Study design and population

The study was a retrospective analysis of prospectively collected data from a cohort from the tertiary referral center (Shuguang Hospital). Consecutive adult patients diagnosed with HHAF undergoing BEAS technique between June 2020 and January 2021 were included. Ethical approval was obtained from the ethics committee of Shuguang Hospital Affiliated with Shanghai University of Traditional Chinese Medicine (Approval No. 2020-823-30-01). Written informed consent was obtained from each participant. All methods were carried out in accordance with relevant guidelines and regulations.

Magnetic resonance imaging (MRI) was performed on every patient, which helped to determine the extent of the HHAF lesion and its relationship with surrounding tissues. The diagnosis of HHAF was made and confirmed by at least two senior imaging specialists.

The inclusion criteria were the following: (1) male or female patients aged 18 to 65years; and (2) patients diagnosed with high cryptoglandular fistula-in-ano (involving more than one-third of the sphincter complex as assessed on MRI and intraoperative examination under anesthesia). Both primary and recurrent horseshoe fistulas were included. Patients with Crohn's disease, cancer, tuberculosis, diabetes, autoimmune diseases or patients receiving long-term steroids or corticosteroid therapy were excluded.

Patient demographics, clinical information, and short-term clinical outcome data were collected through outpatient follow-up, a WeChat questionnaire and telephone follow-up. Forty-one patients were followed-up by WeChat questionnaire and seven patients were followed-up by phone. There is no difference between these methods. The main outcomes included the 6-month cure rate, Visual Analog Scale pain score (VAS-PS) and Cleveland Clinic Florida incontinence score (CCF-IS). The secondary outcomes included the Quality of Life in Patients with Anal Fistula Questionnaire score (QoLAF-QS), Bristol stool chart and postoperative complications. Postoperative pain was measured using an 11-point Visual Analog Scale pain score (VAS-PS)14. The severity of fecal incontinence symptoms was evaluated using the Cleveland Clinic Florida incontinence score (CCF-IS)15. The Quality of Life in Patients with Anal Fistula Questionnaire score (QoLAF-QS) was used to assess the quality of life of patients with anal fistula16. Stool consistency was assessed using the 7-point Bristol stool scale17. Disease recurrence, as was reported by Mei et al., was defined as persistence or recurrence of symptoms or the relapse of the perianal sepsis within or more than 6months following surgical intervention18,19.

SPSS Statistics 25.0 (IBM Inc., IL, USA) software was used for ststistical analysis. Continuous variables are presented as the meanstandard deviation (SD) or median with interquartile range (IQR) based on distribution. The independent t test was used to compare normally distributed continuous variables, and the MannWhitney U test was used to compare nonnormally distributed continuous variables. Categorical data are expressed as the number of cases and percentages. P<0.05 was considered to indicate a ststistically significant difference.

Preparation for surgery begins with a careful evaluation of preoperative MRI to assess the location of the internal opening and the extent of inflammation as well as the relationship between the fistula and the muscles. The imaging also informs about the anatomical structure of anal canal, aiding in operative planning (Fig.1).

The diagrams of preoperative MRI. (a) The cross section of the perianal structure showing the relationship between IAS, EAS and HHAF. (b) The coronal section of the pelvis showing layers of anal sphincter, especially the levator ani muscle, and HHAF. IAS=internal anal sphincter; EAS=external anal sphincter; HHAF=high horseshoe anal fistula.

The patient is given spinal anesthesia and then placed in prone jackknife position. After preparing and draping, the operating table is placed in a 10 to 15 head-side-down position. This allows the muscles and spaces exposed more clearly in posterior aspect of anal canal during the operation. The internal opening, the external opening and the fistula of HHAF is then identified again to begin dissection (Fig.2).

Anatomic Structure of HHAF. (a) View of the outside appearance. The dotted line represents the scope HHAF. (b) Sagittal section of the pelvis. (c) Schematic diagram of posture for surgical exposure. (d) Preoperative visual field. The green shaded part represents HHAF. HHAF=high horseshoe anal fistula.

The dissection is initiated with a curvilinear incision (IS approach) along the intersphincteric groove to identify the internal anal sphincter (IAS) and external anal sphincter (EAS). This incision is directly behind the anal canal, which is approximately 1/41/3 of a quadrant of the anus. Then, the dissection is performed along the plane of the intersphincteric groove to separate the IAS from EAS with an electrical scalpel. The internal opening should be concerned during the dissections. Through both the anal canal and intersphincteric plane, the internal opening can be identified easily. There is barely no blood supply in the intersphincteric plane, therefore it is a safe dissection plane. However, care should be taken to observe the muscle contraction of EAS during this dissection. Because dissection is close to the IAS and EAS, the surgeon should take care during the dissection to avoid inadvertent injury. To avoid complications of incontinence or bleeding, the surgeon should dissect the IAS and EAS strictly along the plane (Fig.3).

The operation diagram of IS approach and LES approach. (a) View of the outside appearance. (b) Sagittal section of the pelvis. The dissection of IS approach is along the intersphincteric plane to separate the IAS from EAS. (c) IS approach. (d) LES approach. The dissection of LES approach is along the outer edge of the EAS to bare the EAS. IS=Intersphincteric; IAS=internal anal sphincter; EAS=external anal sphincter; LES=Lateral-external-sphincteric.

The next step involves the dissection of the EAS, which is initiated with a curvilinear incision (LES approach) along the outer edge of the EAS on one side behind the anal canal. The dissection is performed along the outer edge of the EAS until above the level of the deep EAS so as to bare the EAS. The lateral part of the EAS in the corresponding quadrant is exposed with the traction of a self-retaining retractor (Lone Star, Cooper Surgical, Trumbull, CT). The highest risk for incontinence, which is the most common postoperative complication, may be due to the injury of EAS. The bareness of EAS can completely expose the infection focus of HHAF. In this process, the surgeon should also be mindful of avoiding the anterior displacement of anal canal caused by the injury of anococcygeal ligament (Fig.3).

Once the IAS and EAS are separated, medial to lateral dissection of the muscles are continued along the intersphincteric plane to both sides. Then, the IAS is separated from EAS by a combination of sharp and blunt dissection. Through the IS approach, the suprasphincter anal fistula can be detected above the level of the deep EAS easily. Cephalad dissection is continued above or beneath the levator ani muscle so that the DPIS and the inner part of the EAS could be completely exposed (Fig.4).

The operation diagram of exposure of DPIS and DPAS. (a) View of the outside appearance. (b) Sagittal section of the pelvis. (c) Exposure of DPIS. (d) Exposure of DPAS. Expose DPIS and DPAS to reach the fistula through IS approach and LES approach, respectively. DPIS=deep intersphincteric space; DPAS=deep postanal space.

Continuing the dissection cephalad with the assist of self-retaining retractor along the LES approach reveals the DPAS, which can then be handled at the top of the infection. Both two approaches communicate at the top of the EAS (or at the top point of the pus cavity of the HHAF). Typically, the visualization of these approaches reveals the pus cavity under direct vision. The aim of these dissections is to utilize both the IS approach and the LES approach as a landmark to ensure a complete preservation of the EAS (Fig.4).

After the DPIS, the DPAS, and the pus cavity are irrigated repeatedly with povidone and hydrogen peroxide, the bare EAS is pushed proximally to confirm that the internal opening on the musculomucosal flap could reach the inferior edge of the EAS without tension. After the musculomucosal flap and the EAS advancement are performed, they are sutured and fixed with 20 Polyglactin suture (Coated VICRYL, 20, ETHICON Inc, China) to close the intersphincteric incision in an interrupted manner. At last, the LES approach is kept open and indwelled with povidone gauze to facilitate postoperative drainage (Fig.5).

The operation diagram of musculomucosal flap and EAS advancement. (a) View of the outside appearance. (b) Sagittal section of the pelvis. (c) Musculomucosal Flap and EAS Advancement. (d) Visual field after suture. Perform advancement of the musculomucosal flap and the EAS to confirm the internal opening could reach the inferior edge of the EAS without tension. Then close the intersphincteric incision (IS approach) in an interrupted manner and keep LES approach. EAS=external anal sphincter; IS=Intersphincteric; LES=Lateral-external-sphincteric.

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