Fistula is defined as a track lined by granulation tissue between two epithelial surfaces. In cases of fistula in ano, it is a track that connects deeply the anal canal or rectum to the skin around the anus. The primary opening is deep in the anal canal or rectum and the superficial openings, which may be multiple, are around the perianal skin.
The incidence is 8.6 per hundred thousand population, with male to female ratio of 1.8:1. The mean age of presentation is around 38 to 40 years.
The most common presentation of fistula in ano is a purulent discharge around the anus and from within the anal canal, associated with impaired anal hygiene and soiling. The other presenting symptoms may be pain, swelling, bleeding and skin excoriation etcetera.
The patient usually gives a past history of anorectal abscess that has either discharged spontaneously or has required formal surgical incision and drainage.
Clinical examination in most cases shows an external opening in the perianal skin, but there may not be any obvious external opening, particularly in patients with inter-sphincteric fistulae. Digital rectal examination may reveal a track or cord beneath the skin and it may be possible to express a discharge.
Most fistulae in ano arise secondary to anal gland infection. About six to eight anal glands lie in the inter-sphincteric space and their ducts enter the anal canal to discharge at the dentate line. Infection in these glands leads to perianal or ischiorectal abscess. Drainage of these abscesses may leave a potential communication between the anal canal and skin which, if it persists, leads to a fistula in ano.
Other causes of fistula in ano are secondary to other diseases and are as follows:
l Inflammatory bowel disease, commonly Crohn’s disease, but 7 per cent of patients with ulcerative colitis develop perianal fistulas;
l Sexually transmitted diseases;
l Steroid use;
l Anal canal neoplasm;
l Foreign body around the anal canal;
l Radiotherapy for prostate or rectal cancer;
The most widely used classification is Park’s classification, based on the relationship between the fistula and the external and internal sphincter muscle. This classification describes the following types of fistulae:
l Inter-sphincteric fistula – these account for about 50 per cent to 65 per cent of all fistulae. An inter-sphincteric fistula passes via the internal sphincter into the inter-sphincteric space and then to the perineum;
l Trans-sphincteric fistula – these account for 20 per cent to 25 per cent of all fistulae. A trans-sphincteric fistula passes between internal and external sphincters into the ischiorectal fossa and then to the perineum;
l Supra-sphincteric fistula – these account for 3 per cent to 5 per cent of all fistulae. A supra-sphincteric fistula passes via the inter-sphincteric space to above the puborectalis muscle in the ischiorectal rectal fossa and then to the perineum;
l Extra-sphinteric fistula – these account for 1 per cent to 3 per cent of all fistulae. An extra-sphinteric fistula goes from perineal skin through the levator-ani muscle to the rectal wall, bypassing the sphincter mechanism.
Fistulae can also be classified as low fistula and high fistula, depending on the level of the internal opening below or above the anorectal ring.
In simple cases, Goodsall’s rule states that a fistula with an external opening in relation to the anterior half of the anus tends to follow a straight course to the dentate line, whereas a fistula with an external opening in relation to the posterior half of the anus will follow a curved course and may be of the horseshoe type, which is much more common and enters in the midline.
Accurate diagnosis of the complexity, extent and site of the fistula is vital for adequate and successful treatment. The following diagnostic procedures and investigations are needed for appropriate assessment of patients with fistula in ano:
l Digital rectal examination (DRE) – careful anal inspection and DRE is needed to identify the areas of induration and external opening which may discharge pus during rectal examination;
l Proctoscopy – to identify the internal opening and any associated anal papillae and skin tags;
l Rigid sigmoidoscopy – to rule out primary colorectal disorders like Crohn’s disease;
l Examination under anaesthesia – an examination of the perineum, digital rectal examination and anoscopy are performed. Several techniques have been described to help locate the course of the fistula and, more importantly, identify the internal opening. Inject hydrogen peroxide, povidone-iodine, milk or dilute methylene blue into the external opening and watch for egress at the dentate line. Gentle probing, often a combination of external and internal probing, is necessary to identify the tract;
l Anorectal ultrasound – this can be helpful in outlining the course of intersphincteric fistulae but the 7-MHz probe is not deep enough to image beyond the external sphincter or puborectalis, thus it is not very reliable in delineating complex fistulae or supra-shincteric and extra-sphinteric fistulae and is rarely very satisfactory in most trans-sphincteric fistulae;
l CT scan – this is more helpful when there is perirectal inflammatory disease rather than in the setting of small fistulae, because it is better for delineating fluid pockets that require drainage than for small fistulae;
l MRI – this is regarded as the investigation of choice for the delineation of anorectal fistulae and for appropriate mapping of complex cases. Findings show 80-90 per cent concordance with operative findings when observing a primary tract course and secondary extensions. It has been shown to improve recurrence rates by providing information on otherwise unknown extensions;
l Fistulography – this involves injection of contrast via the internal opening, which is followed by anteroposterior, lateral and oblique x-ray images to outline the course of the fistula tract. The accuracy rate is 16-48 per cent. The procedure is well tolerated but requires the ability to visualise the internal opening.
The goals in the treatment of fistula in ano are to eliminate the septic foci and any associated epithelialised tracks and to do so with least amount of functional derangements.
There are wide varieties of surgical procedures for the treatment of anorectal fistulae. The preferred treatment is fistulotomy for low-lying fistulae and seton fistulotomy for all other fistulae. Some of the surgical techniques are as follows:
Fistulotomy – this is appropriate only for simple intersphincteric and low trans-sphincteric fistulae. The principle of fistulotomy is identification of the fistula track and its subsequent opening. This procedure is inappropriate for supra-sphincteric and extra-sphincteric fistulae as the fistula track lies outside the sphincter mechanism and incision will divide the external sphincter mechanism and render the patient incontinent.
The course of simple fistulae usually obeys the Goodsall rule.
A malleable fistula probe is gently passed into the fistula track and the perianal skin and the anal epithelium is divided. If the internal sphincter is encountered, it is identified and it can be divided but if the external striated sphincter is identified, it should not be divided and a seton should be inserted at this point.
Seton fistulotomy – the principle of seton fistulotomy is that any striated external sphincter muscle that lies superficial to the fistula track is encircled by a seton i.e. a ligature of silk, nylon, prolene, silastic or linen, tied firmly and left in situ for a week or two, so that the external sphincter is slowly divided by a process of ischaemic necrosis.
As the process is slow, the muscle heals behind the process of division and a fibrous tissue is laid down in the bed of the fistula. While the seton remains in situ, it acts as a wick or a drain and allows the acute inflammatory reaction around the track to subside.
The procedure involves identifying the fistula track by passing a probe as in fistulotomy and a substantial amount of fistula is laid open. A seton is threaded through the eye of the fistula probe and delivered around the remaining fistula track and snugly tied with multiple knots. The patient will need to be re-examined under anaesthesia and the seton replaced and tightened on few occasions until it finally cuts through.
Mucosal advancement flaps – this is used particularly for extra-sphincteric and supra-sphincteric fistulae. The surgery is technically demanding and excellent results can be obtained by those surgeons performing these operations frequently.
The technique involves mobilisation of a flap of mucosa with underlying internal sphincter, making sure it has good blood supply and strength, advanced down to cover the internal opening and at the same time opening the external component of the fistula and draining any co-existing sepsis. An alternative is to advance a flap up rather than down.
l Fistulectomy is a technique for excising the fistula track. It is technically difficult involving careful dissection around the fistula track to its internal opening. Side tracks must be excised and in complex fistulae, several tracks will need to be excised;
l Re-routing fistulae is a technique to reroute extra-sphincteric fistula to intersphincteric plane which is a complex operation. It is not always successful has been superseded by seton fistulotomy.