What is the most common complication of a fistula? Understanding the risks

September 30, 2025 •

4 min read

Up to 50% of anal abscesses lead to the formation of a fistula, and for these common cases, the most frequent issue is persistent infection, resulting in the recurrence of painful abscesses. The inability of the abnormal passage to close properly creates a cycle of infection and re-infection that often requires medical intervention.

Quick Summary

Recurrent infection, often leading to a cycle of recurring abscesses, is the most common complication for the majority of fistulas, particularly anal ones. Other serious risks can arise depending on the fistula's location and underlying cause.

Key Points

Recurrent abscess: For the most common type, an anal fistula, the primary complication is a persistent infection that leads to a recurring cycle of abscess formation.
Sepsis risk: Fistulas can lead to systemic infections (sepsis) if the infection spreads beyond the local area, which is a particular danger with enterocutaneous fistulas.
Heart failure: Large arteriovenous (AV) fistulas can cause a dangerous increase in the heart's workload, potentially leading to heart failure over time.
Nutritional issues: Enterocutaneous fistulas that leak intestinal contents through the skin can cause severe malnutrition and electrolyte imbalances due to poor nutrient absorption.
Post-surgical recurrence: Even after surgery, fistulas have a significant recurrence rate, influenced by factors like the fistula's complexity and underlying medical conditions.
Incontinence risk: Surgical treatment for complex anal fistulas carries a risk of damaging the sphincter muscles, which can result in fecal incontinence.
Malignancy: In very rare cases, a long-standing, chronic fistula with persistent inflammation may increase the risk of developing anal cancer.

A fistula is an abnormal, tube-like connection that forms between two organs or vessels that do not normally connect. While fistulas can occur in many parts of the body, such as between an artery and a vein (arteriovenous or AV fistula) or between the intestines and skin (enterocutaneous fistula), the most common type is an anal fistula. The majority of anal fistulas begin with an infection of the anal glands, leading to an abscess, which is a pocket of pus. If this abscess does not heal completely or drains improperly, it can leave behind a channel, or fistula.

The Most Common Complication: Recurrent Infection and Abscess

For anal fistulas, the most prevalent complication is a persistent, cyclical infection that leads to recurrent abscess formation. This happens because the fistula tract, or tunnel, keeps the initial infection active. While the external opening may appear to heal and close, the underlying infection remains. This causes a recurring cycle of pain and swelling as the abscess refills with pus until it eventually reopens to drain.

The Cycle of Infection: An infected anal gland can block and form a painful abscess. This abscess drains, either spontaneously or surgically, but a fistula tract can be left behind. If this tract does not completely heal, it provides a channel for bacteria to persist, causing the abscess to reform and the cycle to repeat.
Challenges in Treatment: The high rate of recurrence poses a significant challenge for both patients and surgeons. Even with surgery, failure rates can range significantly, and many patients may require multiple procedures. Factors influencing recurrence include the fistula's complexity, underlying conditions like Crohn's disease, and the surgical technique used.

Other Significant Complications

While recurring infection is the most common issue for anal fistulas, other types of fistulas can lead to different, and sometimes more severe, complications. It's crucial to understand that complications vary widely depending on the fistula's location and its underlying cause.

Systemic Infection (Sepsis)

Sepsis is a serious and potentially life-threatening complication that can develop from an overwhelming immune system response to an infection. While most perianal infections remain localized, a systemic infection can occur, leading to low blood pressure, organ damage, and in severe cases, death. This is a particular risk with enterocutaneous fistulas.

Heart Failure

Large, untreated arteriovenous (AV) fistulas—connections between an artery and a vein—are most notably associated with heart complications. Blood flows much more quickly and at higher volume through the fistula, forcing the heart to pump harder to maintain proper blood pressure. Over time, this extra strain can lead to an enlarged heart and, eventually, heart failure.

Malnutrition and Electrolyte Abnormalities

Enterocutaneous fistulas connect the intestine to the skin, causing enteric contents to leak out through an opening other than the anus. This leakage can result in significant fluid and electrolyte imbalances and malnutrition, as the body is unable to properly absorb nutrients. Managing these complications is a critical part of treating enterocutaneous fistulas.

Fecal Incontinence

For anal fistulas, especially complex ones, a risk of surgery is damaging the anal sphincter muscles, which are responsible for bowel control. In some cases, this can result in fecal incontinence, causing an inability to control the passing of gas or stool. The risk of incontinence depends heavily on the amount of sphincter muscle involved and the surgical technique used.

Comparison of Fistula Types and Their Complications


Feature	Anal Fistula (Most Common)	Arteriovenous (AV) Fistula	Enterocutaneous Fistula
Most Common Cause	Perianal abscess	Congenital or injury; sometimes surgically created	Post-surgical complication; inflammatory bowel disease
Most Common Complication	Recurrent abscess and persistent infection	Heart failure (for large fistulas)	Sepsis, malnutrition, fluid imbalance
Other Noteworthy Risks	Fecal incontinence, cancer (rarely)	Blood clots, leg pain, stroke	Bleeding, intestinal failure
Typical Presentation	Pain, swelling, drainage of pus/blood near anus	Bulging veins, swelling, decreased blood pressure	Leakage of intestinal content through skin opening

Conclusion

The most common complication of a fistula, particularly an anal fistula, is the persistence of the underlying infection, which frequently leads to recurring abscesses. This can result in a challenging cycle of pain, swelling, and drainage. However, it's essential to recognize that the most serious or prevalent complication depends on the fistula's specific location and cause. While anal fistulas carry the risk of recurring infection and potential incontinence, other types like AV fistulas can lead to heart failure, and enterocutaneous fistulas can cause severe systemic problems like sepsis and malnutrition. Accurate diagnosis and appropriate treatment, often requiring surgery, are crucial to prevent these complications and achieve a permanent cure. For more information, the American Society of Colon and Rectal Surgeons provides extensive resources on anal fistulas and their treatment.

Factors Contributing to Recurrence and Non-Healing

Several factors can increase the risk of a fistula not healing properly and recurring, requiring further medical intervention.

Undetected Complexities: Fistulas can have multiple tracts or extensions that are missed during initial treatment. Failure to identify and treat all parts of the tract is a leading cause of recurrence.
Underlying Medical Conditions: Chronic inflammatory diseases, most notably Crohn's disease, can cause fistulas that are more resistant to healing and have a higher chance of recurrence.
Compromised Immune System: Patients with a compromised immune system, due to conditions like diabetes or the use of immunosuppressive drugs, may experience impaired healing.
Inadequate Surgical Technique: If the surgery fails to completely eliminate the infected tract or properly address the internal opening, the fistula may persist or reappear.
Poor Postoperative Care: Failure to maintain proper hygiene and follow postoperative instructions can hinder the healing process.

Recurrent fistulas pose a significant challenge because they are often associated with a higher risk of re-recurrence and potential damage to the anal sphincters, affecting continence. Therefore, a thorough assessment using imaging techniques like MRI or ultrasound is crucial before planning treatment for a recurrent fistula.

Frequently Asked Questions

Fistulas often recur because the underlying infection or abscess is not completely eliminated. If any part of the fistula tract remains, or if underlying issues like Crohn's disease are not managed, the tract can reopen and cause a new infection cycle.

Yes, a fistula can potentially lead to sepsis, a life-threatening systemic infection. This risk is higher with fistulas that involve deep or abdominal areas, such as enterocutaneous fistulas.

Diagnosing a recurrent anal fistula often involves a review of symptoms and a physical exam, possibly including examination under anesthesia. To map the tract and check for missed branches, advanced imaging such as MRI or endoanal ultrasonography is often used.

The risk of fecal incontinence from anal fistula surgery depends on the complexity and location of the fistula, as well as the surgical technique used. It is a concern when the fistula involves a significant portion of the anal sphincter muscles.

While some minor anal fistulas may appear to heal on their own, the underlying tract often persists. Spontaneous healing is rare, and without proper medical or surgical intervention, the fistula will likely recur.

Signs of a recurring fistula are similar to the initial symptoms, including pain, swelling, and a foul-smelling discharge of pus, blood, or stool from an opening near the anus. The cycle of pain relief followed by recurring symptoms is a strong indicator.

If a large arteriovenous fistula is not treated, it can lead to serious complications including heart failure, blood clots (deep vein thrombosis), leg pain due to lack of blood flow, or internal bleeding.