A fistula is an abnormal tunnel that forms between two body cavities or between an internal cavity and the skin surface. The signs depend on where the fistula develops, but the most common type people search about is an anal fistula, which typically causes persistent pain near the anus, drainage of pus or blood from a small opening in the skin, and recurring infections that seem to heal and then flare up again. Other types, like vaginal or vascular fistulas, have distinct signs of their own.
Symptoms of an Anal Fistula
Anal fistulas account for most fistula-related searches, and they produce a recognizable pattern of symptoms. The hallmark is pain near the anus that’s often intense and throbbing. It tends to worsen when you sit down, have a bowel movement, or cough. The skin around the anus may be swollen, red, and sensitive to touch.
The other major sign is drainage. You may notice pus, blood, or fecal matter leaking from a small spot on the skin near your anus. This drainage can have a noticeable odor. The external opening where fluid escapes is usually red and inflamed, and it can appear anywhere from right next to the anal opening to an inch or two away, depending on the type of fistula.
What makes fistulas particularly frustrating is their cyclical nature. The opening may close on its own, trapping fluid inside the tunnel. Pressure builds, pain and swelling return, and eventually the opening breaks through again to release drainage. This repeating cycle of apparent healing followed by a painful flare-up is one of the strongest clues that you’re dealing with a fistula rather than a simple skin issue. Some people mistake this pattern for recurring abscesses, which is understandable since most anal fistulas actually begin as an abscess that doesn’t fully heal.
Signs of a Vaginal Fistula
A vesicovaginal fistula, which connects the bladder to the vagina, presents very differently from an anal fistula. The classic symptom is continuous, uncontrollable leakage of urine from the vagina. This isn’t the occasional leak you might experience with stress incontinence. It’s a steady, constant flow that soaks through heavy pads and requires frequent changing throughout the day.
This type of fistula most commonly appears 7 to 12 days after pelvic or gynecological surgery. The fluid may have a urine-like color and smell. Doctors can confirm the leakage is urine by testing the fluid’s chemical composition. One diagnostic method involves placing a tampon in the vagina after filling the bladder with blue dye. If the tip of the tampon turns blue, urine is leaking through a fistula between the bladder and vagina.
Signs of a Vascular Fistula
An arteriovenous fistula forms when an artery connects directly to a vein, bypassing the normal capillary network. These can develop after trauma, surgery, or sometimes form on their own. The telltale signs are physical sensations you can hear or feel. A “bruit” is an audible whooshing or buzzing sound caused by turbulent blood flow through the abnormal connection. A “thrill” is a vibration you can feel when you place your fingers over the area.
If the fistula is near your head or neck, you may hear pulsatile tinnitus, a rhythmic whooshing in your ear that matches your heartbeat. Vascular fistulas can also cause swelling, warmth over the affected area, and visible enlarged veins nearby.
Who Is Most at Risk
Anal fistulas most commonly develop from perianal abscesses. When an abscess near the anus drains (either on its own or through surgery), the tunnel it creates sometimes doesn’t close, leaving behind a fistula. People with Crohn’s disease face a significantly higher risk: up to 50% of people with Crohn’s will develop a fistula at some point in their lifetime. These fistulas tend to be more complex, with branching tunnels and multiple openings.
Other risk factors include a history of radiation therapy to the pelvic area, previous anorectal surgery, and infections like tuberculosis or sexually transmitted infections that affect the area. Vaginal fistulas are most often linked to pelvic surgery, particularly hysterectomy, and in some parts of the world, to complicated childbirth.
What a Physical Exam Reveals
A doctor can often identify a fistula during a physical examination. For anal fistulas, the external opening is usually visible as a small, red, inflamed spot on the skin near the anus. Pressing gently on the area may produce drainage. The doctor may also be able to feel the cord-like tunnel running beneath the skin. A digital rectal exam can sometimes locate the internal opening inside the anal canal.
For vaginal fistulas, a speculum exam may reveal the internal opening. For vascular fistulas, listening with a stethoscope can detect the characteristic bruit.
How Fistulas Are Confirmed With Imaging
While many fistulas can be diagnosed on physical exam alone, imaging helps map the full extent of the tunnel, which is critical for planning treatment. This is especially important for complex fistulas with multiple branches or deep tracts that aren’t visible from the surface.
Pelvic MRI is considered the gold standard for imaging anal fistulas. It maps the tunnel’s path, identifies any hidden branches the surgeon can’t feel during an exam, and distinguishes between active infection and scar tissue from old inflammation. MRI detects fistulas with a sensitivity between 81% and 100%, and it’s particularly good at identifying horseshoe-shaped fistulas that wrap around the anus, with accuracy rates of 97% to 100%.
Endoanal ultrasound is another option, using a small rotating probe inserted into the anal canal. It provides detailed images of the anal muscles and surrounding tissue. Studies report sensitivity of about 92% and accuracy of 93% compared with surgical findings. Sometimes hydrogen peroxide is injected into the external opening during the ultrasound to light up the tunnel on the screen, making it easier to trace. The main limitation is that it doesn’t image deep or high fistula tracts as well as MRI does.
An older technique called fistulography, where contrast dye is injected into the external opening and X-rays are taken, has largely been replaced by MRI and ultrasound for anal fistulas.
The Pattern That Sets Fistulas Apart
The single most telling feature of a fistula is the cycle of flare and drain. A skin infection or abscess typically heals and stays healed after treatment. A fistula keeps coming back because the tunnel persists even after the surface appears to close. If you’ve had a perianal abscess that was drained but the area keeps swelling, hurting, and leaking weeks or months later, a fistula is the most likely explanation.
Fistulas do not heal on their own in most cases. The tunnel is lined with tissue that prevents it from closing naturally, which is why surgical treatment is almost always necessary. The specific procedure depends on how deep the tunnel runs and how much of the anal muscle it passes through, but the goal is always the same: eliminate the tunnel while preserving normal function.
If you notice persistent or recurring drainage near the anus, continuous vaginal leakage after pelvic surgery, or a palpable vibration with an audible whooshing sound over a blood vessel, these are the patterns worth getting evaluated. A fistula caught early is generally simpler to treat than one that has had time to branch and become more complex.