A fistula is an abnormal tunnel that forms between two body parts that aren’t normally connected, and its appearance depends entirely on where it develops. The most commonly searched type, an anal fistula, looks like a small hole in the skin near the anus that may ooze pus, blood, or other fluid. But fistulas can also appear on the gums, on the forearm (for dialysis patients), or form internally where they can’t be seen at all. Here’s what each type looks like and what to watch for.
Anal Fistula: A Small Hole Near the Anus
An anal fistula is the outermost portion of a tunnel that connects to an abscess or infection inside the anal canal. On the skin surface, it appears as a small opening near the anus. The hole itself can be quite small, sometimes only a few millimeters across, and the surrounding skin may have raised, thickened tissue that has built up over time as the tract developed.
The most telling feature isn’t the hole itself but what comes out of it. Drainage from an anal fistula can range from clear or slightly blood-tinged fluid to thick yellowish pus or even fecal matter, depending on how deep the internal connection runs. You might notice this fluid staining your underwear, or you might see it when pressing gently on the skin around the opening. The skin surrounding the hole is often red, swollen, or tender to the touch.
Some anal fistulas have periods where the opening appears to close and heal over, only to swell up again as fluid builds behind the sealed surface. This cycle of swelling, draining, and temporary relief is one of the most recognizable patterns. When the area is actively infected, it can look like a small boil or pimple that eventually ruptures and drains on its own.
Dental Fistula: A Pimple on the Gums
A dental fistula forms when infection from a tooth root creates a drainage path through the jawbone and gum tissue. The visible sign is a small, pimple-like bump on the gum, typically right above or below the infected tooth. This bump is called a parulis, and it marks the spot where the internal tunnel reaches the surface.
The bump is usually white, yellow, or reddish and may be anywhere from a few millimeters to roughly the size of a pea. It can burst on its own, releasing a salty or foul-tasting fluid into your mouth, then refill and swell again days or weeks later. In rare cases, the drainage tunnel exits through the skin of the face or neck rather than inside the mouth, appearing as a persistent, draining sore on the cheek or jaw that doesn’t heal with typical skin treatments.
Dialysis Fistula: A Raised, Buzzing Vein
Unlike other fistulas caused by infection or injury, an arteriovenous (AV) fistula is created surgically in the forearm or upper arm for kidney dialysis patients. It connects an artery directly to a vein, causing the vein to enlarge over several weeks so it can handle repeated needle access during dialysis.
A healthy AV fistula looks like a visibly swollen, raised vein running along the forearm. The vein may appear darker or more prominent than surrounding veins and often has a noticeable bulge at the connection point. If you place your fingertips lightly over it, you can feel a continuous buzzing vibration called a “thrill,” which signals that blood is flowing properly through the connection. A strong, distinct heartbeat pulsation felt over the fistula, rather than a gentle buzz, can actually signal a problem with blood flow downstream.
Warning signs that something is wrong include redness, warmth, swelling, or drainage around the fistula site. These suggest infection and need prompt attention, since losing a functioning dialysis fistula is a serious setback.
Internal Fistulas You Can’t See
Several types of fistulas form entirely inside the body, making them invisible from the outside. You know they’re there because of their symptoms, not their appearance.
A rectovaginal fistula creates a tunnel between the rectum and the vagina. There’s no external hole to see, but the connection allows gas and stool to pass into the vagina. The primary signs are foul-smelling vaginal discharge, gas escaping from the vagina, pain during intercourse, and sometimes rectal or vaginal bleeding. These fistulas most commonly develop after childbirth injuries, pelvic surgery, or inflammatory bowel disease.
Other internal fistulas can form between loops of intestine, between the bladder and bowel, or between the stomach and colon. These are typically discovered through imaging rather than visual inspection. On an MRI, a fistula tract shows up as a bright line of fluid running through tissue where no channel should exist. Doctors can also identify areas of active inflammation and small fluid collections along the tract, which helps them map the full extent of the tunnel before planning treatment.
How Fistulas Change Over Time
Fistulas rarely look the same from week to week. An active fistula with ongoing infection tends to be swollen, red, and draining. During quieter periods, the external opening can shrink, flatten, and appear nearly healed. This doesn’t mean the internal tract has closed. The underlying tunnel typically persists, and the cycle restarts when drainage builds up again and has nowhere to go.
Over months or years, the skin around a chronic fistula opening often becomes thickened and scarred. The raised, firm tissue surrounding the hole is a sign that the body has been repeatedly trying to wall off and heal the area without success. Long-standing fistulas may also develop secondary openings, creating multiple small holes in the skin that all connect to the same internal tract or abscess. This is more common with complex anal fistulas associated with Crohn’s disease.
The color and consistency of drainage is one of the most useful indicators of what’s happening inside. Clear or slightly pink fluid suggests a relatively calm tract. Thick, yellow or green pus points to active infection. Brown or fecal-colored drainage from an anal fistula means the tunnel has a direct connection to the bowel. Any sudden change in the amount, color, or smell of drainage is worth noting, as it often reflects a shift in the underlying condition.