Anorectal Fistula

Excerpt

Anal fistula occurs most commonly when the anal glands, which reside in the intersphincteric plane, become occluded and infected, which results in a cryptoglandular abscess. Whether surgically or spontaneously drained, a perirectal abscess may still result in fistula in up to 40% of cases; however, spontaneously draining abscesses tend to have a higher rate, up to 66%, of fistula formation. The mean incidence has reported at 8.6 per 100,000. The presence of acute or chronic anal fistula can be distressing for patients and cause reduced quality of life. They are commonly classified based on their anatomical locations, first described by Parks, Gordon, and Hardcastle, in 1976.

Understanding anorectal anatomy, as well as the classifications of perirectal fistulas, is paramount in their management.

Classification of Anorectal Fistulas

1. A transphincteric fistula

2. A high intersphincteric fistula

3. A suprasphincteric fistula

4. Extrasphincteric fistula

Anorectal fistulas are characterized by their tract location relative to the internal and external sphincters. Parks and Gordon

Intersphincteric Fistulas:

Inevitably, as most abscesses develop in the place between these sphincters, the most common type is an intersphincteric fistula. That is one that crosses the internal sphincter and then has a tract to the outside of the anus leading. A fistulotomy efficiently manages these, or laying open of the fistulous tract and rarely cause incontinence as the treatment does not affect the external sphincter. Intersphincteric fistulas are the most common type of fistula comprising 50-80% of all cryptoglandular fistulas.

Transsphincteric Fistulas:

Trans is a Latin word for “on the other side of.” So a trans-sphincteric fistula is one that crosses to the other side of the external sphincter before exiting in the perianal area and thus involving both sphincters. Transsphincteric fistulas represent a challenge in management because of this and often require more complex or staged treatment. However, the use of a seton to gradually “lower” the tract and make the tract less involved with the external sphincter may allow migration of the tract and a fistulotomy at a later date while preserving the continence of the patient. The extent of involvement of the external sphincter dictates the likelihood of postoperative incontinence as a partial sphincterotomy will usually be tolerated. Still, if the fistula involves the majority of the sphincter, then incontinence will result after a complete division.

Suprasphincteric Fistulas:

These fistula tracts travel superior to the external sphincter and cross the puborectal muscle before changing course caudal to their external opening. Accordingly, they pass the internal sphincter and the puborectal muscle but spare the external sphincter. When these patients typically present with a perirectal abscess, it may not be visible on inspection, but they will have tenderness on the digital rectal exam. Again, because of their high tract, the use of a seton may be considered in these cases before fistulotomy. A fistulectomy is similar to fistulotomy but involves removal of the entire fistula tract either sharply or with cautery. Historically radical fistulectomy was the standard treatment for anal fistula; however, fistulotomy tended to be preferable as it preferred more sphincter function, was a less morbid procedure, and healed faster. However, more recently, with specialists performing the majority of fistula procedures, it seems the outcomes of fistulectomy and fistulotomy are similar. A recent meta-analysis published in 2016 of six randomized controlled trials (RCTs) of fistulectomy versus fistulotomy in low fistulas demonstrated no significant difference in recurrence in five RCTs and no significant difference in postoperative incontinence in four RCTs.

Extrasphincteric Fistulas:

These fistulas often arise in the more proximal rectum rather than the anus and are often sequelae of a procedure. Their external opening is in the perianal area and the tract courses superiorly to enter the anal canal above the dentate line.

St James University Hospital (SJUH) Classification (Imaging-based classification)The SJUH is an imaging-based classification that has five grades based on the anatomic location of fistulas. MRI is more sensitive at delineating soft tissue than CT and has proven to be a reliable method of characterizing the anatomy of anorectal fistula preoperatively as it provides excellent images of the sphincter complex as well as fat in perirectal and supra-levator spaces allowing preoperative identification of involvement of these regions. Furthermore, because MR has multiple potential axial planes, it will enable identification of the internal opening of anal fistulas more readily than other imaging modalities.

A paper published in 2000 by Morris et al. first described the SJUH classification, which is based on MRI findings. Grade 1 fistulas are “simple linear intersphincteric fistula,” which is the same as the “intersphincteric” Parks classification. A grade 1 fistula with the presence of concomitant abscess or an additional fistulous tract is a grade 2 Fistula. Traversement of the external sphincter denotes a grade 3 fistula, which is also known as a trans-sphincteric fistula. A trans-sphincteric fistula with an abscess or an additional tract in the ischiorectal fossa is classified as a grade IV. In contrast, a supra-levator or trans-levator fistula is denoted as grade V. This classification, based on MRI findings of the pelvis, provides an objective preoperative assessment for the surgeon. The diagnostic utilization of MR imaging classification is more predictive of surgical outcome than intraoperative findings.

In general, grade 1 and grade 2 fistulas can be readily managed with fistulotomy or fistulectomy. If an abscess is present in grade I or 2, an incision and drainage should be performed. If a fistula remains after the acute infection resolves, management is then based on the type of fistula present. Grade 3 and 4 involve the external sphincter, so placement of seton may be necessary before fistulotomy or a more specific procedure such a ligation of intersphincteric tract (LIFT) procedure or endoanal advancement flap may be required. Grade 5 fistulas are often indicative of an atypical etiology of the fistula, and further workup and diagnostics should be performed before an operation. Treatment of fistulas in Crohn disease is discussed briefly below but should always be done in the context of the treatment of the underlying condition.