Managing high complex fistulas is challenging, and no gold standard or satisfactory treatment is available. Conventionally, the procedures associated with a higher success rate had higher risks of incontinence, and sphincter-sparing procedures had a low success rate in high complex fistulas [1]. Several new sphincter-saving procedures, such as the over-the-scope clip (OTSC), anal fistula plug, fistula laser closure (FiLaC), fixcision, and video-assisted anal fistula treatment (VAAFT), have been proposed in the last 2 decades [1]. These procedures are as expensive as they are device dependent. The main intention behind innovating these procedures was to find a sphincter-saving procedure for high fistulas, as fistulotomy was gold standard for low fistulas and offered 95% to 100% healing rates [1]. However, these new sphincter-saving procedures had cure rates of 40% to 70% in low simple fistulas, and no study to date has analyzed their efficacy in a cohort of exclusively high complex fistulas. Against this background, a new procedure, transanal opening of the intersphincteric space (TROPIS), first described in 2017 [2], completely preserves the external anal sphincter (EAS) and has shown highly encouraging results in high complex fistulas (85% to 94% in various studies) [3–7] (Fig. 1).

The TROPIS procedure is a paradigm shift in understanding and managing complex anal fistulas [9, 10]. It is perhaps the first procedure that challenges the current practice of attempting to close the internal opening to heal complex high fistulas, as is done in advancement flaps, VAAFT, anal fistula plugs, OTSC, FiLaC, and so forth. The idea and concept of transanally laying open the fistula tract in the intersphincteric space arouse suspicion and apprehension that widening the internal opening might facilitate the passage of fecal matter into the fistula tract, thereby worsening the disease. However, this does not happen because the fistula tract in the EAS is not widened in the TROPIS procedure. The EAS muscle's tone prevents fecal matter from passing into the fistula tract in most cases. Moreover, the high success rate of TROPIS (85%–93%) in high complex fistulas, even in long-term follow-up, allays this fear.

We published our results in an exclusive cohort of high complex fistulas in 2017 (90.4% success rate in 52 patients) [11], in 2018 (84.6% success rate in an exclusive cohort of 26 supralevator anal fistulas) [2, 12], in 2021 (87.6% in 325 patients, 86.0% in 408 patients) [13], and in 2023 (86.4% in 650 patients) [3]. In 2022, Li et al. [4] reported a success rate of 86.5% in 41 patients with high fistulas, and in 2021, Huang et al. [5] published a success rate of 93.7% in 48 fistula patients. A recent meta-analysis [6] highlighted that of all sphincter-sparing procedures utilized to treat high fistulas, TROPIS has the highest success rate.

The TROPIS procedure has a minimal impact on continence [4–6, 13]. Although the mean incontinence scores deteriorated after TROPIS, these changes were insignificant [4, 13]. The increase in the number of patients with incontinence after TROPIS ranged from 0% [5] to 2% [4, 13], and the reported incontinence was mainly gas and urge incontinence [13]. Even this minor deterioration in continence was corrected and fully compensated by doing Kegel (pelvic floor) exercises in the postoperative period [13]. The reason for this could be that the IAS is responsible for maintaining resting anal pressure (tone), while EAS is mainly responsible for squeezing or peak anal pressure. The slight deterioration in resting anal pressure due to the partial division of the IAS is perhaps compensated by strengthening of EAS by Kegel exercises [13].

The routine drainage of high intersphincteric and supralevator abscesses through the transanal route was described 5 decades ago. Still, it had never been advocated for the definitive management of abscesses or fistulas. The routine utilization of TROPIS in the definitive management of high complex fistulas was first described by us in 2017 [11]. The TROPIS procedure has been demonstrated to have a high success rate in high transsphincteric, suprasphincteric (Figs. 2, 3), supralevator, and horseshoe fistulas (Figs. 6, 7). It has also been shown to be effective for the definitive single-stage management of fistulas associated with acute anorectal abscesses [13].

The sphincter complex (IAS and EAS) has different dimensions in different positions of the anal canal. The sphincter complex and intersphincteric plane are quite well-developed posteriorly, slightly less developed laterally, and least developed anteriorly. The sphincter complex in the anterior anal region in female patients is even more attenuated. Therefore, while performing the TROPIS procedure in anterior fistulas, caution must be exercised to avoid damaging the EAS. It needs to be kept in mind that due to the poorly developed intersphincteric space, anterior fistulas in females are more often totally transsphincteric and have minimal or no intersphincteric component. Therefore, the TROPIS procedure may not be possible in many anterior fistulas, especially in female patients.

Few procedures similar to the TROPIS procedure have been described in the past. However, there are significant differences between them and the TROPIS procedure. Parks [7] described a procedure that involved the limited division of the lower half of the IAS to drain the infected anal gland, in addition to coring out or curetting the outer portion of the fistula tract. This procedure was replicated by Sumikoshi et al. [8] and was further developed by Tsuji et al. [14]. A possible shortcoming of Parks procedure was limiting the internal sphincterotomy to below the dentate line. Cases in which an infected anal gland or part of its duct is located at a higher plane in the intersphincteric space might be missed, leading to a high risk of recurrence. In the TROPIS procedure, the internal anal sphincterotomy is extended as high as the tract goes up in the intersphincteric space to ensure eradication of the origin of infection. There is also a conceptual difference between the 2 procedures. The primary aim of Parks procedure was to remove the infected anal gland, whereas in the TROPIS procedure, the aim is both to remove the infected anal gland and to lay open the fistula tract in the intersphincteric space, allowing it to heal by secondary intention. Deroofing the fistula tract in the intersphincteric space is perhaps the easiest way to tackle the sepsis there. In Parks procedure, the infected anal gland is removed, but the sepsis in the intersphincteric space is not completely eradicated, as in intersphincteric horseshoe fistulas. This residual sepsis would increase the chances of recurrence. Therefore, the internal sphincterotomy is higher and wide in the TROPIS procedure than in Parks procedure. In Hanley procedure, the EAS is divided, unlike the TROPIS procedure. Goligher [15] developed a procedure that combined the advantages of Parks procedure and Hanley procedure, but as in Parks procedure, the sepsis in the intersphincteric space (other than the primary infected gland) was not taken care of. Ligation of the intersphincteric tract (LIFT) deals with the infected anal gland and the sepsis in the intersphincteric space. However, unlike the TROPIS procedure in which healing in the infected intersphincteric fistula tract is allowed to occur by secondary intention, the intersphincteric space is allowed to close by primary intention in the LIFT procedure. It is well-known that in the presence of infection, healing by secondary intention yields better results than healing by primary intention.

TROPIS has a higher success rate than LIFT (85%–93% vs. 40%–92%), although LIFT has been more widely utilized and published than TROPIS. Only time and more experience from across the globe will confirm which procedure is better, although both have advantages and disadvantages. LIFT preserves the IAS and thus does not pose any risk of incontinence. However, LIFT is technically difficult, has a longer learning curve, and is difficult to perform in high intersphincteric, suprasphincteric, and intersphincteric horseshoe fistulas. Nonetheless, both these sphincter-preserving procedures are complementary and are likely to be essential parts of the surgeon’s armamentarium to manage high complex anal fistulas.