Anorectal Abscesses and Fistulas

An Anal Fistula is a recurrent infection affecting the anus and occasionally the lower rectum. It causes considerable discomfort and disability to the sufferer of this chronic condition.

In Singapore, this condition is also known as the 'rat hole' (LaoShudong) amongst the Chinese community and traditional Chinese medical practitioners. It commonly affects economically active adults in the 3rd to 5th decades of life.

Although Anal Fistulas have been recognised for thousands of years, the treatment of the complex or recurrent Anal Fistula continues to challenge surgeons till today.

A fistula is a medical term describing an abnormal tract with two openings (internal and external) communicating between any two epithelium lined surfaces.

Therefore, an Anal Fistula is an abnormal tract communicating between the perianal area to the anal canal, and occasionally to the lower rectum.

Anal Fistula

Anal Fistulas and anal abscesses are separate phases along the spectrum of a common infective condition affecting the anus. The abscess represents the acute stage whereas the fistula is the representative of the chronic phase.

The anal canal is a muscular tube approximately 3 to 4 cm long, connecting the rectum to the anal verge. It is made up of 2 cylindrical muscle tubes called the outer and inner anal sphincter muscles. Anal glands are found in-between these two muscle tubes at the level of the dentate line (approximately 2 cm from the anal verge).

Anal Abscess Fig. 1

Anal Abscess Fig. 2

The role of these anal glands in humans is unknown.

It is believed that the duct from the anal glands is blocked resulting in stasis, infection and subsequently abscess formation. Persistence of the infected anal gland and its duct results in the formation of an Anal Fistula.

As the duct from the infected anal gland leading into the anal canal is blocked, the accumulated pus may spread in other directions. It usually follows the path of least resistance. The path created will determine the type of Anal Fistula formed.

90% of all anal abscesses result from non-specific infection of the anal glands. We do not know why certain individuals are prone to this infection but approximately 25% to 30% of these patients with anal abscesses will eventually end up with an Anal Fistula.

Only people with susceptible anal glands can develop anal abscess and eventually Anal Fistula. The formation of Anal Fistulas is totally unpredictable. The only way to prevent the disease is to reduce the incidence of an infection of the anal glands, which is the precursor to an Anal Fistula. Good perianal hygiene is an important step to prevent this uncomfortable condition.

As stated earlier, anal fistulas and anal abscesses are two different cases in the spectrum of a common infective condition affecting the anus. Therefore, a patient with an anal fistula will often recount a history of an anal abscess that has been drained either surgically or spontaneously.

Usually, patients complain of intermittent anal pain and anal swelling followed by discharge and resolution of these symptoms. There is occasionally a low grade fever during these episodes. Patients may also notice a small lump in the perianal region with occasional bleeding associated with defecation.

Additional bowel symptoms may be present when the Anal Fistula is associated with other conditions (e.g. proctocolitis, Crohn's disease, cancer, tuberculosis, actinomycosis).

The general principles of fistula management are to:

• Eliminate the fistula
• Prevent recurrence
• Preserve anal sphincter function

Surgery is the mainstay of management of Anal Fistula. The role of medical management is mainly in the usage of antibiotics to treat recurrent low grade infections associated with the Anal Fistula especially in patients with immunocompromised medical conditions (e.g. Diabetes Mellitus).

There are several surgical methods used in the eradication of the fistula tract and the method selected depends on the type of fistula tract present.

• Lay-Open Technique - This is the most common procedure for simple anal fistula (low intersphincteric or transphincteric fistula). The tract is incised to deroof it and to allow adequate drainage for secondary healing to take place.

Lay Open Technique

• Seton - A seton is a foreign material inserted into a fistula tract to encircle the anal sphincter muscles. It is used to treat high anal fistulas where simple lay-open methods might render the patient incontinent.
• Fistulectomy - The fistula tract is totally excised up to the anal sphincter muscle complex, leaving the wound for secondary healing.

Seton Insertion

• Anorectal Advancement Flap - This is a more complicated technique used when the conservative laying-open method is not appropriate e.g. in anterior fistulas in women, high complex fistulas and previous multiple anal sphincter surgeries. The internal opening is excised and then closed with a full-thickness flap of rectal mucosa.

Anorectal Advancement Flap

• Fibrin Glue - This is a non-invasive approach where the fistula tract is closed using fibrin glue.
• Bioprosthetic Fistula Plug - This is a relatively new technique using a bioprosthetic plug to plug the fistula tract and close it. However, results are equivocal and costs are high.

After surgery, patients are advised to take a high-fibre diet with plenty of fluid. They are prescribed stool bulking agents, stool softener and analgesia. Patients are also instructed to take frequent sitz baths to ensure perianal hygiene. Daily wound dressing at nearby clinics is usually required in the first week after surgery.