Fistula in ano is a condition characterized by pain and discharge of purulent secretions from an orifice in the anorectal region. There are two stages of the disease: The acute stage of abscess formation and the chronic stage of fistulization.[1, 2]
The most common cause of anal fistula is inflammation of the anal glands. This is why this type of fistula is also called a cryptoglandular fistula in ano. The second most common cause of anorectal fistula is chronic inflammatory bowel disease, particularly Crohn disease. Fistulas resulting from malignancies or surgical procedures in the lesser pelvis are less common. Fistulas in infancy are a separate entity and often can be traced to a congenital origin.[3, 4]
Anal abscess and fistula in ano are seen primarily in younger adults between the ages of 30 and 50 years.[5] Men are affected more often than women. In Europe, the incidence of this condition is 1–2 cases per 10,000 population per year and varies depending on the population.[6]
Cryptoglandular anal fistulas originate in the anal glands in the intersphincteric plane.[7, 8] The glands cluster posteriad and number higher in males than females.[9, 10] The excretory duct of these glands terminate in the anal canal at the level of the dentate line. Purulent inflammation in these glands may induce local necrosis surrounded by granulation tissue (abscess capsule). Depending on the virulence of the pathogens, the abscess spreads in the direction of least resistance and breaches the skin. The junction of the abscess with the body surface is lined with granulation tissue and is called a fistula.
The acute inflammatory stage, with no discharge of secretion, is perceived as anal abscess, while the chronic course with putrid secretion in the presence of an external orifice is called anal fistula or fistula in ano.
In chronic inflammatory disorders, on the other hand, the fistulas result from transmural inflammation, which sometimes involves the perianal and perirectal tissue planes and respects anatomic structures only to a limited extent.[11] It is not uncommon to find intersphincteric anal fistulas in conjunction with anal fissures.
Microbiological testing has no bearing on the treatment of anal fistulas and usually reveals a mixed microflora.[12, 13] The literature does not describe any risk factors for abscess formation and thus for the development of an "ordinary" anal fistula (in contrast to IBD).
The treatment of anal fistula is mainly surgical.
Seton drainage
One procedure commonly employed in surgery of anal fistula is seton drainage. Here, a strong, braided non-absorbable suture such as silk or a plastic tether ("vessel loop") is pulled through the fistula tract.[14-17] In Germany, seton drainage is mainly used to prepare a fistula tract prior to definitive secondary treatment.
There are three types of seton drainage:
· Marking and drainage of a fistula by means of a loosely knotted suture as part of abscess treatment prior to additional planned measures
· Formation of a stable fistula tract by means of a fibrosing suture as long-term drainage prior to subsequent therapeutic measures
· Controlled transection of the sphincter muscle by a so-called cutting seton
Loose seton drainage
The objective of using sutures as a drain (known as a "loose seton") is to provide long-term drainage of the abscess cavity, thereby preventing the external orifice of the fistula from closing prematurely. The seton is removed later on to allow the fistula to heal spontaneously. Some authors perform this procedure in combination with primary fistula excision or internal sphincterotomy.[18-23] The rate of success varies from 33% to 100%, and continence disorders range from 0% to 62%.
Cryptoglandular anal fistulas are unlikely to be definitively repaired by temporary seton drainage alone. Most cases require further surgery. In fistulas in Crohn disease with local inflammation, the success rate of surgical measures is lower. However, in these cases, seton drainage can be used as a definitive measure after successful abscess incision.[24-26]
Fibrosing seton
Fibrosing setons is mostly used as primary or secondary measures during the treatment of acute or chronic inflammation. Once the inflammatory process has subsided, the seton leads to consolidation of the fistula tract before further surgical measures are taken. In Germany, fibrosing asetons are mainly employed in trans-/ suprasphincteric fistulas where, once the acute inflammation has resolved, the aim is to achieve definitive repair sparing the sphincter muscle. It is unclear, however, whether priming the fistula with seton drainage has a favorable effect on the success of definitive fistula surgery. Some studies suggest that seton drainage may reduce the rate of recurrent abscess formation.[27]
In general, setons can be left in place as a long-term drain, especially in patients with inflammatory bowel disease who are oligosymptomatic or display high levels of inflammatory activity.
Cutting seton
The objective of the "cutting seton" procedure is to gradually transect those parts of the sphincter surrounding the fistula tract after the inflamed area has been eradicated. Various materials may be used as "setons". On the one hand, elastic setons can be used, which gradually transect those parts of the spincter muscle involved in the fistula. On the other hand, regular readjustment of the seton may become necessary. Although this technique is preferred in the Anglo-American world [28, 29], it carries a significant risk of impairment of postoperative continence [30], and therefore should be employed only in exceptional cases that are well justified. Despite the high rate of success, the decision to use this technique should be carefully deliberated.