An anal fistula is an abnormal tunnel that forms between the inside of the anus and the skin around it, usually as a result of an infection. This condition affects thousands of people each year and can cause persistent discomfort, drainage, and repeated infections if left untreated. Understanding how anal fistulas develop, their symptoms, and available treatment options can help those affected make informed decisions about their care.

What Is an Anal Fistula?

An anal fistula is a small tunnel that develops between the anal canal, which is the end part of the intestinal tract where stool exits the body, and the skin near the opening of the anus. This passageway should not exist under normal circumstances. The fistula typically starts in the upper part of the anus, where small glands are located between the internal and external anal sphincter muscles, which are the muscles that control bowel movements.^[1]

The tunnel usually has two openings: an internal one inside the anal canal and an external one visible on the skin near the anus. The external opening may look like a hole in the skin and often oozes fluid, including pus, blood, or sometimes even stool. This drainage happens because the fistula connects the infected area inside to the outside, creating a pathway for discharge.^[1]

Fistulas are classified based on how they pass through the sphincter muscles. Some are superficial and stay outside these muscles, while others, called transsphincteric fistulas, pass through them. Complex fistulas may branch and twist in multiple directions, making them more challenging to treat.^[5]

Epidemiology: Who Gets Anal Fistulas?

Anal fistulas are relatively common conditions affecting the anorectal region. They occur twice as often in men as in women, making males more susceptible to this condition. The typical age range for developing an anal fistula is between 30 and 50 years old, though people of any age can be affected.^[1]

About half of all people who develop an anal abscess, which is a pocket of pus caused by infection near the anus, will go on to develop a fistula. In fact, infected anal glands that form abscesses are responsible for approximately 75% of all anal fistulas. This strong connection between abscesses and fistulas means that anyone who has had an anal abscess faces a significant risk of developing a fistula afterward.^[1]

As many as 50% of people with an abscess eventually get a fistula, even after proper treatment of the initial infection. This high rate of progression highlights the importance of careful monitoring and follow-up care after an abscess has been drained.^[4]

Causes: How Anal Fistulas Form

The vast majority of anal fistulas develop as a complication of an anal abscess. Inside the anus are small glands that produce mucus to help with bowel movements. These glands have openings into the anal canal. When one of these glands becomes blocked, bacteria can accumulate and cause an infection. The body responds to this infection by forming a collection of pus, creating an abscess.^[4]

When an abscess forms, the pressure from the accumulated pus needs to be released. The abscess may drain on its own, creating a pathway through the tissues, or a healthcare provider may surgically drain it. Either way, once the pus has drained, a tunnel or tract may remain behind. This tunnel is the fistula. It connects the original site of infection inside the anal canal to an opening on the skin outside.^[2]

Less commonly, anal fistulas can develop from other causes. Inflammatory bowel diseases, particularly Crohn’s disease, can lead to fistula formation because these conditions cause ongoing inflammation in the digestive tract. People with Crohn’s disease affecting the intestines or rectum are at higher risk of developing perianal fistulas.^[1]

Other uncommon causes include sexually transmitted infections affecting the anal area, tuberculosis involving the anus, traumatic injuries to the anal region, complications from previous anal surgery, and radiation therapy for cancer in the pelvic area. Certain rare conditions such as actinomycosis, a bacterial infection that causes abscesses, and hidradenitis suppurativa, a chronic skin disease that produces abscesses in sweat glands, can occasionally affect the perianal region and lead to fistula formation.^[1]

Risk Factors: Who Is Most at Risk?

Several factors increase a person’s likelihood of developing an anal fistula. Being male is a significant risk factor, as men develop fistulas twice as often as women. Having a current or previous anal abscess dramatically increases risk, since about half of all abscesses lead to fistulas.^[1]

People with inflammatory bowel diseases, especially Crohn’s disease, face a much higher risk. The chronic inflammation characteristic of these conditions can affect the tissues around the anus and rectum, making fistula formation more likely. Similarly, individuals with diverticulitis, which is an infection of small pouches that can form in the colon, may develop fistulas as a complication.^[3]

Those with weakened immune systems due to conditions like HIV infection or diabetes may be more susceptible to the infections that lead to abscesses and subsequent fistulas. Patients who have undergone radiation therapy for pelvic cancers have damaged tissues that heal poorly and are more prone to developing fistulas.^[4]

Symptoms: Recognizing an Anal Fistula

The symptoms of an anal fistula can range from mildly annoying to severely disruptive. The most common symptom is anal pain, which is often intense and throbbing. This pain typically worsens when sitting, having a bowel movement, coughing, or even moving around. The area around the anus becomes very sensitive to touch, making everyday activities uncomfortable.^[1]

Swelling and redness around the anal opening are also common. These are signs of active infection and inflammation under the skin, a condition called cellulitis. The inflamed area may feel warm to the touch and appear visibly red and puffy.^[1]

Fluid drainage from an opening near the anus is a hallmark symptom of anal fistulas. This discharge may include pus, which can be thick and foul-smelling, blood, or even small amounts of stool. The drainage often keeps the area around the anus constantly wet and irritated. Some people notice that pressing on the skin near the anus causes more discharge to come out.^[2]

The visible opening of the fistula on the skin may appear as a small hole near the anus. Sometimes this opening closes temporarily, trapping drainage inside. When this happens, pressure builds up, causing increased pain and swelling until the opening reopens to allow drainage again. This cycle of closing and reopening can repeat over weeks or months if the fistula is not treated.^[1]

Less commonly, people with anal fistulas may experience fever and chills, especially if there is active infection. Some report pain when urinating if the fistula is located in a position that affects nearby structures. In certain cases, people have difficulty controlling their bowel movements, experiencing leakage or urgency.^[1]

The persistent discharge and irritation can cause itching and skin breakdown around the anus. The constant moisture and presence of bacteria lead to skin maceration, where the skin becomes soft, damaged, and prone to further irritation. This can create an unpleasant odor and make hygiene challenging.^[5]

Prevention: Reducing Your Risk

While not all anal fistulas can be prevented, several measures can reduce the risk of developing the infections that lead to them. Maintaining good anal hygiene is fundamental. After bowel movements, gently clean the area with water or fragrance-free wipes rather than dry toilet paper, which can be abrasive. Keeping the anal area clean and dry helps prevent bacterial growth that could lead to gland infections.^[3]

Preventing constipation is important because straining during bowel movements can damage the anal lining and contribute to the blockage of anal glands. Eating a diet rich in fiber, including plenty of fruits, vegetables, and whole grains, helps keep stools soft and easy to pass. Drinking adequate amounts of water, typically eight to ten glasses daily, also promotes regular, comfortable bowel movements.^[13]

Avoiding prolonged sitting on the toilet and not straining excessively during bowel movements reduces pressure on the anal area. When you feel the urge to have a bowel movement, respond promptly rather than delaying, as waiting can make stools harder and more difficult to pass.^[18]

For people with inflammatory bowel diseases like Crohn’s disease, working closely with healthcare providers to manage the underlying condition is crucial. Keeping inflammation under control with appropriate medications and lifestyle modifications can reduce the risk of perianal complications, including fistulas.^[14]

If you develop an anal abscess, seeking prompt medical treatment is essential. Proper drainage and care of an abscess may reduce, though not eliminate, the chance of it progressing to a fistula. Following all post-treatment instructions carefully, including completing any prescribed antibiotics and maintaining proper wound care, gives the best chance of complete healing.^[4]

Pathophysiology: What Happens in the Body

The development of an anal fistula involves a series of events that begin with the normal anatomy of the anal canal. The anus contains small glands located in the space between the internal and external sphincter muscles. These glands produce mucus and drain into the anal canal through tiny openings called crypts.^[5]

When an anal gland becomes clogged, bacteria that are normally present in stool can enter and multiply within the blocked gland. This bacterial invasion triggers an immune response. White blood cells rush to the area to fight the infection, and the accumulation of dead bacteria, white blood cells, and tissue debris forms pus. As more pus accumulates, it creates an abscess, a walled-off pocket of infection in the space between the sphincter muscles.^[4]

The body naturally tries to drain this abscess. The pus may work its way through the tissues, creating a path of least resistance from the infected gland through the anal wall and eventually to the skin surface near the anus. This process essentially bores a tunnel through the tissue. Once the abscess drains, either spontaneously or with medical help, a tract lined with inflammatory tissue remains. This tract is the fistula.^[2]

The lining of the fistula tract consists of granulation tissue, which forms as part of the body’s healing response to chronic inflammation. However, because the fistula connects the inside of the anal canal, where bacteria are constantly present, to the outside, the tract remains continuously infected. This ongoing contamination prevents the fistula from healing on its own. The body cannot close the tunnel as long as bacteria keep entering from the anal canal.^[5]

If the external opening of the fistula temporarily closes while the internal opening remains open, new drainage from the anal canal accumulates in the tract, essentially re-creating an abscess. This causes pain and swelling until the pressure forces the external opening to reopen and drain. This cycle can repeat indefinitely without surgical intervention.^[1]

In people with Crohn’s disease, the pathophysiology is somewhat different. The chronic inflammation characteristic of this condition can directly damage the tissues around the anus and rectum. Inflamed, weakened tissues are more prone to forming abnormal connections. Fistulas in Crohn’s disease may be more complex, with multiple branches, and are more likely to recur even after treatment.^[14]