Fistula in ano

A fistula in ano is a track, lined by granulation tissue, which connects deeply in the anal canal or rectum and superficially on the skin around the anus. It usually results from an anorectal abscess which burst spontaneously or was opened inadequately (Fig. 61.34). The fistula continues to discharge and, because of constant reinfection from the anal canal or rectum, seldom, if ever, closes permanently without surgical aid. An anorectal abscess may produce a track, the orifice of which has the appearance of a fistula, but it does not communicate with the anal canal or the rectum. By definition this is not a fistula, but a sinus.

Types of anal fistulae

These are divided into two groups, according to whether their internal opening is below or above the anorectal ring.

Low-level fistulae open into the anal canal below the anorectal ring.

High-level fistulae open into the anal canal at or above the anorectal ring.

As an alternative to the common anatomical classification illustrated in Fig. 61.38, Parks produced another based on the origin of the fistula from an abscess in an anal gland situated in the plane between the internal and external sphincters (the ‘anal intersphincteric plane’) (Fig. 61.39).

The importance of deciding whether a fistula is a low or a high-level type is that a low-level fistula can be laid open usually without fear of permanent incontinence (from damage to the anorectal bundle), while a high-level fistula can be treated only by ‘staged’ operations, often with the use of a protective colostomy to prevent septic complications and to shorten healing time between the stages.In probing a high fistulous track, great care must be taken not to create an internal opening into the rectum where none existed previously. Such a disaster could convert a relatively straightforward ‘intersphinctenic’ track into a high ‘pelvirec­tal’ fistula that might prove very difficult to cure.

By the standard classification, a high fistula refers to both a high anal [Fig. 61.38(4)] and a pelvirectal fistula [Fig. 61.38(5)]. By the Parks’ classification, both a high trans sphinctenic and a supralevator fistula would qualify as high, with the intersphincteric falling into either category depending on whether an internal opening was present at all, and at what level it entered the anal canal [see Fig. 61.38(1)].

Low-level fistulae

Clinical features. Commonly, the principal symptom is a per­sistent seropurulent discharge that irritates the skin in the neighbourhood and causes discomfort. Often the history dates back for years. So long as the opening is large enough for the pus to escape, pain is not a symptom, but if the orifice is occluded pain increases until the discharge erupts. Frequently, there is a solitary external opening, usually situated within 3.5—4 cm of the anus, presenting as a small elevation with granulation tissue pouting from the mouth of the opening. Sometimes superficial healing occurs, pus accumulates and an abscess reforms and discharges through the same opening or a new opening. Thus there may be two or more external openings, usually grouped together on the right or left of the midline but, occasionally, when both ischiorectal fossas are involved, an opening is seen on each side, in which case there is often intercommunication between them (Fig. 61.40). As a rule there is much induration of the skin and subcutaneous tissues around the fistula.

Goodsall’s rule. Fistulae with an external opening in relation to the anterior half of the anus tend to be of the direct type (Fig. 61.41). Those with an external opening or openings in relation to the posterior half of the anus, which are much more common, usually have curving tracks, and may be of the horseshoe variety. Note that posteriorly situated fistulae may have multiple external openings which always connect to a solitary internal orifice, usually midline (Fig. 61.41).

Digital examination. Not infrequently an internal opening can be felt as a nodule on the wall of the anal canal. Irrespec­tive of the number of external openings, there is almost invariably only one internal opening.

Proctoscopy sometimes will reveal the internal opening of the fistula. A hypertrophied papilla is suggestive that the internal orifice lies within the crypt related to the papilla (Fig. 61.42). Probing. In the past, it was the universal practice to probe a fistula in the ward or the out-patient department. Such manoeuvres accomplish nothing, are painful and are liable to reawaken dormant infection. Furthermore, if probing is performed without the utmost gentleness, or if the patient, experiencing pain, makes a sudden jerk, a false passage may result which complicates the condition still further. Probing should he postponed until the patient is under an anaesthetic in the operating theatre.

The injection of lipiodol, or other opaque medium, along the sinus, before radiography has little to recommend it. The radiographs thus obtained are seldom illuminating, and the procedure is likely to cause a recrudescence of inflammation. Endoluminal ultrasonography and magnetic resonance imaging are being developed as techniques for ‘mapping’ complex fistulae.

Radiography of the thorax should be undertaken and the possibility of pulmonary tuberculosis considered, despite the fact that today it will be found in only a small proportion of patients with fistula in ano — usually of Asian origin (see below).

I— Special clinical types of fistulae in ano

Fistula connected with an anal fissure. Unlike the usual fistula in ano, pain (due to the fissure) is a leading symptom. The fistula is very near the anal orifice, usually posterior, and the external opening is often hidden by the sentinel pile.

Fistula with an internal opening above the anorectal ring is due, almost invariably, to penetration by a foreign body or probing and interference with a high abscess. A supralevatot fistula arising spontaneously will be seen only once or at most twice in a surgical career.

Granulomatous infections and Crohn’s disease. If induration around a fistula is lacking, if the opening is ragged and flush with the surface, if the surrounding skin is discoloured and the discharge is watery, or if the external openings are multiple, tuberculous or Crohn’s disease should be considered. In more than 30 per cent of patients suffering from pul­monary tuberculosis, virulent tubercle bacilli are present in the rectum. About 2—3 per cent of fistulae in the UK are due to Crohn’s disease or tuberculosis. In Asian communities, the incidence of tuberculosis is higher. Crohn’s disease should also be suspected if there are other stigmata, and a small bowel meal may be necessary. If tuberculosis is suspected, a chest radiograph and sputum cultures are mandatory. However, the diag­nosis can usually only be made on histological examination of biopsy material from the track. If due to tuberculosis, the fistula will usually respond to antituberculous drugs alone..

Fistulae with many external openings may arise from tuberculous proctitis, Crohn’s disease of colon or ileum, bilharziasis and lymphogranuloma inguinale with a fibrous rectal stricture. Crohn’s disease is the most frequent cause seen in this country from this group.

Carcinoma arising within perianal flstulae. Colloid carcinoma may complicate fistulae in ano and a colloid carcinoma of the rectum is notoriously liable to be complicated by perianal fistulae. In some instances, the fistulous condition, with its discharge of colloid material, overshadows the primary carcinoma, and not a few unfortunate patients have had their condition diagnosed for a time as an inflammatory fistula in ann. If a primary tumour is present in the rectum, usually it can be detected and its nature established by biopsy. Dukes established conclusively that colloid carcinomatous fistulae can develop without a primary neoplasm in the rectum. He regarded such cases as examples of colloid carcinoma developing in a reduplicated portion of the intestinal tract. Both adenocarcinoma and squamous-cell carcinoma are known to arise within chronic fistulous tracks. The former can develop from the anal granular tissue; the latter is an example of true malignant change of squamous epithelium lining the wall of the track.

Hidradenitis suppurativa. This is a chronic infection of apocrine glands around the anal margin giving rise to numerous sinuses. The mons pubis and groin can also be affected. After excision of the area, granulation and healing ate accelerated by using Silastic foam dressing (Hughes).

Treatment. That the fistulous track must be laid open from its ter­mination to its source was a rule promulgated by John of Arderne more than 600 years ago.

The operation can best be described in stages:

Step 1. Preoperative cleaning enemas are necessary. When the patient has been anaesthetised, he or she is placed in the lithotomy position or in the prone jack-knife position, according to the preference of the operator. Using bidigital palpation under anaesthesia, it is often possible to obtain more information concerning a fistula than can be learned from probing; it is surprisingly easy to push a probe through the wall of the track. Unfortunately, many inexperienced operators find it more reassuring to create a false passage than to risk criticism for not being able to demonstrate the internal opening. Careful bidigital palpation of the perianal tissue will often reveal a cord-like induration, representing the track, which will lead the intra anal finger towards the proximal opening. Rather than insert a probe through the distal orifice at this stage, it is better to endeavour to find the internal opening via a proctoscope. If the internal opening still cannot be seen, the insertion of a probe termgradely into an anal crypt, especially one with a nearby hypertrophied papilla, often reveals the internal portion of the track (Fig. 61.42). The injection of dilute methylene blue or other dye into the external mouth of the fistula to establish the site of the internal opening is occasionally necessary, but is not recommended as a routine.

Step 2. A probe-pointed director (Fig. 61.43) is inserted into the distal orifice, and it is advanced delicately until it reaches a point where it does not pass readily. The track is opened along the director and bleeding is controlled.

Step 3. If it is not at once evident in which direction the track passes, granulations are wiped away with gauze (it is seldom necessary to use a curette). Often this will leave a granulation-filled spot at one site only. Gentle probing at this spot frequently will give the clue to the continuation of the fistula. The director is reinserted, and again followed with the knife for a short distance. This procedure is repeated until the entire track, and any side channels, are laid open. As far as possible, all muscle is divided at right angles to its fibres. In the rare event of the track passing above the anorectal ring, cutting should cease at the level of the dentate line, and from thenceforth the operation is conducted as sug­gested below. In most instances, probing and laying open the track can be repeated until the entire track is laid open. Pursuing this course, if there is no internal opening, the track will become bereft of granulations on wiping it. As a rule, the internal opening can be demonstrated either by direct inspection through a proctoscope, or by a bent probe inserted into an anal crypt. In the latter circumstance, the internal portion of the track is excised in continuity.

Step 4. The edges of the track are trimmed, 1—3 mm of tissue being removed — a step that makes postoperative packing unnecessary after the first 24—36 hours. Hughes advocated primary split skin grafting of the wound resulting from fistulotomy. The grafts are taken from the inner aspect of the thigh and applied to the anal wound, being stitched to the skin edges and to each other in the depths of the wound. Tulle gras is then superimposed and a firm pack of cotton wool applied. The first dressing is done on the 5th postoperative day.

When skin grafting is not employed, digital dilatation of the anus, or the passage of a St Mark’s Hospital3 dilator every other day, prevents pocketing or bridging of the granulating wound.

Biopsy. Always send a piece of track for biopsy.

High-level fistulae

The treatment of these cases is difficult. If the track is laid open as for low-level fistulae, incontinence will follow. There are four types (Parks).

Supralevator fistula — secondary to local disease [Fig. 61.39(3)]. It occurs as a result of Crohn’s disease, ulcerative colitis, carcinoma, a foreign body perforating the rectal ampulla from above or trauma. This fistula is quite unrelated to the ordinary type and the treatment is that of the cause. A traumatic fistula usually needs a colostomy. None of these fistulae requires to be laid open, which would in any case cause incontinence.

Trans-sphincteric fistula [Fig. 61.39(2)] with perforating secondary track. The condition starts as an intersphincteric track [Fig. 61.39(1)], often with a high secondary track in the ischiorectal fossa up to the levator ani. Here lies the danger. Although the anal opening may be low, during exploration of the high secondary track, unless great care is taken, the probe can be pushed through the levator ani into the rectal ampulla, thus converting a low fistula into a high-level type. Treatment should first of all be directed to the low trans-sphinctenic flstula and healing of the upper track may follow. If it fails to do so, or if the opening into the rectum is of any size or near the anorectal bundle, a colostomy must sometimes be done before sound healing will take place. High tracks often require staged operations.

A seton — a time-honoured device — (i.e. a ligature of silk, nylon, silastic or linen) is helpful when the internal opening is near the anorectal ring. Insertion of a seton and subsequent re-examination of the patient without anaesthesia will establish whether the internal opening is situated so near to the anorectal ring that incontinence would result if the track were laid open. Under these conditions, a staged operation and a covering colostomy would be the proper treatment. While the seton remains in situ it acts as a wick/drain and allows the acute inflammatory reaction around the track to subside: this can greatly simplify subsequent surgery. In expert hands, primary repair of divided sphincter muscle can preserve continence when a high-level track is laid open.

Intersphincteric fistula. The track starts as a primary anal gland abscess (Fig. 61.34a), and it runs between the internal and external sphincter along the plane of the longitudinal muscle fibres (see Fig. 61.39, Type 1). It may have an opening into the rectum above the anorectal ring and below at the site of a perianal abscess (Fig. 61.34b). Providing it is recognised it is easy to treat. The internal sphincter is divided and the whole track is laid open without fear of incontinence.

Suprasphincteric flstula. Occasionally, the intersphincteric track passes over the top of the sphincter before passing down again in the ischiorectal fossa. Treatment of this type is very difficult and is sometimes best done by an indwelling seton.