A fistula is an abnormal communication between two epithelial surfaces.
An epithelial surface is any part of the body that is covered by a special type of
cells called “the epithelium”. Examples of such surfaces are the skin, the mucosa
of the mouth, the mucosa of the bowel, the mucosa of the anal canal, the vagina
A fistula in ano is an abnormal sinus track (or a fine tube), between the skin
(external orifice of the fistula) and either an abscess or the anus / rectum
(internal orifice of the fistula).
There is a relation between a perianal abscess and a fistula in ano: almost
always a fistula develops as a result of an abscess.
As we describe in the chapter of “perianal abscess” any abscess tries to drain
itself automatically to the nearest exit point. This could be the interior of the
anus/rectum or the skin.
Once it is formed, a fistula will stay in place for as long as there is pus that
requires drainage through its track. Therefore a chronic infection that drags
on also perpetuates the related fistula.
This is why the main treatment of the fistula is an attempt to treat the infection or
drain the abscess that feeds it. It is also necessary to eliminate the track by
“laying it open” (fistulotomy). If the fistula track is superficial it can sometimes be
completely excised (fistulectomy).
There are two problems that make the treatment of fistula in ano difficult:
a. The anus is an area that can never be completely clean or sterile.
b. Often we cannot completely lay open or excise the fistula because
it lies too close or goes through the anal sphincter muscles.
In those cases, in order to completely lay open or remove the fistula we may
require to cut the anal sphincter muscle. This creates a risk of incontinence.
When a fistula lies either too close to the sphincter or goes through it then it is
not possible to lay it open or remove it without the risk of incontinence. In those
cases the treatment consists of drainage of any abscess or infection plus
placement of a seton.
A seton a is thread of nylon, prolene, rubber or other material that is non
absorbable and is placed through the fistula track with the purpose of keeping it
open for a certain period of time. It was first described by Hippocrates in ancient
The principle of seton is that no fistula will close permanently if the
“feeding” abscess or infection does not drain completely. After a partial
excision of a fistula the external (skin) orifice has a tendency to close much faster
than the internal orifice. The internal orifice is inside either an abscess cavity or
the anus/rectum (which contains faeces and plenty of bacteria). Thus, early
closure of the external fistula orifice will “trap” infection inside the fistula track and
will result in a recurrent abscess/ infection. By keeping the fistula track patent
and draining for a long time we allow the gradual complete drainage and clearing
of the infection.
A seton may stay in place for a long time: 3 -12 months or more. Although it may
be slightly uncomfortable the first few days, most patients get used to it very
quickly and are not even aware of its presence.
There are two types of seton: the tight or “cutting” seton which was
invented by Hippocrates. The tight seton cuts (“cheese-wires”) very slowly
through the sphincter muscle. Because this process is extremely slow (occurs
over a period of months) the cut muscle is gradually replaced by scar tissue.
Therefore the seton slowly and gradually advances through the muscle,
eliminating on its way the fistula track as well. The seton becomes more and
more superficial and at some point it either completely cuts through and falls off
or is removed by the surgeon.
The loose seton is usually a rubber sling whose purpose is mainly drainage of
the pus but does not cut through the muscle. A variety of ayurvedic setons has
been described – those setons are soaked in chemical caustic substances and
cut through the muscle much faster. There are however some concerns about
those because fast cheese-wiring through the muscle may lead to incontinence.
Diagnosis of Fistula in Ano
Most fistulas have an easily identifiable external orifice which discharges pus or
bloody fluid. However this orifice may close from time to time and on first
examination may be missed. A lump of the skin around the anus often hides an
underlying fistula orifice.
The methods to diagnose and image the fistula are:
- Examination under Anaesthetic (EUA)
- MRI Scan of the perineum
- Endorectal Ultrasound
Surgical Treatment of Fistula in Ano
- Fistulotomy: laying open of the fistula track.
- Fistulectomy: excision of the fistula track
- Advancement flap repair: this is a complex form of surgery, performed
only by specialist colorectal surgeons and it consists of creation of a “flap”
of rectal tissue which is used to cover the internal orifice of the fistula. It is
performed only for persistent high fistulas and it carries a high risk of
After either fistulotomy or fistulectomy there is an open wound left which is
packed for a few days. Nursing care is required for a varied period of time which
depends on the size of the wound and the degree of associated infection.
At some point dressings stop being necessary and the patient can continue
treatment with salt baths.
There are two main complications after any surgery for fistula in ano:
a. Recurrence: in many cases a fistula will recur despite surgery. This is
because, as explained above, infections in this area are difficult to eradicate and
surgery is limited by the risk of incontinence.
Recurrence is common and can sometimes be very frustrating for both patient
and surgeon. It is not unusual for some unfortunate patients to have to undergo
many repeat examinations under anaesthetic (EUA) and fistulotomies. The
placement of seton helps with keeping the local infection under control without
having to do many repeat surgical drainages.
b. Incontinence: Any type of surgery in the anal area can result in
incontinence. This can be mild incontinence (for flatus only) or more severe
(incontinence for stools). Incontinence is the result of either surgical damage or
severe infection which destroys the sphincter.
The risk of incontinence has to be discussed with all patients prior to surgery.
This risk is usually quite low (around 2-5%) but in case of complex high fistulas
can be higher. Also the “flap advancement” operation may have a much higher
risk of incontinence (around 20%).
Colostomy is sometimes necessary as a temporary measure if the infection
does not settle. A colostomy for a few months diverts the flow of the faeces from
the anus and allows for the sepsis to be treated. It can then be closed. Very few
patients with fistula will need a colostomy.
Fibrin glue is a new treatment that has been useful in some cases. A biological
glue is prepared either from the patient’s blood or from bovine blood (the latter is
commercially available). Fibrin glue contains natural blood clotting products and
when applied it forms a plug that seals off the fistula. Although some satisfactory
results have been reported, its usefulness is limited by infection. In the presence
of infection the glue fails to close the fistula.
Factors predisposing to fistula in ano are Crohn’s disease, radiotherapy,
cancer etc., however in most cases no specific cause is found.
The life with a fistula can be normal if sepsis/infection is avoided. A patient can
wear the seton for many months or a year or two without severe symptoms apart
from occasional small discharge. Hot salt baths help to soothen and irrigate the
Although surgery is absolutely necessary at some point for every fistula, multiple
operations without a specific aim, such as draining sepsis or laying open the
complete fistula, do no always help and can lead to incontinence.
An MRI of the perineum-anus should be performed whenever new symptoms
or serious flare-ups occur in order to identify new abscess cavities.
Most fistulas eventually settle, however they may take a long time until
they do so!
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Fistula in Ano