1. What is a Crohn’s disease anal fistula? Anal fistula is an abnormal passage between the anal canal and the skin, and is a common perianal complication of Crohn’s disease. About 1/4 to 1/3 of patients with Crohn’s disease have a complication of anal fistula, and about 1/3 of them will have recurrent attacks, which seriously affects the quality of life of patients. Anal fistulas can occur at any point in the course of the disease, and about 10% of patients have an anal fistula as the first manifestation, even years before bowel symptoms. The high location of Crohn’s fistulas, with multiple fistulas and multiple internal openings, makes them more likely to persist and their presentation more complex than that of common fistulas. In addition to infection of the anal canal glands, the pathogenesis is also related to ulcer penetration, immunity and genetics. 2.What are the examination methods for Crohn’s disease fistula? The most commonly used local examination methods are finger examination (physical examination), anorectal ultrasound, anal canal/pelvic magnetic resonance imaging (MRI), and finger examination under preoperative anesthesia. When the doctor sees the patient, a physical examination is routinely performed, which includes a perianal examination. If leakage from the external orifice of the anal fistula with localized pressure is found, it is indicative of a symptomatic (active) anal fistula. An accurate understanding of the anatomy of the fistula will help in surgical treatment and evaluation of prognosis. There are three main diagnostic methods for fistula anatomy: pelvic/anal magnetic resonance imaging (MRI), endorectal ultrasound and preoperative finger examination under anesthesia. The diagnostic accuracy of MRI, endorectal ultrasound, and preoperative anesthesia fingerprinting has been shown to be 87%, 91%, and 91%, respectively, while the combination of any two of these methods results in 100% accuracy. MRI has high tissue resolution, shows fistulas and internal orifices well, is non-invasive and radiation-free, and is the examination of choice for current clinical workup. No bowel cleansing is required prior to the examination, but evacuation of rectal contents with an open plug is required. To perform the examination, it is required that the bladder is moderately full after half an hour of urinary evacuation. The advantage of endorectal ultrasound is that it is simple, inexpensive, and quick, but it is less accurate in judging very complex anal fistula structures. Also, it may be difficult to perform in patients with severe perianal lesions due to pain. It also requires evacuation of the rectal contents before the examination. For patients who need surgery, preoperative anal finger examination under anesthesia can confirm what is seen on imaging and may also detect fistulas and abscesses that are missed on imaging, thus minimizing the possibility of leaving untreated lesions. 3. How is Crohn’s fistula treated? The traditional treatment of Crohn’s disease fistula is dominated by surgery, but it is ineffective and prone to recurrence after surgery. In recent years, it has been recognized that it is the control of Crohn’s disease that is the primary means of treating anal fistula. Medical drug therapy is the treatment strategy for Crohn’s disease fistulas based on adjuvant surgical treatment. The choice of surgical procedure for the treatment of Crohn’s disease fistulas depends on the type and location of the fistula and the presence of active proctitis. Asymptomatic fistulas do not require treatment. Surgical incision and drainage is necessary in the presence of an abscess formation. Simple low-grade fistulas without active proctitis may be considered for fistulotomy or excision, but attention should be paid to postoperative wound non-union. The most common procedure for complex fistulas is non-incisional ligature, and the goal of treatment is to provide symptomatic relief with unobstructed drainage and create conditions for Crohn’s disease medication and natural healing of the fistula. In cases of severe symptoms, high and complex fistulas (especially in those with deep abscesses that cannot be drained), active rectal inflammation, and refractory cases, temporary ileostomy or colostomy may be considered to allow fecal diversion to facilitate rectal mucosal healing and fistula closure. Medication plays an important role in maintaining remission of the fistula. Glucocorticoids or mesalazine are effective for intestinal Crohn’s disease but have no effect on the fistula itself. Antibiotics such as metronidazole and ciprofloxacin help improve anal fistula symptoms and promote healing. Thiopurines have a slow onset of action and are mainly used as maintenance medication, requiring close monitoring for the occurrence of drug side effects. Biologics can promote healing of anal fistulas more quickly, but whether the long-term efficacy differs from that of azathioprine is still being studied.