This is a pro- and retrospective cohort study to evaluate the natural history and outcome of various treatment options of patients with fistula. At each clinical visit parameters describing activity of a fistula, potential underlying Crohn's disease, investigations as well as the current treatment will be entered into a database. Selected patients can also be contacted by phone in the form of a structured interview. Data will be analyzed to determine treatment outcomes, characterize the natural history as well as risk factors for complications of treatment and an unfavorable disease course. The same data will be obtained and analyzed retrospectively from patients who visited the clinic prior to January 2017.

Fistulizing disease remains a frequent and severe problem for patients with Crohn's disease (CD). Life-time prevalence of fistula for patients with CD has been estimated to be 17 - 50% [1-5] with a cumulative incidence of 33% and 50% after 10 years and 20 years disease duration, respectively. These numbers have been confirmed by population-based studies [6, 7].

Patients with CD and fistula have a decreased quality of life. Reasons include perianal pain, discharge from the vagina, abdominal wall, perianal region and urinary tract infections. Fistula are frequently associated with abscess formation [8]. Unfortunately, surgical and medical treatment options for fistula for CD patients remain limited. Treatment with the tumor-necrosis factor (TNF) antibody infliximab remains the best medical treatment option available: At least temporary fistula closure is observed for 55% of treated fistula (compared to 12% for placebo). However, treatment success is frequently limited to a short time period of 3 month [9, 10]: 34% of all fistula patients experience recurrent fistula despite therapy [6] and successful treatment is limited to one third patients. For these reasons, up to 82% of all fistula need surgical treatment. Surgical options include non-cutting setons, application of fibrin glue as a "fistula plug", ligation of the intersphincteric fistula tract (LIFT) and reparative surgery including mucosal advancement flaps. However, surgical therapy for fistula patients can be complicated by slow wound healing, recurrent disease and additional problems [11].

Combined medical and surgical treatment for patients with fistula and CD can significantly improve clinical outcomes [12-15]. However, the best combination of clinical and surgical methods has yet to be determined [16]. Open questions include timing of seton removal and outcomes of the various surgical procedures for different patient groups.

At the University Hospital Zurich (USZ) the investigators recently started a combined surgical and gastroenterological clinic for patients with fistula. In the new clinic patients are seen by gastroenterologists and surgeons and the investigators are aiming for joint treatment decisions. Due to this combination of expertise and improved access to gastroenterological and surgical resources the investigators are expecting better treatment results for the patients and improved satisfaction for patients and referring physicians.

With the current study the investigators want to achieve the following objectives:

• Continuous monitoring of the quality of medical and surgical treatment
• Determining the outcome of various fistula therapies and calculating risk factors for a favorable and unfavorable outcome
• Understanding the natural history of perianal fistula and calculate risk factors for a favorable and unfavorable outcome.

The investigators are therefore proposing a pro- and retrospective database containing data regarding the clinical history, patient symptoms, examinations, lab values as well as other parameters. The database will be used for quality management regarding the daily work at USZ and to improve understanding regarding treatment options and the natural history of fistula. The investigators expect significant benefits for future patients from their analysis.

Inclusion Criteria:

• Fistula
• written informed consent

Exclusion Criteria:

• age under 18
• retrospective analysis: documented rejection of clinical research

Patients with fistulae