Subcutaneous incision of the fistula tract and internal sphincterotomy (SIFT‐IS): a novel surgical procedure for transsphincteric anal fistula

Abstract Aim The aetiology of anal fistula has not been fully clarified. One of the causes of anal fistulas may be the markedly deep crypts that characterize the primary openings. We developed subcutaneous incision of the fistula tract and internal sphincterotomy (SIFT‐IS) to eradicate these deep crypts. The aim of this study was to evaluate outcomes in patients with anal fistula treated with SIFT‐IS. Method A retrospective study was performed over a 2‐year period. Patients with transsphincteric anal fistula who underwent SIFT‐IS were enrolled. The primary endpoint was the anal fistula healing rate at 16 weeks postoperatively. The secondary endpoints were healing time, postoperative complications and clinical continence status. Results One hundred and fifty one patients were enrolled. Primary healing was accomplished in 129 patients (85%). There were 17 patients (11%) with a remnant fistula and five (3%) with a recurrence. The remnant fistulas healed spontaneously at more than 16 weeks postoperatively in seven patients. The median healing time was 6 (3–96) weeks. Surgical intervention was required in seven patients with a remnant fistula and four with recurrence. At the final follow‐up, the wounds had healed in 148 patients (98%). No significant postoperative complications or incontinence were observed. Conclusion Subcutaneous incision of the fistula tract and internal sphincterotomy is a promising surgical option for transsphincteric anal fistulas, with a satisfactory healing rate.

These SSPs are also based on the theory that anal fistulas are caused by cryptoglandular infection.
The aetiology of anal fistulas is still not completely clarified.
Perianal abscesses and fistulas are considered two sequential phases of the same anorectal infectious process. Perianal abscesses arise from cryptoglandular infections and represent the acute phase, while anal fistulas represent the chronic phase; however, perianal abscesses do not always progress to anal fistulas; the frequency ranges from 16% to 41% [9][10][11][12][13]. The cryptoglandular infection theory is unable to explain why some perianal abscesses develop into anal fistulas and others do not. This suggests that there are other factors that promote the progression of perianal abscesses to anal fistulas.
The primary openings of anal fistulas comprise wide and thick pockets referred to as deep crypts ( Figure 1A). The shape of these deep crypts may promote the entry of faecal contents from the primary opening and lead to persistent infection. Deep crypts are considered to cause anal fistulas in infants [14,15], but there have been no reports of deep crypts as the aetiology of anal fistula in adults.
Clinical observations suggest that deep crypts are likely to be formed as the anorectal infection progresses from cryptoglandular infection to anal fistula. Contraction of the fibrous tissue attached behind the primary openings accentuates and opens the crypts to form deep crypts ( Figure 1B). Pathological examination has also revealed inflammatory fibrosis in the subepithelial layer around anal fistulas [16]. Once deep crypts are created, they may promote continuous inflow of faecal contents into the fistula, causing chronic inflammation and preventing the fistula from healing. We considered that eradicating these deep crypts was the key to treating anal fistulas. Thus, we developed a surgical procedure consisting of two techniques, namely subcutaneous incision of the fistula tract (SIFT) and internal sphincterotomy (IS) (Video S1).
The aim of SIFT is to flatten deep crypts by dissecting fistula tracts and fibrous subepithelial tissue connecting the deep crypt, which prevents faecal contents from entering the internal openings.
The role of IS is to restore the compliance and distensibility of the fibrotic internal sphincter, which complements the effects of SIFT.
The purpose of the current study was to assess the feasibility of SIFT-IS for treating transsphincteric anal fistulas.

Patients and methods
This retrospective study was conducted at the Coloproctology Center, Tokyo Yamate Medical Center, Japan, from January 2016 to December 2017. RS and KM worked there until March 2020. The patients provided written informed consent for the procedure, and the study was approved by the hospital's institutional review board.
Patients with transsphincteric fistula were included in this study.
In accordance with the Parks classification [17], transsphincteric anal fistulas were defined as fistulas crossing both the internal and external sphincters. Transsphincteric anal fistulas were identified via digital examination and proctoscopy and confirmed by intraoperative findings. Magnetic resonance imaging was used to differentiate transsphincteric anal fistulas from complicated fistulas (suprasphincteric and extrasphincteric fistulas) when necessary. The following were excluded: patients with multiple fistulas, recurrent fistula, horseshoe fistula and fistulas associated with Crohn's disease.

Operative technique
All patients were administered a laxative suppository (New

RE SULTS
There were 1720 patients with anal fistula during the study period.
Six hundred and ninety one patients were assigned to RS. Among Patient characteristics are shown in Table 1.
No patients received seton placement prior to SIFT-IS because we only perform seton placement for patients with a complicated fistula or Crohn's disease.

Outcomes of SIFT-IS
Primary healing was achieved in 129 patients (85%). The median duration of follow-up was 10 (3-131) weeks. There were 17 patients (11%) with a remnant fistula and five (3%) with a recurrence. The outcomes are summarized in Table 2.

Postoperative complications
Four patients had postoperative bleeding from the drainage wound that required haemostasis with bipolar forceps; none of these patients experienced changes in vital signs or required infusion or blood transfusion. There were no postoperative complications related to the surgical wound, such as abscess formation, severe pain or dermatitis. No patient experienced changes in their clinical continence.

DISCUSS ION
SIFT-IS is a novel surgical procedure for anal fistula. The essential feature of SIFT-IS is to eliminate the structural abnormalities of primary openings that cause persistent infection of the anal fistula. Our study showed that SIFT-IS achieved primary healing in 85% of patients.
The Parks classification categorizes anal fistulas as intersphincteric, transsphincteric, suprasphincteric or extrasphincteric [1]. We focused on transsphincteric fistulas because these are the most common fistula type. Fistulotomy is the most reliable procedure for treating transsphincteric fistula in terms of healing rate, but postoperative incontinence occurs in up to 54% of cases [3,4]. The risk of postoperative incontinence depends on the amount of sphincter muscle involved in the fistula [4,18,19]; nevertheless, incontinence, including mild continence disorders, was observed in 24% even in F I G U R E 2 (A) The fistula is dissected from the secondary opening and then an incision is made on the lower edge of the anoderm. (B) The subepithelial layer is dissected through the incision. (C) Internal sphincterotomy is performed on the fibrotic internal muscle which is adjacent to the fistula. (D) The fistula is divided on the surface of the internal sphincter without ligation.
patients with a low fistula (less than one-third of the external sphincter division) [19]. It is necessary to consider a less invasive procedure for sphincter muscles in the treatment of transsphincteric anal fistula.
The primary healing rate with SIFT-IS was 85% at 16 weeks, reaching 89% at 24 weeks without further surgical intervention.
These results are comparable to the SSPs adopted by many surgeons.
Previous studies have reported healing rates of 66%-83% after the endoanal advancement flap procedure [20][21][22] and 68%-88% after LIFT [23][24][25][26]. Our results suggest validity of the concept that SIFT-IS restores the structure of the primary opening, leading to healing of the anal fistula. We started to develop the SIFT-IS procedure in 2007. The tract stumps were initially supposed to be ligated, but we found that anal fistulas healed without ligation in patients in whom the fistula tracts running through the subepithelial layer were too thin and fragile to be ligated. Thus, we stopped ligating the fistula tracts and verified that they still healed. In anal fistula treatment, it has previously been considered necessary to close the primary opening where the faecal contents enter. However, we assume that flattening the primary opening prevents faecal contents from flowing into the crypt, which performs the same role as primary opening closure. Consequently, ligation of the fistula tract is unnecessary.
Our study may provide new insights into the aetiology of anal fistula. Cryptoglandular infection is considered the first step in the development of a perianal abscess, which is the preliminary stage of anal fistula formation. However, the cryptoglandular infection theory is not sufficient to explain the mechanism by which a perianal abscess transforms into an anal fistula because the majority of perianal abscesses will not develop into anal fistulas [9][10][11][12][13]. The largest and the most recent series demonstrated that anal fistulas occurred in only 16% of patients with perianal abscesses [13]. Therefore, there must be other factors that allow persistent infections and promote the formation of an anal fistula from a perianal abscess. We speculate that the transformation of a perianal abscess to an anal fistula is contingent on LIFT is a recent and widely performed SSP in which the anal fistula is divided and ligated in the dissected intersphincteric layer [8].
LIFT is associated with several risks while dissecting the intersphincteric layer that can lead to impaired anal function because the conjoined longitudinal muscle runs between the internal and external sphincters.
Contraction of the longitudinal muscle plays a role in shortening and widening of the anal canal during the process of defaecation [27], and LIFT may harm the function of the muscle. The impairment of fine nerve networks and generation of scar tissue in the intersphincteric layer in LIFT may also interfere with the coordinated movement of sphincteric muscles. Postoperative evaluation of incontinence using surveillance of Cleveland Clinic Florida Faecal Incontinence scores detected minor incontinence after LIFT [28,29].
In contrast, since SIFT-IS does not affect the intersphincteric layer, it would not be expected to damage the coordinated movement of the anal sphincter complex.
In the present study, no patient had faecal incontinence. SIFT-IS involves cutting the internal sphincter, but IS seems to have only a minor effect on internal sphincter function because the cutting length of the IS is less than 1 cm and the depth is very shallow.
Furthermore, IS restores the compliance and distensibility of the fibrotic internal sphincter, which may even be beneficial to anal function. However, the incidence of faecal incontinence based on the medical records may have been underestimated. Validated continence scoring systems and anal manometry are required to accurately evaluate anal competence after SIFT-IS.
Another beneficial characteristic of SIFT-IS is that treatment

CON CLUS ION
SIFT-IS is a novel and promising procedure for treating anal fistula.
We hope that our research will contribute to providing new insights into the incompletely understood aetiology of anal fistula.

AUTH O R CO NTR I B UTI O N S
RS developed the surgical procedure, collected the data and contributed to the design of this study. MK analysed the data and wrote the manuscript. KM collected and interpreted the data and prepared the figures and tables. IK contributed to the interpretation of the results and critical revision of the manuscript.
All authors discussed the results and approved the final version of the manuscript.

ACK N OWLED G EM ENTS
The authors thank Kelly Zammit, BVSc, from Edanz for editing a draft of this manuscript.

CO N FLI C T O F I NTE R E S T
The authors have no conflicts of interest relevant to the content of this manuscript.

DATA AVA I L A B I L I T Y S TAT E M E N T
The data that support the findings of this study are available on request from the corresponding author. The data are not publicly available due to privacy or ethical restrictions.

E TH I C S A PPROVA L
The patients provided written informed consent for the procedure, and the study was approved by the Institutional Review Board of Tokyo Yamate Medical Center.

PATI ENT CO N S ENT S TATEM ENT
Informed consent was obtained from all individual participants included in the study.

PER M I SS I O N TO R EPRO D U CER M ATER I A L FRO M OTH ER SO U RCE S
None.