The project was initiated in September 2023 and the first introductory video meeting was held on September 14, 2023. At the beginning, 90 identified experts in laser proctology were invited to join the project. Following the second and third invitations, 50 international experts responded, signaling interest to participate, including 21 that were involved in the project until its completion. These 21 panelists from 13 nations constituted the recommendation development group (RDG); five from China, two from Germany, Spain, UK, and Croatia, as well as one participant from Italy, the Netherlands, France, Cypress, Hungary, Slovakia, Montenegro, and Switzerland.

The RDG defined 25 DQs (Q1–Q25) concerning FiLaC which were discussed and commented upon based on the current literature and expert opinion, resulting to the generation of 25 responses (R) or Delphi statements. The 25 DQs and commented statements were voted upon by 21 panelists. All 25 items scored above 75% in the first Delphi round; thus, voting was terminated (Fig. 1).

Fig. 1

Results of the first Delphi round

Q1: What are the minimum requirements in laser surgery and is a workshop attendance mandatory?

Laser surgery in proctology has rapidly evolved and many surgeons across Europa and Asia are adopting this new technique [23]. The need for a formal training in laser surgery has so far not been addressed. It is common sense that the outcome of any surgical procedure largely depends on the expertise of the operating surgeon. This simple rule holds for laser surgery. Although no data has been published on this aspect of FiLaC, the RDG uniformly agrees on the need for a well-defined training program for laser-assisted proctological procedures, especially FiLaC. The RDG is very clear on the fact that an observation alone is not enough and that at least one workshop with optimally hands-on training represents a minimum requirement for surgeons prior to performing FiLaC. Therefore, besides being trained in the management of coloproctological pathologies and having experience in the management of fistula in ano, the completion of at least one FiLaC workshop represents the minimum requirements for performing FiLaC. The RDG encourages mentoring of surgeons during their first cases of FiLaC or direct training with a mentor.

R1: Besides being a trained proctologist, completion of at least one workshop, optimally with hands-on training, represents the minimum requirements prior to performing FiLaC. Active mentoring of the first FiLaC cases is very much encouraged.

Q2: Which laser wavelength should be used for FiLaC?

The current literature is inconsistent about the laser settings for FiLaC. The most used wavelengths currently in laser proctology are 980 nm and 1470 nm. The 980 nm wavelength has been reported in only a few series for FiLaC. Giamundo et al. used the 980 nm wavelength to manage the first eight FiLaC cases in their primary publication on this technique in 2014 and subsequently switched to the 1470 nm wavelength for the same indication [9, 24, 25]. The current literature is vastly homogenous on the use of 1470 nm [9, 25,26,27,28,29,30]. Although these are all retrospective studies, they are all based on the 1470 nm wavelength. Thus, the RDG recommends that FiLaC should be performed with 1470 nm wavelength.

R2: The 1470 nm wavelength is recommended for FiLaC.

Q3: How much power (watts) is appropriate for FiLaC?

The current literature is rather heterogenous with respect to the amount of laser energy used during FiLaC. In the first publication on FiLaC by A. Wilhelm, the power of the 1470 nm emitting laser was set at 13 W [7]. Since then, a variety of power settings have been reported in the literature. Low power of 10 W was used by some authors [31, 32], while other groups reported using 12 W [30, 33,34,35] and 13 W to perform FiLaC [28, 36, 37]. The outcome of FiLaC depends, at least to some degree, on the amount of laser energy used [38]. Low energy on one hand may be insufficient to successfully close the tract. Two studies using 10 W by Stijns et al. and Bhushan and Joshi reported very low healing rates of 20% and 40%, respectively. Very high energy on the other hand leads to carbonization of the epithelium with increased risk of treatment failure. On the basis of the available literature, the RDG unanimously recommends 12 W as the standard power for FiLaC. Alternative power settings may be chosen as needed on the basis of the surgeon’s expertise.

R3: 12 W is recommended for standard FiLaC.

Q4: What kind of fiber is optimal for/in performing FiLaC?

The literature is very homogenous in this regard. All publications on FiLaC reported the use of a 360° emitting fiber. Therefore, the fiber for FiLaC must be a 360° emitting ring-like fiber with light guidance. Various names have been used in the literature to describe this fiber, e.g., radial fiber [39, 40] or bare fiber [41]. FiLaC should be performed under visual guidance via the indicator light of the fiber, irrespective of the method of fiber introduction, i.e., with or without the aid of a seton.

R4: A 360° emitting ring fiber should be used in performing FiLaC.

Q5: What is the role of tactile feedback or ultrasound/MRI images in setting the energy for FiLaC?

There is hardly any data looking at the relationship between findings from preoperative imaging, either by MRI or via endoanal ultrasound (EAU) and the amount of energy used during FiLaC in the current literature. Preoperative imaging may help make the decision on the timing of FiLaC regarding the size of the fistula tract, the presence or absence of accessory tracts and inflammatory collections, supported by clinical judgement [42]. See comment on seton (changing the caliber of the fistula tract into a FiLaC-friendly tract, Q9 and Q10).

Tactile feedback, especially regarding the fiber adhering inside the tract, should not be very relevant in FiLaC. According to expert opinion, the fiber may not always adhere to the wall of the tract. If, however, the fiber gets stuck, withdrawal must be done in a very gentle and careful manner, to prevent leaving back islands of untreated tract secondary to a forceful fiber withdrawal. The same caution should be exercised about re-intubation of the fistula tract. Equally, there is no data on this aspect of FiLaC. Proponents of this maneuver suggest a gentle teasing rather than a forceful re-intubation trial, because of the risk of reopening the sealed tract. The expert opinion is that the energy used in FiLaC should be independent of findings from preoperative imaging and tactile feedback.

R5: The energy used in FiLaC should be independent of findings from preoperative imaging and/or tactile feedback.

Q6: Which type(s) of fistula according to Park’s classification can be treated with FiLaC?

FiLaC belongs to the spectrum of sphincter-preserving techniques for the management of fistula in ano [43,44,45]. Thus, all fistula types that may compromise sphincter function may be managed with FiLaC. Park’s classification may not be an appropriate tool on which to base the indication for FiLaC. From a clinical standpoint, the etiology of the fistula may become a relevant aspect to consider. Thus, the RDG looked at cryptoglandular fistulae, Crohn’s fistula, and rectovaginal fistula. On the basis of this, indications for FiLaC for cryptoglandular fistula may include high fistulae with either a transsphincteric or suprasphincteric course [46]. Besides, cryptoglandular fistulae in female patients independent of fistula type based on Park’s classification, as well as fistulae in patients with prior compromised sphincter function, represent good indications for FiLaC. A huge advantage for FiLaC compared to other sphincter-sparing procedures is that there is hardly any damage to the sphincter following FiLaC, even if the procedure fails. Also, the RDG strongly recommends FiLaC for all forms of perianal Crohn’s fistula [26, 47, 48]. However, in this group, FiLaC should not be performed in patients with active intestinal disease and proctitis.

A good argument for the use of FiLaC in high fistulae, e.g., high transsphincteric (Park’s type 3) and suprasphincteric (Park’s type 4) fistulae, is the possibility of downgrading the fistula tract to a less complex fistula, which can then be easily managed. In the paper published by Wilhelm et al., distalization (downgrading) of the fistula was seen in 50% of cases with treatment failure following FiLaC for high fistulae [29]. This interesting finding was also reported by Giamundo et al. [9, 24]. Irrespective of the type of fistula, the RDG highly recommends ruling out a diverticulum at the apex of the fistula tract in patients with supralevator and suprasphincteric fistulae, which may largely be responsible for treatment failure in such cases.

R6: Cryptoglandular fistulae with a risk of postoperative compromise of the continence, Crohn’s perianal fistulae, and fistulae in female patients represent possible indications for FiLaC.

Q7: What are the contraindications for FiLaC?

Acute perianal inflammation with collections represents an absolute contraindication for FiLaC (see commentary on seton, Q9 and Q10) [49]. Also, simple fistulae, although not formally regarded as a contraindication for FiLaC, have higher healing rates with low risk of sphincter injury following fistulectomy. FiLaC may not work well in cases with active Crohn’s disease, both intestinal inflammation and proctitis. Although there are no publications on the use of FiLaC in rectovaginal fistula, the RDG sees rectovaginal fistulae as a contraindication for FiLaC. The expert opinion is that the probability for success is lower due to the relatively short fistula tract and possibly low tissue density over the anterior aspect of the tract.

R7: Acute cellulitis and inflammatory collections or abscess formation, active Crohn’s, and rectovaginal fistula represent contraindications for FiLaC.

Q8: Could FiLaC be a first-line treatment?

The healing rate and the risk of continence disturbance represent two relevant outcome measures following fistula closure [50]. This is especially true regarding the management of complex or high fistula with involvement of a relevant portion of the sphincter muscle. The current literature suggests that healing rates of FiLaC (ca. 63–75%) are comparable with those of the currently used sphincter-sparing techniques for fistula closure. Moreover, FiLaC shows better results with respect to postoperative complications, especially continence disturbance [51]. Therefore, FiLaC has a place as a first-line option in fistula surgery. This is in line with the Danish publication by Nordholm-Carstensen et al. that reported FiLaC as a first-choice treatment [28].

For simple fistula, where there is hardly any risk of continence disturbance following conventional surgical techniques, lying the fistula open is associated with high healing rates [52, 53]. This outcome is better than the data for FiLaC [8]. Thus, FiLaC may not be a first-line option in the management of such cases. Moreover, factors to be considered in the decision-making, e.g., sex, fistula location (anterior vs. posterior), prior surgeries, and low resting tone, may tip the scale towards FiLaC in some cases with simple fistula. Also, other well-known advantages of FiLaC, e.g., less pain, smaller wound, early return to work, should be discussed with the patient.

R8: FiLaC is one of the sphincter-sparing treatment options for patients with complex fistula and is therefore a first-line treatment option. FiLaC can be offered to selected patients with simple fistula in case of risk factors for postoperative continence disturbance.

Q9: Should a draining seton be used prior to FiLaC and for how long?

Drainage of any collection and management of cellulitis represent the initial treatment for patients presenting with anorectal abscess with or without fistula [54, 55]. The use of a draining seton has been shown to improve the healing rates following FiLaC [9]. A statistically significant higher healing rate was reported by Giamundo et al. in the group with draining seton vs. no draining seton (70.4% vs. 51.5%) [9]. As is the case with other fistula closure procedures, the use of a draining seton is strongly recommended [6]. The duration of the draining seton has not been well defined in the literature. For example, some leave the seton in place for 2 months [28]. The expert opinion is to keep the seton for at least 2 months. Thereafter, the timing of FiLaC could be determined based on both clinical judgement and findings from imaging (MRI and/or EAU).

R9: A draining seton is recommended for at least 2 months. Thereafter, the timing of FiLaC can be decided upon based on clinical judgement and findings from imaging.

Q10: What to do if there is still purulent secretion despite seton placement?

Copious purulent secretion after a reasonable period (e.g., 4–6 weeks) following seton placement should prompt further investigation. Common reasons for this event could be an occlusion of the fistula tracts e.g., by the knot on the seton, if this is too large, or secondary to undrained accessory tracts. Large amounts of secretion in patients with Crohn’s fistula may be suggestive of proctitis [56, 57]. Such patients need an endoscopic evaluation and if needed medical optimization to achieve mucosal healing prior to FiLaC. Patient education and counseling about wound care including documentation (e.g., in a diary) of the amount of secretion from the fistula are recommended. Definitive surgery (FiLaC) should be postponed and the need to perform any kind of imaging should be made following clinical judgement.

R10: FiLaC should be postponed in case of copious purulent secretion.

Q11: What is the role of imaging? Should MRI or EAU always be performed before FiLaC?

The timing of fistula closure has been shown to have a crucial effect on healing rates. This is especially the case regarding the cleanliness of the fistula tract. Traditionally, the absence of pus, associated with minimal secretion, may be indicative of a good time to perform fistula closure. While these clinical judgements remain unquestionable, the use of imaging has evolved as a meaningful adjunct to clinical judgement. Preoperative imaging with either MRI [58, 59] or EAU [60, 61] represents a standard aspect of preoperative workup prior to fistula closure. This is not different in the case of FiLaC (see comments to Q9).

R11: Preoperative imaging either with MRI or EAU is strongly encouraged prior to FiLaC.

Q12: Is preoperative bowel preparation necessary? If so, what kind of preparation?

The need for bowel prepping and the kind of prepping to perform prior to FiLaC have not been systematically analyzed in the current literature. While some authors completely omit bowel prepping [30], using an enema to empty the rectum has been suggested by some surgeons [28, 62]. Opponents to enema see soiling secondary to enema as a problem. However, a clean rectum secondary to prepping is usually of advantage when there is a need to change the procedure, e.g., to advancement flap. According to the RDG, the decision to prep the bowel or not is at the discretion of the individual surgeon because this aspect of the procedure does not affect healing rate.

R12: Bowel prepping prior to FiLaC can be omitted or performed based on surgeon’s discretion.

Q13: What is the role of pudendal block during FiLaC?

Postoperative pain following fistula surgery represents a relevant outcome measure and thus addressing this outcome is of interest. A major advantage of FiLaC in comparison with other surgical techniques is the reduced pain associated with the laser procedure [37]. Many studies in the current literature used either general or spinal anesthesia [27, 63]. In such cases, pudendal block in combination with general and regional anesthesia seems irrational. On the other hand, pudendal block could be a good add on to local anesthesia. The RDG voiced some reservation regarding the use of pudendal block in a potentially infectious condition (despite draining seton), especially looking at the expected low pain level after FiLaC. Therefore, the need for pudendal block should be well chosen and should consider the type of anesthesia for any individual patient.

R13: Pudendal block may be omitted in patients undergoing FiLaC in general and regional anesthesia, and should be considered as an adjunct to local anesthesia or based on surgeon’s expectation of pain.

Q14: What to do when the fistula tract is too wide?

The principle of fistula treatment with FiLaC is based on the use of laser energy to denature the epithelized fistula tract, which then collapses and closes. Therefore, the results of FiLaC largely depend on the amount of energy that gets onto the wall of the fistula tract [24, 31, 38]. The probability that enough laser energy to achieve closure would reach the wall of a large tract is small. Therefore, the risk of failure in such cases may be high. Expert opinion is that the caliber of the fistula tract will usually adapt to that of the draining seton. However, this adaptation may take some time. A persistently large tract should warrant a clinical control and imaging in selected cases. Importantly, the patient should be counseled about the need for prolonged drainage in such cases. According to the RDG downsizing the seton may be a reasonable next step after waiting for about 3–6 months.

R14: A persistently large fistula tract should prompt a downsizing of the draining seton.

Q15: What is the role of cochleation (coring out) and curettage during FiLaC?

Coring out a fistula has been shown to be a very good treatment strategy with high healing rates [64]. However, the involvement of the sphincter limits the extent of cochleation. Coring out the external fistula tract, however, represents an important step in FiLaC. This procedure basically widens the external opening to enhance drainage. Therefore, chochleating (coring out) the external opening is strongly recommended during FiLaC.

Performing a curettage of the fistula tract has been reported by some groups [26, 62]. However, this aspect of the FiLaC procedure is far from being standard. This is also true for fistula tract irrigation [63] as well as the combination of curettage and irrigation [32]. The current literature does not provide sufficient data to support either form of manipulation. The RDG, however, expressed concern about widening the fistula opening secondary to curettage. Furthermore, bleed/blood secondary to curettage may absorb part of the laser energy, thereby reducing the efficacy of the procedure. Should curettage be performed, achieving optimal hemostasis and flushing out clots is strongly recommended by the RDG. Therefore, while coring out the external opening is strongly recommended, irrigation and/or curettage can be performed at surgeon’s discretion.

R15: Coring out (cochleating) the external opening during FiLaC is strongly recommended. Curettage and irrigation can be done at the surgeon’s discretion.

Q16: Can FiLaC be combined with other techniques?

While FiLaC has largely been used as a stand-alone procedure, many studies have reported a combination of FiLaC with other techniques. In the initial publication by Wilhelm et al., FiLaC was done in combination with advancement flap [29]. Since then, the use of some form of flap during FiLaC has also been reported by other authors [32, 63].

Recently, some studies on the combination of FiLaC with video-assisted anal fistula treatment (VAAFT) and ligation of the intersphincteric tract (LIFT) have been published [65, 66]. Some authors indicate the advantage of VAAFT in the visualization of the fistula tract with identification of accessory tracts, which obviously would predispose to treatment failure or recurrence. However, the role of cofounders should always be considered when many different techniques are used simultaneously to manage the same condition. Moreover, it remains questionable to what degree each procedure contributed to healing. Besides, one major advantage of FiLaC, low pain, may be scarified following the addition of other surgical techniques to FiLaC. According to the RDG, adding other procedures to FiLaC, besides closure of the internal opening (see Q17) and widening of the external opening should not be part of the standard FiLaC procedure.

R16: The standard FiLaC is a stand-alone procedure.

Q17: Are the shape and diameter of the internal ostium relevant for closure?

The size and shape of the internal opening have not been widely investigated with regard to FiLaC. An analysis of 51 failed FiLaC cases by De Bonnechose et al. indicated a significantly higher failure rate of 84.6% in 16 cases with wide internal orifice compared to 50.6% in 41 cases with a narrow internal opening [67]. While this finding suggests that a wide internal opening may be a risk factor for failure, its clinical meaning is limited by some possible flaws in the surgical technique used by the authors. First, curettage of the fistula tract, which was performed in this study, may have widened the size of the internal opening. Second, the size of the internal opening was subjectively estimated by the operator, and finally the internal orifice was not closed. The expert opinion is that the shape and size of the internal ostium probably may not affect healing rate as this can be closed using a simple Z-stitch (figure of eight suture). In certain instances, the internal orifice is of such substantial size that achieving closure using a simple Z-stitch may present a significant challenge. This scenario is exemplified by highly inflammatory fistulas in Crohn’s disease with a very large internal orifice. An advancement flap can serve as a viable alternative to the simple Z-stitch, and when employed in conjunction with the FiLaC technique, it can yield optimal results.

R17: The size and shape of the internal option should not influence FiLaC. However, in some cases, a very large internal orifice may require more than a simple stitch.

Q18: Should the internal opening be excised?

Excision of the internal opening during FiLaC has so far not been reported in the literature. According to the RDG excising the internal opening bears a risk of increasing the diameter of the fistula tract at this point as well as bleeding. These two complications may negatively affect the efficacy of FiLaC. Moreover, closure of a wide internal opening following excision may be more challenging.

R18: Excision of the internal opening is not recommended.

Q19: What is the role of closing the internal opening and what is the recommended closure method/technique?

Whether to close the internal opening of the fistula track during FiLaC or not is a topic of discussion. While some authors did not close the internal opening [35], or did so only in individual cases [34], closing the internal opening has been used by many authors as part of the FiLaC procedure [8, 32, 63]. The technique employed in closing the internal orifice, however, has been very heterogenous. While some authors used a simple stitch (Z-stitch or figure of eight suture) [30, 36], flaps were constructed and used to close the internal opening by some others [29, 32].

In a study by Giamundo and De Angelis [9], the healing rate was higher following the closure of the internal opening (74% vs. 66%). Although this aspect of the surgical technique has not been systematically analyzed, closing the internal opening seems to be a relevant determinant for healing. While the RDG acknowledges this finding, closing the internal opening, especially using a flap, represents a definitive closure technique on its own. Therefore, it would be impossible to tell which technique (FiLaC or flap) contributed to what degree to the successful closure. Besides, extensive additive procedures like flaps may marginalize the low pain level following with FiLaC. Expert opinion of the RDG, therefore, is to close the internal opening with a simple stitch.

R19: Closure of the internal opening with a simple stitch is recommended.

Q20: What is the optimal withdrawal speed for FiLaC?

The speed of withdrawal of the bare fiber is equally as crucial as the energy setting of the laser machine. A very high withdrawal speed may result in inlands of untreated tract epithelium while a very slow withdrawal speed may lead to carbonization of the tract. Both extremes may lead to treatment failure. Therefore, the speed of withdrawal is a relevant determinant for healing. Withdrawal rates of 1 mm/s, 1 cm/3 s, and 1 cm/6 s have been reported in the literature [