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.
Fistulectomy with primary anal sphincter repair
Fistulectomy with primary sphincter repair (FPSR) is a safe and promising method, particularly for distal and intermediate transsphincteric fistula in ano, and may also be an option in proximal fistulas.[16, 21, 22]
Suture dehiscence, a complication of high fistulas, is associated with an increased risk of postoperative disorders of fecal continence, which is why a clinical examination should be performed 2 to 4 weeks after the FPSR so that the repair can be revised promptly, if needed.[14] Specialized centers can have healing rates of 88% for FPSR, and up to 96% following revision.[15, 16]
Varying rates of fecal continence impairment have been reported. A study from 2012 reported that preoperatively continent patients did not suffer any deterioration in fecal continence function after FPSR. Preoperatively incontinent patients even experienced improved continence performance.[17] A systematic review from 2015 (14 studies, 666 patients) compared the outcomes of fistulotomy with those of FPSR.[18] FPSR was followed by a higher incidence of mild and severe postoperative fecal incontinence (mild: 8.6% vs. 15.4%, severe 1.1% vs. 2.7%). In patients with unimpaired preoperative fecal continence, the rate of postoperative continence disorders was 12.4%. A randomized study did not find any differences between FPSR and advancement flap in terms of functional outcome and recurrence rate.[19]
De Hous et al. describe that FPSR avoids unfavorable anal keyhole deformities in most cases and has a high healing rate of almost 96%.[20] FPSR offers low morbidity, good healing rate and good postoperative fecal continence outcome.[23–26]
However, the data from the studies is not enough to provide a definitive answer to the question of which surgical procedure is best suited for the treatment of higher or complex anal fistulas. In particular, there is a lack of further randomized controlled trials. Even though FPSR is now an established procedure, it should be reserved for proctological centers, as the individual expertise of the surgeon is also key.[21]