An anal fistula typically looks like a small opening or hole in the skin near the anus, often surrounded by red or inflamed tissue, that oozes pus, blood, or stool. It can be surprisingly subtle, sometimes no larger than a pinhead, or it can present as a raised mound of tissue with visible drainage. What you see on the outside is only part of the picture: beneath the skin, a narrow tunnel connects that external opening to the inside of the anal canal.
The External Opening
The most visible sign of an anal fistula is a small opening on the skin somewhere around the anus. It often appears as a tiny hole or dimple, sometimes with a ring of reddened, irritated skin around it. In well-established fistulas, the tissue around this opening tends to look heaped up or slightly raised, almost like a small bump or nodule. The surrounding skin may appear swollen, pink, or darker than the skin nearby due to chronic irritation.
This external opening is where drainage escapes. You may notice pus (whitish or yellowish fluid), blood, or even small amounts of stool on your underwear or when wiping. The drainage often has a noticeable smell. It can be constant or come and go. Some fistulas close temporarily on their own, so the opening may look sealed over for days or weeks before reopening and draining again.
If you or a doctor presses on the area between the external opening and the anus, you may be able to feel a firm, cord-like structure under the skin. That’s the fistula tract itself, the tunnel running beneath the surface.
What’s Happening Beneath the Skin
The visible opening is just one end of a tunnel that runs from the skin surface into the anal canal. Inside, the tunnel is lined with a type of healing tissue called granulation tissue, which the body produces in response to ongoing infection or irritation. This lining is what prevents the tunnel from closing on its own in most cases.
The internal opening, where the tunnel connects to the anal canal, sits at or just above a ridge called the dentate line, roughly 1 to 2 centimeters inside the anus. During a clinical exam, a doctor can sometimes feel this internal opening as a small pit or depression. In about 72% of cases, this internal opening sits at the back (posterior) midline of the anal canal. In roughly 20% of cases, it’s at the front (anterior).
Most fistulas follow a relatively straight path between these two openings. The tunnel may pass through or between the ring-shaped muscles that control the anus (the sphincters). The depth and path of the tunnel determine how it’s classified and treated, but from the outside, a deep fistula can look identical to a shallow one.
How It Differs From an Abscess or Hemorrhoid
A perianal abscess, which is often the precursor to a fistula, looks quite different. An abscess appears as a swollen, tender lump near the anus that’s warm to the touch, often bright red, and usually very painful. It tends to come on quickly and may cause fever. There’s no open drainage hole because the infection is still contained under the skin.
A fistula, by contrast, is what sometimes remains after an abscess drains (either on its own or through surgery). The acute swelling and redness resolve, but a persistent opening with intermittent drainage stays behind. The pain is generally less intense than an abscess, though sitting and bowel movements can still be uncomfortable.
Hemorrhoids look different still. External hemorrhoids appear as soft, bluish or skin-colored lumps right at the anal opening. They may bleed, but they don’t produce pus or fecal drainage, and there’s no tunnel or cord-like structure beneath the skin.
Signs of Worsening Infection
A fistula can flare up if the tunnel becomes blocked and a new abscess forms along its path. When this happens, you’ll notice increasing redness that spreads outward from the opening, more significant swelling, escalating pain, and possibly fever. The drainage may change from a slow ooze to a more sudden, larger release of pus if the abscess bursts. Spreading redness and warmth across the surrounding skin suggest the infection is extending beyond the fistula tract and needs prompt medical attention.
Simple vs. Complex Fistulas
A simple fistula has one external opening, one internal opening, and a single tunnel that runs a relatively shallow course through the sphincter muscles. This is the most common type and is usually straightforward to identify on exam.
Complex fistulas look and behave differently. They may have multiple external openings scattered around the anus, each draining independently. The tunnels can branch, curve, or form a horseshoe-shaped path that wraps partially around the anus beneath the skin. Complex fistulas are more common in people with Crohn’s disease or other inflammatory bowel conditions. If you notice drainage coming from more than one spot, or from an opening that seems far from the anus (on the buttock, thigh, or even near the vagina), that suggests a more complicated fistula network.
What Imaging Reveals
Because much of a fistula is hidden beneath the skin, imaging often provides the most complete picture. On an MRI, an active fistula tract appears as a bright line on certain scan sequences, reflecting the fluid and inflammation inside the tunnel. The tissue surrounding the tract may also appear bright due to swelling. If an abscess has formed along the tract, it shows up as a bright pocket of fluid with an inflamed rim around it.
An inactive or healing fistula looks darker on imaging because the inflammation and fluid have subsided. This distinction matters because a fistula can look closed on the skin surface while the underlying tunnel remains open, which is why recurrence is common. In one large study, roughly 21% of surgically treated fistulas either persisted or came back, with a recurrence rate of about 13% over a median follow-up of nearly two years.
What to Watch For at Home
If you’re checking yourself, the key signs of a fistula are a small, persistent or recurring opening near the anus that produces drainage. The opening may be easy to miss when it’s not actively draining. Look for staining on underwear (yellowish, brownish, or blood-tinged), skin irritation that doesn’t resolve with normal hygiene, and a tender cord-like area you can feel under the skin when you press gently between the opening and the anus. Pain that worsens with sitting or bowel movements, combined with any visible drainage, is a strong signal that what you’re seeing is a fistula rather than a simple skin irritation.