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

Anal Fistula, Rectal Fistula, Crohn Disease

mechanical bowel preparation, fistula-in-ano, LIFT, endorectal advancement flap, platelet-rich fibrin, endoanal ultrasound

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

Background Surgery for anal fistula aims to close the fistula while preserving continence, and healing rates after sphincter-preserving techniques vary widely. Current guidelines (ESCP 2024) give no recommendation on bowel preparation before anal fistula surgery, and no randomised evidence exists on its effect on fistula healing. The hypothesis is that emptying the bowel before surgery delays the first postoperative bowel movement and reduces faecal contamination of, and mechanical strain on, the repair during early healing. Design Single-centre, open-label, randomised controlled trial with two parallel arms and 1:1 allocation. The surgical technique (LIFT, autologous platelet-rich fibrin \[Obsidian\] or endorectal advancement flap) is chosen individually by the surgeon as the most appropriate for each fistula, and documented before randomisation. Allocation is computer-generated using permuted blocks stratified by surgical technique. Each participant undergoes one fistula operation within the study. Interventions Experimental arm: mechanical bowel preparation with Plenvu, two doses taken the evening before surgery. Control arm: no bowel preparation. Both arms receive a single oral dose of trimethoprim/sulfamethoxazole 160/800 mg and a single oral dose of metronidazole 1.2 g before surgery, and otherwise identical perioperative care. Follow-up Participants are assessed at baseline and at 6 and 12 months after surgery. Baseline data include age, sex, height, weight, body mass index, smoking, diabetes, cardiovascular, pulmonary, renal and chronic inflammatory disease, albumin, haemoglobin, C-reactive protein, ongoing immunosuppressive treatment, fistula aetiology, Parks classification and previous fistula surgery. For participants with Crohn's disease, the Montreal classification and the Perianal Disease Activity Index (PDAI) are recorded. Blinding Participants and surgeons are not blinded. Clinical healing is assessed by the surgical team, who are aware of allocation. Sample size A total of 116 participants (58 per arm) is planned to detect an absolute difference of 25 percentage points in clinical healing at 6 months, with an alpha of 0.05 and a power of 80%, allowing for 10% loss to follow-up. Statistical analysis Dichotomous Variables (Healing Rates): The overall proportion of healed patients between groups will be compared using a Chi-square test. A multivariable logistic regression model will be used to adjust for the surgery type stratum. The treatment effect will be reported as an odds ratio (OR) alongside 95% confidence intervals. Continuous Variables (Score Changes): Changes in continuous scores from baseline will be evaluated using a multiple linear regression model. The model will compare the mean differences between the two treatment groups while adjusting for baseline scores and the stratum of the surgery type. If the assumptions of linear regression are strictly violated, appropriate data transformations or nonparametric alternatives will be applied. Missing data: To strictly adhere to the intention-to-treat (ITT) principle, all randomised participants will be included in the final analysis in their originally assigned groups. To handle missing data, Multiple Imputation (MI) may be utilised as the primary analytical approach, but a complete case analysis will also be conducted. The imputation model would include the stratification variable (surgery type), baseline scores, and available intermediate follow-up data.

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