3. Therapeutic Approaches to CD-Related Fistulae
Perianal fistulizing CD is a particularly challenging form of CD, presenting in up to one-third of the patients 
; in a tiny percentage of cases, it can be the only manifestation of the disease and may precede by several years intestinal manifestations of CD in up to 10% of the patients 
. The presence of fistulae is often associated with an aggressive form of CD, with chronic course and disappointing rates of long-lasting remission 
As a consequence, patients commonly experience a negative impact on quality of life, including intimate and social relationships and a frequent need for hospital admissions and medical observations 
The establishment of biologic therapy has dramatically improved the efficacy of medical treatment of CD fistulae compared to the previous use of traditional immunomodulators, and, at present, anti-TNFα represent the therapy of choice in these patients 
. The effectiveness of biologic therapy basically depends on the capability of these drugs to reduce tissue inflammation, which is the driving mechanism for fistulae development.
Medical therapy alone has demonstrated remission rates around 60%, and its combination with surgery improves reaction, recurrence rate, as well time to recurrence 
Anyway, immunosuppression by anti-TNF agents needs to evaluate the presence of abscesses (and the possibility to resolve them by drainage and antibiotics), due to their potential septic complications.
The use of anti-TNFα has also been studied as to its local/topic injections, with the aim to potentiate the efficacy on fistula healing 
. Although several reports have shown promising results, this technique has not been standardized yet, therefore it can be intended as a supportive tool in case other approaches have failed or are not available.
The use of biologics other than anti-TNFα (ustekinumab and vedolizumab) is not currently recommended as first-line therapy and should be considered only in case of contraindications to anti-TNFα 
The closure of the fistula tract can be attempted by using different techniques, either endoscopic or surgical, and materials including fibrine glue, plugs, and n-butyl-2-cyanoacrylate (Histoacryl). Among these, fibrin glue injection is the most common technique, with a good safety profile and limited costs, although penalized by limited efficacy 
. The insertion of fistula plugs has also been tested; the procedure consists of the application of a bio-absorbable xenograft which should promote tissue regeneration and fistula closure. This technique has demonstrated a success rate equal to seton drainage 
, hence it is not recommended.
Considering endoscopic techniques, fistula closure can be achieved by clipping with either through-the-scope or over-the-scope clips 
. Clipping is effective in fistula closure, being a safe and simple procedure in the hands of trained endoscopists.
The advancement flap is probably the most used among the surgical techniques. The procedure was first developed for treating cryptoglandular fistulae but is now routinely applied also in CD patients. Fistula healing is complete in about half of the patients 
Of comparable efficacy, ligation of the intersphincteric fistula tract (LIFT), which was also initially described in the treatment of cryptoglandular fistulae and has been then transferred to CD associated fistulae 
. Consisting of the opening of the intersphincteric groove, dissection, and isolation of the fistulous tract, ligation of the tract with interrupted sutures and closure of the perianal wound LIFT is a demanding procedure and should always be performed by dedicated surgeons. Compared to advanced flap procedure, LIFT has demonstrated lower incontinence rates (7.8% versus 1.6%) 
In order to preserve the sphincter functionality, the video-assisted anal fistula treatment (VAAFT) consists of the video-assisted inspection of the fistula followed by a precise cauterization of the fistula tract from the external towards the internal margin, and closure of the internal opening. Although not yet routinely applied in CD fistula surgery, this technique appears promising, especially for the benefits of sphincteric preservation 
. Similar to VAAFT, the fistula laser closure technique applies laser instead of electrocautery and is not performed under direct vision. This technique has similar efficacy to VAAFT but shorter learning curves 
Overall, surgery for CD fistulae should always be performed by expert operators in high-flow centers after adequate study of the clinical case. Local availability and expertise should guide the choice of the technique.
The limited success rate of combined medical and surgical therapy, although being slightly improved, has promoted the research of novel methods. One of the most promising is the injection in the fistula tract of mesenchymal stem cells (MSCs), aimed at tissue regeneration using a minimally invasive procedure 
MSCs are non-hematopoietic multipotent cells, which can be set apart from connective tissues, like adipose tissue, and from the bone marrow. These cells have been studied in fistula treatment due to their immunomodulatory, immunosuppressive, and regenerative properties 
. Their use as treatment of CD fistulae has been described by Panés et al. in the ADMIRE trial 
with promising results in terms of efficacy and safety, which paved the way to their entrance also in the last ECCO guidelines 
The progressive in-depth analysis on the pathogenetic mechanisms of CD fistulae has allowed hypothesizing new promising therapeutic tools, such as anti-MMP antibodies. Studies about anti-MMP drugs start from the assumption that MMP-9, a type IV collagenase, has a central role in tissue remodeling and is upregulated in crypt abscesses and around fistulae 
. A study by Fontani et al. 
showed in vitro that N-acetylcysteine and curcumin were able to downregulate MMP-3 in high oxidative state conditions, and specifically in TNFα stimulated cells, suggesting that such antioxidants may have a therapeutic use for the prevention and treatment of fistulae in the gut of CD patients.
Another study by Goffin et al. 
was conducted in vitro from human specimens and in mice xenograft, confirming in the patients affected by penetrating CD the upregulation of MMP-9, and showing in mice a protective effect of anti-MMP antibodies with respect to intestinal fibrosis. Albeit in literature only animal- and in vitro studies are available, the future application of such molecules could revolutionize the treatment of perianal fistulae.
The understanding of the crucial role of inflammation in fistula development has sustained the still limited but promising application of hyperbaric oxygen therapy as supportive treatment in patients affected by perianal CD 
. The treatment consists of breathing 100% oxygen under increased atmospheric pressure, provoking tissue hyperoxygenation and oxidative stress which has been associated to stem cell mobilization and upregulation of growth factors and ultimately to anti-inflammatory effects. Considering its safety and limited costs if the equipment is available, this treatment appears a valid supportive method for patients with otherwise unsatisfactory healing