in both
men and women (90% of all fissures) is the midline posteriorly in the anal
canal, the part of the anus nearest the spine. Fissures are more common
posteriorly because of the configuration of the muscle that surrounds the anus. This
muscle complex, referred to as the external and internal anal sphincters, underlies
and supports the anal canal. The sphincters are oval-shaped and are best
supported at their sides and weakest posteriorly. When tears occur in the
anoderm, therefore, they are more likely to be posterior. In women, there also is
weak support for the anterior anal canal due to the presence of the vagina
anterior to the
anus. For this reason, 10% of fissures in women are anterior, while only 1%
are anterior in men. At the lower end of fissures a tag of skin may form, called
a sentinal pile.
When fissures occur in locations other than the midline
posteriorly or anteriorly, they should raise the suspicion that a problem other
than trauma is the cause. Other causes of fissures are anal cancer, Crohn's
disease, leukemia as well as many infectious diseases including tuberculosis,
viral infections
(cytomegalovirus or
herpes), syphilis, gonorrhea, chlamydia
, chancroid (Hemophilus
ducreyi), and human immunodeficiency virus (HIV). Among patients with Crohn's
disease, 4% will have an anal fissure as the first manifestation of their
Crohn's disease, and half of all patients with Crohn's disease eventually will
develop an anal ulceration that may look like a fissure.
Studies of the anal canal in patients with anal fissures consistently
show that the muscles surrounding the anal canal are contracting too
strongly (they are in spasm), thereby generating a pressure in the canal that
is abnormally high. The two muscles that surround the anal canal are the external
anal sphincter and the internal anal sphincter (already discussed). The
external anal sphincter is a voluntary (striated) muscle, that is, it can be
controlled consciously. Thus, when we need to have a bowel movement we can
either tighten the external sphincter and prevent the bowel movement, or we can
relax it and allow the bowel movement. On the other hand, the internal anal
sphincter is an involuntary (smooth) muscle, that is, a muscle we cannot
control. The internal
sphincter is constantly contracted and normally prevents small amounts of stool
from leaking from the rectum. When a substantial load of stool reaches the
rectum, as it does just prior to a bowel movement, the internal anal sphincter
relaxes automatically to let the stool pass (that is, unless the external anal
sphincter is consciously tightened).
When an anal fissure is present, the internal anal sphincter is
in spasm. In addition, after the sphincter finally does relax to allow a bowel
movement to pass, instead of going back to its resting level of contraction and pressure, the internal
anal sphincter contracts even more vigorously for a few seconds before it goes
back to its elevated resting level of contraction. It is thought that the high
resting pressure and the "overshoot" contraction of the internal anal sphincter
following a bowel movement pull the edges of the fissure apart and prevent the
fissure from healing.
The supply of blood to the anus and anal canal also may
play a role in the poor healing of anal fissures. Anatomic and microscopic
studies of the anal canal on cadavers found that in 85% of individuals that
the posterior part of the anal canal (where most fissures occur) has less blood flowing
to it than the other parts of the anal canal. Moreover, ultrasound
studies that measure the flow of blood showed that the posterior anal canal had
less than half of the blood flow of other parts of the canal. This relatively poor
flow of blood may be a factor in preventing fissures from healing. It also is
possible that the increased pressure in the anal canal due to spasm of the
internal anal sphincter may compress the blood vessels of the anal canal and further reduce the flow of
blood.
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