A fistula-in-ano is an abnormal communication between the anal canal or rectum and the perianal skin. This simple definition is complicated, however, by the fact that there may be no internal opening, there may be no external opening, and the track itself may be very complex. The most common cause of fistula in ano is anal gland sepsis ( Fig. 1(a 1170 and b)). A resulting perianal abscess may drain through the perianal skin and heal spontaneously or may persist as a fistula-in-ano. A small number of such fistulae-in-ano are caused by trauma, anal surgical injury or, in the case of extrasphincteric fistula, diverticular disease or foreign body perforation of the rectosigmoid. In addition, diseases such as ulcerative colitis, Crohn's disease, tuberculosis and, occasionally, anal canal carcinoma may cause an irregularity or distortion of the opening of the anal glands, predisposing to a fistula with ensuing cryptoglandular infection. Rarely the threadworm Enterobius vermicularis may cause anal gland infection and fistula formation.

The most widely used classification is that of Parks et al. (1976); that of Eisenhammer (1978) is very similar. These classifications emphasize the relationship between the fistula and the external sphincter muscle. The frequencies of the various types of fistula are shown in Table 1 351.

The management in fistula in ano aims to eradicate the sepsis in the anal gland whilst preserving the maximum amount of anal function.

The first step in successful management is careful assessment (Table 3) 353. Inspection of the perianal area may or may not reveal an obvious perianal sinus representing the external opening of a fistula (Fig. 2) 1171. On palpation there is the important sign of induration, not only in the skin around the external opening, but also along the course of the fistula track. Any degree of sepsis in the intersphincteric plane will cause considerable thickening in that region. Induration at the junction between the pelvic floor and the upper anal canal, so called ‘supralevator induration’, is an important physical sign of uncontrolled sepsis spreading up to the supralevator space from the intersphincteric plane. Goodsall's law may be helpful, indicating the most likely position for the internal opening (Fig. 3) 1172. It is worth remembering that the abscess in the intersphincteric plane can extend in a horseshoe around the circumference of the anal canal. The anal canal and rectum should be carefully examined by rectoscopy and proctoscopy to exclude any other local lesion. Occasionally an internal opening, sometimes disclosed by a small bead of pus will be seen, but this is unusual.

Palpation should be repeated under anaesthesia and the extent of the tracks defined using probes designed by Lockhart Mummery: lacrymal probes are too soft (Fig. 4) 1173. A probe should be passed through the external opening whilst ‘feeling’ with the examining index finger in the anal canal (Fig. 5) 1174. Injection of methylene blue has the disadvantage that staining of tissues may make appreciation of the true extent of the tracks more difficult. Injection of hydrogen peroxide into the external opening of the fistula will not stain tissues, but the appearance of bubbles in the anal canal with an anal retractor in place will confirm a fistula and define the position of the internal opening. If probing does not reveal the extent of the track the superficial part of the fistula should be laid open, looking for the characteristic granulation tissue of the lining of the track. The opened track is probed again, and if necessary the incision is extended until the anatomy of the fistula is clear.

A number of imaging techniques have been introduced but these are still being developed and are no substitute for careful palpation. Fistulography may be helpful in the investigation of the rare extrasphincteric fistula, but is rarely necessary in simpler fistulae. CT and magnetic resonance imaging scanning have both been used in the assessment of fistula-in-ano but without any convincing evidence of improvement in management. Anal ultrasonography is not yet widely available, but can demonstrate secondary tracks and abscesses, and the use of hydrogen peroxide to show up bubbles in the fistula track which have a characteristic echo-rich appearance may improve its accuracy. Breaches in the internal sphincter and changes in the intersphincteric plane can be seen.

A superficial fistula is treated simply by laying it open (Fig. 6) 1175: at the most this may involve dividing the distal internal anal sphincter. Care is needed in the midline posteriorly, where the guttering caused by this partial sphincterotomy can sometimes cause problems with incomplete closure of the anal canal and soiling. Having laid open the track the granulation tissue is carefully curetted out, a biopsy is taken for histology, and the wound is cleaned and dressed with a superficial dressing rather than by tight packing. Postoperative care is just as important as the operation: the patient should take daily baths and the anal wound should be reviewed soon after the procedure.

The standard treatment (Fig. 7) 1176 is to lay open the intersphincteric track by dividing the distal internal anal sphincter, curetting out the anal gland and then draining any secondary tracks. Again, care must be taken in dividing the internal sphincter in the midline posteriorly to avoid guttering and subsequent continence problems.

Similar principles apply in the management of trans-sphincteric fistula (Fig. 8) 1177. The intersphincteric abscess should be drained, together with any secondary tracks. Occasionally a secondary track passes upwards to the top of the ischiorectal fossa or through the pelvic floor to the supralevator space. A primary track below the anal valves should be drained by dividing the external anal sphincter; if above the anal valves it is drained using a seton. Great care must be taken in the judicious division of muscle. In the study by Marks and Ritchie (1977) 164 of 796 fistulae in ano were trans-sphincteric; 7 per cent of these had tracks below the anal valves, and in 14 per cent the primary track was at or above the level of the anal valves.

These are fortunately rare. The track passes straight upwards in the intersphincteric space, over the puborectalis muscle and down through the ischiorectal fossa to the skin. There may be an associated horseshoe abscess in the supralevator space.

Division of the track would result in certain incontinence. The intersphincteric space is drained and the anal gland curretted out. A seton is then passed around the external anal sphincter. Healing may take many months.

Linen, silk and nylon thread have been used as a seton but a brightly coloured soft plastic vessel loop is easy to identify and comfortable for the patient to sit upon. There is some debate over the method of action of a seton: some clinicians feel that it works as a slow elastic ligature and therefore must be tightened periodically. A second school feel that the seton acts by ensuring chronic drainage of the fistula track (Fig. 10) 1179. In a series reported by Thomson and Ross (1989) the sphincter was preserved in 44 per cent of patients after a prolonged period of seton drainage. The seton could then be removed and the fistula healed. In the other 56 per cent, the external sphincter was divided after prolonged seton drainage allowing subsequent healing. A seton certainly allows time for any sepsis and induration to settle before a decision about further treatment is made. As a general principle it is important to leave the seton in place for many months and to be prepared to perform repeated examinations of the area under anaesthesia when the anatomy of the fistula is not clear. Some have suggested that the use of a seton in the management of less complex fistulae may avoid any muscle division at all, but recurrence rates after this approach have not been reported.

There are two further alternative approaches to the trans-sphincteric or suprasphincteric fistula that allow for optimum preservation of sphincter muscle. Aguilar et al. (1985) have described a technique which preserves the internal sphincter by using an endoanal flap advanced over the internal opening (Fig. 11) 1180. The intersphincteric abscess is, of course, drained and the external part of the track in the perianal area is cleaned out. Wedell et al. (1987) used a flap including muscle while Mann and Clifton (1985) have described a procedure in which the external sphincter is divided and the fistula rerouted inside it. The external sphincter is immediately repaired to allow for complete excision of the fistula without the undivided support of any sphincter muscle (Fig. 12) 1181. Patients treated by these procedures have not been followed for long periods, but there is an increasing trend towards maximum preservation of sphincter muscle in fistula surgery.

A defunctioning colostomy is not necessary in the management of most fistula patients but they should undergo meticulous bowel preparation and systemic antibiotics. Where uncontrolled sepsis complicates a trans-sphincteric or suprasphincteric fistula, a colostomy may be indicated.

The two crucial outcomes of fistula surgery are anal sphincter function and recurrence of the fistula. In a study by Belliveau, et al. (1983) anal canal pressures in a group of patients in whom the external sphincter had been divided were compared with a group in whom the external sphincter had been preserved and with a control group. The resting anal canal and voluntary contraction pressures were significantly diminished after external sphincter division. Of those in whom the sphincter had been preserved, 83 per cent had full continence whereas only 32 per cent of those who had undergone sphincter division were fully continent, and some of these needed a subsequent sphincter repair. In a study by Girona et al. (1987) incontinence for solids, liquids, and gas was seen in 4.3 per cent, 25 per cent, and 21 per cent respectively, of a group of patients with high trans-sphincteric fistulae 3 years after treatment. Parnaud (1987) reported incontinence rates varying between 16 and 26 per cent for patients with intersphincteric and trans-sphincteric fistulae.

Fortunately these difficult trans- and suprasphincteric fistulae are seen in a minority of patients, but expert surgical management is required in order to avoid recurrence and postoperative incontinence.

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