An anorectal fistula
(Fistula-in-Ano) is an abnormal communication between the anus and the perianal
skin. Anal canal glands situated at the dentate line afford a path for
infecting organisms to reach the intramuscular spaces. Fistulas can occur
spontaneously or secondary to a perianal (or perirectal) abscess.
A fistula-in-ano is an
abnormal hollow tract or cavity that is lined with granulation tissue and that
connects a primary opening inside the anal canal to a secondary opening in the
perianal skin; secondary tracts may be multiple and can extend from the same
primary opening.
Most fistulas are
thought to arise as a result of cryptoglandular infection with resultant
perirectal abscess. The abscess represents the acute inflammatory event,
whereas the fistula is representative of the chronic process. Symptoms
generally affect quality of life significantly, and they range from minor
discomfort and drainage with resultant hygienic problems to sepsis.
References to
fistula-in-ano date to antiquity. The fascination fistula-in-ano has exerted
for more than 2000 years is manifested by the numerous papers and books on the
subject. Hippocrates, in about 430 BCE, made reference to surgical therapy for
fistulous disease, and he was the first person to advocate the use of a seton
(from Latin seta "bristle").
In 1376, the English
surgeon John Arderne (1307-1390) wrote Treatises of Fistula in Ano;
Haemmorhoids, and Clysters, which described fistulotomy and seton use.
Historical references indicate that Louis XIV was treated for an anal fistula
in the 18th century. Salmon established a hospital in London (St. Mark's)
devoted to the treatment of fistula-in-ano and other rectal conditions.
In the late 19th and
early 20th centuries, prominent physician/surgeons, such as Goodsall and Miles,
Milligan and Morgan, Thompson, and Lockhart-Mummery, made substantial
contributions to the treatment of anal fistula. These physicians offered
theories on pathogenesis and classification systems for fistula-in-ano.
Since this early
progress, little has changed in the understanding of the disease process. In
1976, Parks refined the classification system that is still in widespread use.
Over the past few decades, many authors have presented new techniques and case
series in an effort to minimize recurrence rates and incontinence
complications, but despite more than two millennia of experience,
fistula-in-ano remains a perplexing surgical disease.
Treatment of
fistula-in-ano remains challenging. No definitive medical therapy is available
for this condition, though long-term antibiotic prophylaxis and infliximab may
have a role in recurrent fistulas in patients with Crohn disease. Surgery is
the treatment of choice, with the goals of draining infection, eradicating the
fistulous tract, and avoiding persistent or recurrent disease while preserving
anal sphincter function.
For patient education
information, see the Digestive Disorders Center, as well as Anal Abscess,
Rectal Pain, and Rectal B
A thorough understanding of
the pelvic floor and sphincter anatomy is a prerequisite for clearly
understanding the classification system for fistulous disease.
(See the image
below.)
Anatomy of the anal canal and
perianal space.
The external sphincter muscle is a striated muscle under
voluntary control by three components: submucosal, superficial, and deep
muscle. Its deep segment is continuous with the puborectalis and forms the
anorectal ring, which is palpable upon digital examination.
The internal sphincter muscle is a smooth muscle under
autonomic control and is an extension of the circular muscle of the rectum.
In simple cases, the Goodsall rule can help anticipate the
anatomy of a fistula-in-ano. This rule states that fistulas with an external
opening anterior to a plane passing transversely through the center of the anus
will follow a straight radial course to the dentate line. Fistulas with their
openings posterior to this line will follow a curved course to the posterior
midline (see the image below). Exceptions to this rule are external openings
lying more than 3 cm from the anal verge. These almost always originate as a
primary or secondary tract from the posterior midline, consistent with a
previous horseshoe abscess.
Etiology
In the vast majority of cases, fistula-in-ano is caused by a
previous anorectal abscess. Typically, there are eight to 10 anal crypt glands
at the level of the dentate line in the anal canal, arranged circumferentially.
These glands penetrate the internal sphincter and end in the intersphincteric
plane. They provide a path by which infecting organisms can reach the
intramuscular spaces. The cryptoglandular hypothesis states that an infection
begins in the anal canal glands and progresses into the muscular wall of the
anal sphincters to cause an anorectal abscess.
After surgical or spontaneous drainage in the perianal skin,
a granulation tissue–lined tract is occasionally left behind, causing recurrent
symptoms. Multiple series have shown that formation of a fistula tract after
anorectal abscess occurs in 7-40% of cases.
Other fistulas develop secondary to trauma (eg, rectal
foreign bodies), Crohn disease, anal fissures, carcinoma, radiation therapy,
actinomycoses, tuberculosis, and lymphogranuloma venereum secondary to
chlamydial infection.
Epiediology
The true prevalence of fistula-in-ano is unknown. The
incidence of a fistula-in-ano developing from an anal abscess ranges from 26%
to 38%. One study showed that the prevalence of fistula-in-ano is 8.6 cases per
100,000 population. In men, the prevalence is 12.3 cases per 100,000
population, and in women, it is 5.6 cases per 100,000 population. The
male-to-female ratio is 1.8:1. The mean patient age is 38.3 years.
The classification system developed by Parks, Gordon,
and Hardcastle (generally known as the Parks classification) is the
one most commonly used for fistula-in-ano. This system (see the image
below) defines four types of fistula-in-ano that result from cryptoglandular
infections, as follows :
- Intersphincteric
- Transsphincteric
- Suprasphincteric
- Extrasphincteric
Parks classification of
fistula-in-ano.
Aintersphincteric fistula-in-ano
is characterized as follows:
It is the result of a perianal abscess Common course - It begins at the dentate line, then tracks
via the internal sphincter to the intersphincteric space between the internal
and external anal sphincters, and finally terminates in the perianal skin or
perineum
Incidence - 70% of all anal fistulas Other possible tracts - No perineal opening; high blind
tract; high tract to lower rectum or pelvis.
A transsphincteric fistula-in-ano is characterized as
follows:
In its usual variety, this fistula results from an
ischiorectal fossa abscess
Common course - It tracks from the internal opening at the
dentate line via the internal and external anal sphincters into the
ischiorectal fossa and then terminates in the perianal skin or perineum Incidence - 25% of all anal fistulas
Other possible tracts - High tract with perineal opening;
high blind tract
A suprasphincteric fistula-in-ano is characterized as
follows:
It arises from a supralevator abscess
Common course - It passes from the internal opening at the
dentate line to the intersphincteric space, tracks superiorly to above the
puborectalis, and then curves downward lateral to the external anal sphincter
into the ischiorectal fossa and finally to the perianal skin or perineum Incidence - 5% percent of all anal fistulas
Other possible tracts - High blind tract (ie, palpable
through rectal wall above dentate line)
An extrasphincteric fistula-in-ano is characterized as
follows:
It may arise from
foreign body penetration of the rectum with drainage through the levators, from
penetrating injury to the perineum, from Crohn disease or carcinoma or its
treatment, or from pelvic inflammatory disease
Common course - It runs
from the perianal skin via the ischiorectal fossa, tracking upward and through
the levator ani muscles to the rectal wall, completely outside the sphincter
mechanism, with or without a connection to the dentate line Incidence - 1% of all
anal fistulas.
Current procedural terminology codes
classification
Current procedural terminology
coding includes the following:
1. Subcutaneous
2. Submuscular (intersphincteric, low
transsphincteric)
3. Complex, recurrent (high
transsphincteric, suprasphincteric and extrasphincteric, multiple tracts,
recurrent)
4. Second stage
Unlike the current procedural terminology coding, the Parks
and colleagues classification system developed by Parks et al does not include
the subcutaneous fistula. These fistulas are not of cryptoglandular origin but
are usually caused by unhealed anal fissures or anorectal procedures.
History
Patients often provide a reliable history of
previous pain, swelling, and spontaneous or planned surgical drainage of an
anorectal abscess. Signs and symptoms of fistula-in-ano, in order of
prevalence, include the following:
- Perianal discharge
- Pain
- Swelling
- Bleeding
- Diarrhea
- Skin excoriation
- External opening
Important points in the
patient’s history that may suggest a complex fistula include the following:
- Inflammatory bowel disease
- Diverticulitis
- Previous radiation therapy for prostate or rectal cancer
- Tuberculosis
- Steroid therapy
- HIV infection
A review of symptoms may
reveal the following in patients with a fistula-in-ano:
- Abdominal pain
- Weight loss
- Change in bowel habits
Physical examination
No specific laboratory
studies are required in the diagnosis of fistula-in-ano (though the normal
preoperative studies are performed, based on age and comorbidities). Instead,
physical examination findings remain the mainstay of diagnosis.
The examiner should
observe the entire perineum, looking for an external opening that appears as an
open sinus or elevation of granulation tissue. Spontaneous discharge of pus or
blood via the external opening may be apparent or expressible on digital rectal
examination.
Digital rectal
examination (DRE) may reveal a fibrous tract or cord beneath the skin. It
also helps to delineate any further acute inflammation that is not yet drained.
Lateral or posterior induration suggests deep postanal or ischiorectal
extension.
The examiner should
determine the relationship between the anorectal ring and the position of the
tract before the patient is relaxed by anesthesia. The sphincter tone and
voluntary squeeze pressures should be assessed before any surgical intervention
to determine whether preoperative manometry is indicated. Anoscopy is usually
required to identify the internal opening. Proctoscopy is also indicated in the
presence of rectal disease (eg, Crohn disease or other associated conditions).
Most patients cannot tolerate even gentle probing of the fistula tract in the
office, and this should be avoided.
Fistula-in-ano.
Goodsall rule.
Imaging Studies
Radiologic studies are
not performed for routine fistula evaluation, because in most cases, the
anatomy of a fistula-in-ano can be determined in the operating room. However,
such studies can be helpful when the primary opening is difficult to identify
or when recurrent or persistent disease is present. In the case of recurrent or
multiple fistulas, such studies can be used to identify secondary tracts or
missed primary openings. Several imaging diagnostic modalities are
available to evaluate fistula-in-ano. The efficacy of each modality is
reviewed.
Fistulography
This technique involves
injection of contrast via the internal opening, which is followed by
anteroposterior, lateral, and oblique radiographic images to outline the course
of the fistula tract.
Fistulography is
relatively well tolerated but it can be painful when injecting the contrast
material into the fistulous tract. It requires the ability to visualize the
internal opening. Questions have been raised about its accuracy, which has been
reported to range from 16% to 48%.
Because of these
limitations, fistulography is generally reserved for cases in which there is a
concern about a fistulous connection between the rectum and adjacent organs
such as the bladder, where it may be slightly more useful than a careful
examination under anesthesia.
Endoanal or endorectal ultrasonography
Endoanal or endorectal
ultrasonography involves the passage of a 7- or 10-MHz
ultrasound transducer into the anal canal to help define the muscular
anatomy and thereby help differentiate intersphincteric from transsphincteric
lesions. A standard water-filled balloon transducer can facilitate evaluation of
the rectal wall for any suprasphincteric extension.
Investigations have
shown that the addition of hydrogen peroxide via the external opening can aid
in outlining the course of the fistula tract. This may be useful for helping to
identify missed internal openings.
Endoanal/endorectal
ultrasonography has been reported to be 50% better than physical examination
alone in helping to detect an internal opening that is difficult to
localize. This modality has not been used widely for routine clinical fistula evaluation.
Magnetic resonance imaging
Findings on magnetic
resonance imaging (MRI) show 80-90% concordance with operative findings when a
primary tract course and secondary extensions are observed. MRI is becoming the
study of choice for the evaluation of complex fistulas and recurrent fistulas.
It has been shown to reduce recurrence rates by providing information on
otherwise unknown extensions.
Computed tomography
Computed tomography (CT)
is more helpful in the setting of perirectal inflammatory disease than in the
setting of small fistulas because it is better for delineating fluid pockets
that require drainage than for delineating small fistulas. CT requires
administration of oral and rectal contrast. Muscular anatomy is not well
delineated.
Barium enema/small bowel series
These studies may be useful
for patients with multiple fistulas or recurrent disease to help rule out inflammatory
bowel disease.
By Dr. Vikas Gupta
Master Trainer Ano Rectal Surgeon
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