Anal fistula is one of the most common anal diseases with a prevalence of 2 cases per 10,000 people/year [1]. There are several causes of anal fistula, but 90% of the cases have cryptoglandular origin [2]. In this disease, the anal crypt is blocked by inspissated debris or stool causing an infection in the anal gland, thereby forming abscess and fistula [3].

To date, various methods of surgery for anal fistulas have been introduced. The most common procedures are fistulotomy and fistulectomy [4]. Although these procedures can be performed easily and conveniently, they have the disadvantage of having a high possibility of causing fecal incontinence due to sphincter injury [5]. In particular, in the case of extrasphincteric type fistulas, up to 83% of the patients have been reported to experience incontinence. Therefore, various procedures such as sphincter-preserving surgery are performed to prevent incontinence. One of these is ligation of intersphincteric fistula tract (LIFT), a procedure introduced by Rojanasakul [6] in 2009. A recently published meta-analysis [7] reported that the overall success rate of this procedure was 76.5% and that the rate of complications was 13.9%. Video-assisted anal fistula treatment is another sphincter-preserving technique that was introduced by Meinero and Mori [8]. The recurrence rate following this procedure is reportedly 17.7% and the complication rate is 4.8% [9]. However, the reported success rates of these techniques are inconsistent, and their long-term outcomes have not been sufficiently studied.

Coring-out fistulectomy is a type of sphincter-preserving procedure that enables accurate resection of the fistula tract alone and thus reduces the possibility of missing a secondary tract. [10] This technique was first described by A. Lewis [10] who reported that fistula recurrence was noted only in 2 of the 100 patients with low fistula who underwent coring-out fistulectomy between 1972 and 1986. However, only a few cases of this procedure have been reported in the literature [111213]. Therefore, we aimed to report the surgical outcomes of coring-out fistulectomy for cryptoglandular fistula at a single center over a period of 20 years.

This study was approved by the Institutional Review Board (IRB) of Seoul National University Hospital and was performed in accordance with the IRB’s guidelines and regulations (No. 2004-063-1116). The IRB waived the requirement for informed consent because of the retrospective nature of the study. However, written informed consent was obtained for publication of the accompanying images. The work has been reported in line with the STROCSS (Strengthening the Reporting of Cohort, Cross-sectional and Case-control Studies in Surgery) criteria [14].

After excluding patients with inflammatory bowel disease (n = 21), those who were lost to follow up (n = 25), and those who lacked the description of tract information (n = 4), 184 patients were finally included in our study.

The baseline characteristics of the patients are described in Table 1. The average age of the patients was 44 years (range, 16–75 years), and 88.0% were male. The average body mass index was 25.3 kg/m² (range, 16.0–38.7 kg/m²), and 24 (13.0%), 13 (7.1%), and 68 patients (37.0%) underwent operation for recurrent fistula, multiple tracts, and complex type fistula, respectively.

We demonstrated that the healing rate and recurrence rate of coring-out fistulectomy for cryptoglandular fistula were 92.4% and 8.8%, respectively, during an average of 307 days of follow-up. In particular, the healing and recurrence rates for complex cryptoglandular fistula were 83.8% and 22.8%, respectively. Moreover, when the closure was not performed because the internal opening was not detected, the recurrence and nonhealing rates were statistically significantly high with an OR of 4.00 (95% CI, 1.35–11.59).

There have been few studies on coring-out fistulectomy in the literature [1011121317]. In these studies, the healing rate was 74%–95%, and the recurrence rate was 3%–45%. Most of these studies were conducted mainly on complex fistulas, and our results were similar to those reported in these studies. However, these studies, excluding the study by Tobisch et al. [17], have a disadvantage in that the sample size was small.

So far, various sphincter-saving techniques have been published for cryptoglandular fistula. One of the commonly used techniques is LIFT [4]. Since LIFT was introduced by Rojanasakul [6] in 2009, the surgical outcomes have been reported from several studies. However, it is difficult to use it as a standard technique because, in the existing literature, this procedure was used mostly to treat transsphincteric fistulas and was rarely performed to treat intersphincteric or supra- and extrasphincteric fistulas. Moreover, there have only been 2 randomized control studies, and only the midterm success rate has been reported [7].

Most importantly, the advantage of coring-out fistulectomy is that it can accurately excise the primary fistula tract by lowering the probability of missing a secondary tract and thereby confirming the relationship between the external sphincter and track and obtaining a complete specimen [18]. However, in the case of a recurrent or severe complex fistula, after coring-out fistulectomy, simple anatomical closure may not be properly achieved due to a large defect and excessive scarring. Therefore, other sphincter-preserving methods might be additionally performed [19].

The recurrence rate of complex fistulas (22.8%) was higher than that of simple fistulas (1.8%), and the healing rate of complex fistulas was lower (83.8%) than that of simple fistulas (97.4%) after coring-out fistulectomy. In particular, among the complex types, suprasphincteric or extrasphincteric type showed a high frequency of nonhealing or recurrence. Therefore, if a fistula surgery is planned for such patients, a sufficient explanation is needed, and subsequently, attention should be paid in the follow-up period.

Fistulotomy can be easily used for a simple cryptoglandular fistula, and the frequency of recurrence after the operation has been reported to be 4%–7% [5]. Moreover, since the fecal incontinence frequency after surgery has been reported to be 0%–64% [2021], sphincter-preserving is necessary even for a simple fistula surgery. Therefore, according to the results of our study, saving the sphincter using coring-out fistulectomy can be an alternative for the treatment of simple fistula.

As reported in our study, complex fistula is a fistula type that is difficult to handle because of its higher recurrence rate and lower healing rate than those of simple fistula [2]. In complex fistulas, it is important to prevent fecal incontinence. Therefore, various techniques for this purpose have been reported. If unacceptable incontinence is expected after simple fistulotomy, a technique using advancement flap can be performed, but the success rate has been reported to be only 60% [22]. Alternatively, fibrin glue or fibrin plug is used through a fistula tract, but this cannot be applied to all complex fistulas because of the variable success rates [22]. Among the recently published surgical techniques, LIFT has commonly been used in complex fistulas. However, only 2 randomized control trials have been reported, and the majority of the literature reported only midterm success rates. Thus, its use is limited to a standard technique for complex fistula treatment [7]. Our study showed that the recurrence rate was 21.1% and the healing rate was 83.8% in the subgroup analysis of complex fistulas, which is noninferior to the 76.5% success rate reported in the meta-analysis by Emile et al. [7] as well as the results of existing literature on coring-out fistulectomy [11121317].

In our study, it was confirmed that recurrence was significantly increased in the group that did not undergo internal opening closure because no internal opening was detected. Previous studies have already reported that the recurrence rate may increase if internal openings are not found during surgery [2324]. One hypothesis is that the probability of recurrence increases because the original source of sepsis is not removed if the internal opening is not found [25]. Therefore, it is necessary to find and resolve the internal opening during anal fistula surgery.

Surgical treatment for anal fistula remains a challenge for surgeons dealing with the colorectum and anus [22]. Accordingly, various surgical techniques have been developed; however, coring-out fistulectomy as a simple and traditional method can be easily performed by anyone and can achieve similar results compared to other techniques. However, as it was difficult to detect internal openings in 31.5% of patients in our study, the detection of internal openings during surgery remains challenging. Moreover, in the case of complex fistulas, the results were similar to those of the meta-analysis of LIFT [7] and video-assisted anal fistula treatment [9]; however, the development of new surgical methods that can easily be performed by anyone and can increase the treatment success rate are necessary.

This study has some limitations. First, there was a possibility of bias because this was a retrospective study carried out at a single institution. Second, the follow-up period was relatively short (about 1 year). Nevertheless, our study is meaningful in that it summarized the experiences of a long period (about 20 years), and the analysis was conducted with a relatively large number of samples. Third, incontinence was not accurately evaluated using manometry or a certified questionnaire but was evaluated based on the patients’ self-reported symptoms. However, all patients were asked questions during the follow-up period to confirm the presence or absence of subjective symptoms. Fourth, we did not perform magnetic resonance imaging or anal endosonography. However, in most patients, the path of the fistula tract was expected as a preoperative examination. As seen in our results, it was uncommon to have a deep complex fistula tract (12.5%) such as a suprasphincteric or extrasphincteric tract. The lack of such imaging data is attributed to the cost-effectiveness of performing magnetic resonance imaging or anorectal sonography in Korea as anorectal sonography and magnetic resonance imaging were not covered by the Korean National Health Insurance Service until 2019.

In conclusion, coring-out fistulectomy is a simple and feasible technique applicable to both simple and complex fistulas. In particular, it is important to detect the internal opening and core-out to the internal opening. Furthermore, the possibility of recurrence and nonhealing after surgery for complex fistulas should be considered. However, due to the limitations of a retrospective study design conducted at a single institution, it is necessary to confirm long-term outcomes in a future multicenter study.