Mechanical Bowel Preparation Prior To Sphincter-Sparing Surgery For Complex Anal Fistulae

The goal of this clinical trial is to learn whether bowel preparation with Plenvu before surgery improves healing of complex anal fistula in adults aged 18 to 64 years with a fistula of cryptoglandular origin or associated with Crohn's disease. The main question it aims to answer is: \- Does bowel preparation before surgery increase the proportion of participants whose fistula is clinically healed 6 months after surgery? Researchers will compare participants who prepare the bowel before surgery with participants who receive no bowel preparation. All participants receive the same oral antibiotic regimen and one of three standard sphincter-preserving operations: ligation of the intersphincteric fistula tract (LIFT), autologous platelet-rich fibrin (Obsidian), or endorectal advancement flap. The operation is chosen individually by the surgeon as the most appropriate for each fistula, before randomisation. Participants will: * Be randomly assigned to bowel preparation or no bowel preparation * Undergo one fistula operation within the study * Attend follow-up visits 6 and 12 months after surgery, including clinical examination and 3D endoanal ultrasound at 12 months after surgery * Complete questionnaires on anal continence and quality of life

Inclusion Criteria: * Age 18 to 64 years * Complex anal fistula of cryptoglandular origin or associated with Crohn's disease * Fistula suitable for repair with ligation of the intersphincteric fistula tract (LIFT), autologous platelet-rich fibrin (Obsidian) or endorectal advancement flap * Patients with or without previous attempts at fistula closure * Signed written informed consent Exclusion Criteria: * Patients aged \< 18 years. * Pregnant patients. * Fistulae with aetiologies other than cryptoglandular or Crohn's disease. * Complex fistulae associated with cavities and/or secondary tracts. * Patients with fistulae who underwent faecal diversion before fistula surgery. * Patients with ASA classification of IV or higher. * Patients with severe conditions such as renal failure that predisposes to dehydration or hydroelectrolytic disorders. * Patients who have previously experienced allergic reactions to NER1006 or any of its chemical components.