An anal fissure does not directly transform into a fistula, but a fissure can set off a chain of events that leads to one. The key intermediate step is infection. When a fissure doesn’t heal properly, bacteria can invade the wound and infect the small glands inside the anal canal, forming an abscess. That abscess, if it drains incompletely, can burrow a tunnel from inside the anal canal out to the skin near the anus. That tunnel is a fistula.
This progression is uncommon for simple fissures that heal on their own or with basic treatment. But understanding how it happens, what raises the risk, and what warning signs to watch for can help you catch problems early.
How a Fissure Can Lead to a Fistula
A fissure is a shallow tear in the lining of the anal canal. It’s a surface wound. A fistula, by contrast, is an abnormal tunnel that connects the inside of the anal canal to the skin outside. These are fundamentally different problems, and one doesn’t simply “become” the other. Instead, the path from fissure to fistula involves an infection that develops in between.
Here’s the sequence. The anal canal contains small glands that open into tiny pockets called crypts along the inner wall. Normally these glands drain without issue. But when a fissure creates an open wound nearby, bacteria can reach these glands more easily. If a gland becomes blocked and infected, pus collects and forms a perianal abscess. The abscess grows by spreading along the tissue planes around the sphincter muscles. If that abscess eventually drains (either on its own or through a surgical procedure), the drainage path can persist as a permanent tunnel, a fistula, connecting the infected gland inside to an opening on the skin outside.
So the critical link in this chain is the abscess. Without an abscess forming first, a fissure won’t produce a fistula. Not every fissure leads to an abscess, and not every abscess leads to a fistula, but the risk exists when infection takes hold and isn’t fully resolved.
Who Is Most at Risk
Most acute fissures heal within a few weeks with conservative care like fiber supplementation, sitz baths, and stool softeners. The people most vulnerable to complications are those whose fissures become chronic (lasting longer than 6 to 8 weeks) or who have underlying conditions that impair healing.
Crohn’s disease is the biggest risk factor. Perianal fistulas affect between 17% and 50% of people with Crohn’s, depending on disease severity and location. Crohn’s causes chronic inflammation that weakens the tissue of the anal canal, making it far more susceptible to deep infection and abnormal tunneling. People with Crohn’s affecting the colon or both the colon and small intestine are at particularly high risk. The chronic, relapsing nature of the disease means that what starts as a simple fissure can become a recurring problem that eventually progresses to abscess and fistula formation.
Other factors that raise your risk include a weakened immune system, diabetes, repeated trauma to the anal area from chronic constipation or diarrhea, and any prior perianal abscess that wasn’t completely drained.
How to Tell a Fissure From a Fistula
Fissures and fistulas both cause anal pain, but the quality of that pain and the accompanying symptoms differ in ways you can recognize.
A fissure typically causes sharp, cutting pain during a bowel movement, often described as passing broken glass. You may see bright red blood on the toilet paper. The pain can linger for minutes to hours afterward but tends to follow a predictable pattern tied to bowel movements.
A fistula produces a deeper, throbbing pain that you feel when sitting, coughing, or pooping. The hallmark sign is drainage: fluid leaking from a spot near (but not inside) the anus. This fluid can be pus, blood, or foul-smelling discharge. You may also notice swelling, redness, and skin that’s warm and tender to the touch, all signs of active infection beneath the surface. Some fistulas go through cycles where the outer opening closes temporarily, pressure builds, and pain increases until it reopens and drains again.
If your symptoms shift from sharp pain with bleeding during bowel movements to persistent throbbing with drainage between bowel movements, that’s a signal something deeper is going on.
How Fistulas Are Diagnosed
A doctor can often identify a fistula during a physical exam by spotting the external opening on the skin and feeling the tunnel beneath. But imaging is frequently needed to map the full path of the tunnel before treatment, especially when the fistula is complex or involves the sphincter muscles.
The two main imaging tools are endoanal ultrasound and MRI. Ultrasound uses a small probe inserted into the anal canal and is particularly good at detecting fistulas that pass between or through the sphincter muscles, with an overall accuracy of about 82%. MRI doesn’t require insertion and is better at identifying fistulas that extend higher up into the pelvic tissues. In a study of 126 patients, ultrasound was more sensitive overall at detecting fistulas and abscesses, while MRI had an edge for deeper, more complex tracts. Your doctor will choose based on what they suspect about the fistula’s location and complexity.
Treating the Fissure to Prevent Complications
The most effective way to prevent a fissure from ever progressing toward abscess and fistula is to heal it early. Most acute fissures respond to conservative treatment: increasing fiber and water intake to soften stools, taking warm sitz baths to relax the sphincter muscle, and using topical treatments that improve blood flow to the wound. These measures resolve the majority of fissures within several weeks.
When a fissure becomes chronic and stops responding to conservative care, a minor surgical procedure called a lateral internal sphincterotomy is the standard next step. This involves making a small cut in the internal sphincter muscle to reduce the spasm that keeps the fissure from healing. It’s highly effective, but like any procedure near the anal canal, it carries a small risk of complications. In about 3% of cases, wound-related issues like bleeding, abscess, or even fistula formation can occur after the surgery itself.
If an abscess has already formed, it needs to be drained promptly. Incomplete drainage is one of the most common reasons an abscess progresses to a fistula. Surgeons typically make the drainage incision as close to the anal opening as possible specifically to minimize the length of any fistula tract that might develop afterward.
What Happens if a Fistula Does Form
Fistulas rarely heal on their own. Once a tunnel has formed and lined itself with tissue, it tends to persist, cycling between periods of drainage and painful swelling. Treatment almost always involves surgery, though the type depends on how the tunnel relates to the sphincter muscles that control bowel continence.
Simpler fistulas that involve only a small amount of muscle can be laid open in a single procedure, allowing them to heal from the inside out over several weeks. More complex fistulas, particularly those that pass through a significant portion of the sphincter, require more careful approaches to avoid damaging continence. These may involve placing a small thread (called a seton) through the tunnel to keep it draining while it heals gradually, or using tissue flaps to close the internal opening.
Recovery timelines vary. Simple fistula repairs typically heal within 4 to 6 weeks, while complex cases may require staged procedures over several months. Recurrence rates depend on the type and complexity of the fistula, so follow-up care matters.