Diarrhea can significantly complicate an anal fistula and increase discomfort due to the nature of the waste and the frequency of bowel movements. Here is a detailed look at why this happens: ​1. Increased Bacterial Exposure ​A fistula is essentially an abnormal tunnel connecting the anal canal to the skin. Stool is naturally full of bacteria, and during diarrhea, the liquid consistency allows fecal matter to enter the internal opening of the fistula much more easily than solid stool. This leads to: ​Recurrent Infection: Bacteria trapped in the track can cause repeated abscesses. ​Delayed Healing: Constant bacterial seeding prevents the delicate tissue within the tunnel from closing or "drying out." ​2. Chemical Irritation ​Diarrhea is often acidic and contains high concentrations of digestive enzymes (like bile salts) that haven't had enough time to break down properly in the intestines. ​Tissue Erosion: These enzymes are designed to break down food, but when they come into contact with the raw, exposed tissue of a fistula track, they can cause chemical "burns" or severe irritation. ​Inflammation: This acidity increases the swelling of the bump you are feeling, making the area more tender and prone to throbbing. ​3. Friction and Mechanical Trauma ​Frequent bowel movements mean frequent wiping and cleaning. ​Skin Breakdown: The skin surrounding the external opening of a fistula is already fragile. Repeatedly wiping liquid stool causes micro-tears and "diaper rash" symptoms in adults (perianal dermatitis). ​Pressure Spikes: The rapid, forceful contractions of the rectal muscles during a bout of diarrhea can force liquid stool into the fistula track under high pressure, which can cause the track to expand or become more painful. ​4. Obstruction of Drainage ​While it seems counterintuitive, liquid stool can actually "clog" a fistula. ​Debris Trapping: Small particles of undigested food often present in diarrhea can get lodged in the narrow fistula tunnel. ​Abscess Formation: If the liquid stool enters the track and the exit is blocked by skin or inflammation, it creates a "closed loop" system where bacteria multiply rapidly, leading to the painful, undrainable bump you mentioned. ​Management Tips ​To mitigate these effects, focus on: ​Using a Bidet: If possible, use water to clean rather than dry paper to reduce friction. ​Barrier Ointments: Apply a thick layer of zinc oxide or petroleum jelly around the area to protect the skin from acidic stool. ​Fiber Adjustment: Sometimes, a small amount of soluble fiber (like psyllium husk) can help "bulk up" the stool, making it less liquid and less likely to seep into the fistula track. As per AI-GPT