Anal fistula disease is a condition characterized by pain and discharge of purulent secretion from an opening in the anorectal region. There are two stages of the disease: abscess formation as the acute stage and fistula formation as the chronic stage [1, 2].
The most common cause of anal fistulas is inflammation of the proctodeal glands. For this reason, this type of fistula is also referred to as a cryptoglandular anal fistula. The second most common cause of anorectal fistulas is chronic inflammatory bowel diseases, particularly Crohn's disease. Less commonly, fistulas can occur after surgical procedures in the pelvis or in cases of malignancies. Fistulas in infants are a distinct entity and often have a congenital cause [3, 4].
Anal abscesses and fistulas primarily occur in younger adults aged between 30 and 50 years [5]. Men are more frequently affected than women. The incidence of this disease in Europe is 1-2 cases per 10,000 inhabitants per year and varies depending on the population [6].
The development of cryptoglandular anal fistulas begins in the proctodeal glands in the intersphincteric space [7, 8]. The number of glands is more concentrated posteriorly and is higher in men than in women [9, 10]. The duct of these glands opens in the anal canal at the level of the dentate line. Purulent inflammation in these glands can cause local necrosis, surrounded by granulation tissue (abscess capsule). Depending on the virulence of the germs, the abscess spreads in the direction of least resistance and breaks through the skin. The connection of the abscess with the body surface is lined with granulation tissue and is referred to as a fistula.
The acute inflammatory stage, where no secretion drains, is perceived as an anal abscess, while the chronic course with putrid secretion in the presence of an external opening is referred to as an anal fistula.
In chronic inflammatory diseases, fistulas arise from transmural inflammation, which may also include perianal and perirectal tissue and only partially respect anatomical structures [11]. It is not uncommon to find intersphincteric anal fistulas also associated with anal fissures.
Microbiological examinations are irrelevant for the treatment of anal fistulas and usually yield a mixed flora [12, 13]. Risk factors for the development of an abscess and thus a "common" anal fistula (as opposed to IBD) are not described in the literature.
The treatment of anal fistula is primarily surgical.
Seton Drainage
A commonly used procedure in the surgery of anal fistulas is the placement of a seton drainage. A strong, braided, and non-absorbable thread such as silk or a plastic loop ("vessel loop") is used as the material [14-17]. In Germany, seton drainage is mainly used to prepare a fistula tract before definitive secondary treatment.
Three different procedures are distinguished:
- Marking and drainage of a fistula using a loosely tied thread as part of abscess sanitation before further planned measures
- Creation of a stable fistula tract through a fibrosing thread as long-term drainage before later therapeutic measures
- Controlled division of the sphincter muscle by a so-called cutting thread
Loose Seton Drainage
The goal of using threads as drainage (known as "loose seton") is long-term drainage of the abscess cavity to prevent the external fistula opening from closing prematurely. Later, the thread is removed to allow spontaneous healing of the fistula. Some authors perform this procedure in combination with a primary fistulectomy or an internal sphincterotomy [18-23]. The healing rate varies between 33 and 100%, with continence disorders occurring at a frequency of 0 to 62%.
It is unlikely that cryptoglandular anal fistulas can be definitively healed by temporary seton placement alone. Typically, further intervention is required. In Crohn's fistulas with local inflammation, the success rate of surgical measures is reduced. However, a seton drainage can be used as a definitive method after successful abscess incision [24-26].
Fibrosing Thread
The use of fibrosing threads is usually primary or secondary during the treatment of acute or chronic inflammation. Once the inflammatory process has subsided, the thread leads to consolidation of the fistula tract before further surgical measures are taken. In Germany, fibrosing threads are mainly used for trans- or suprasphincteric fistulas, where after healing of the acute inflammation, definitive sanitation is sought that spares the sphincter muscle. However, it is unclear whether the preparation of the fistula with a seton drainage positively influences the success of a definitive fistula operation. Some studies suggest that the placement of a seton drainage can reduce the rate of recurrent abscesses [27].
In general, seton drainage can be left as long-term drainage, especially in patients with chronic inflammatory bowel diseases who are oligosymptomatic or exhibit strong inflammatory activity.
Cutting Seton
The goal of the "Cutting Seton" procedure is to gradually cut through the sphincter parts surrounding the fistula tract after the inflamed area has been cleared. Various materials can be used as "thread." On the one hand, elastic threads can be used that gradually cut through. On the other hand, regular readjustment of the thread may be necessary. Although this technique is preferred in the Anglo-American region [28, 29], it carries a significant risk of postoperative continence disorders (30), so it should only be used in exceptional cases that are well justified. Despite the high healing rate, the decision for this method should be carefully considered.