Perianal Abscess & Anal Fistula: Causes, Symptoms & Treatment

What Are Perianal Abscess and Anal Fistula?

A perianal abscess is an acute, painful collection of pus that forms in the tissue surrounding the anal canal. An anal fistula is a chronic abnormal tunnel connecting the inside of the anal canal to the skin near the anus. These conditions are closely linked - most anal fistulas develop as a complication of a perianal abscess that didn't heal properly.

Understanding the relationship between perianal abscesses and anal fistulas is crucial for effective treatment. The anal canal contains small glands (anal glands) that secrete mucus to help with bowel movements. When one of these glands becomes blocked and infected, the infection can spread into the surrounding tissue, forming an abscess. If the abscess drains but the infected gland tract remains, it can form a fistula - essentially a persistent tunnel through which infection continues to drain.

Perianal abscesses are classified by their location relative to the anal sphincter muscles. The most common type is the perianal abscess, which occurs in the superficial tissue just beneath the skin around the anus. Ischiorectal abscesses occur deeper in the fatty tissue beside the rectum. Intersphincteric abscesses form between the internal and external sphincter muscles. The rarest and most serious type is the supralevator abscess, which occurs above the pelvic floor muscles.

The condition affects approximately 2-3 people per 10,000 annually, with men being affected 2-3 times more often than women. The peak incidence occurs in people aged 30-40 years, though the condition can occur at any age. While anyone can develop a perianal abscess, certain factors increase the risk significantly, including diabetes, inflammatory bowel disease (especially Crohn's disease), immunosuppression, and previous anorectal surgery.

The Connection Between Abscess and Fistula

The relationship between perianal abscesses and anal fistulas follows what's known as the cryptoglandular theory. According to this well-established theory, infection begins in one of the 6-10 anal glands located at the level of the dentate line (the junction between the rectum and anal canal). When an anal gland becomes blocked, bacteria multiply and form an abscess. If this abscess drains (either spontaneously or surgically) but the original infected gland tract persists, a fistula forms.

Research shows that 30-50% of patients who have a perianal abscess drained will develop an anal fistula. This is because simple drainage addresses the acute infection but may not eliminate the underlying source of infection. Some surgeons perform a fistulotomy (opening of the fistula tract) at the time of abscess drainage if a fistula is identified, though this approach remains somewhat controversial for complex cases due to the risk of sphincter damage.

What Are the Symptoms of a Perianal Abscess?

The hallmark symptoms of a perianal abscess include severe, constant throbbing pain near the anus that worsens with sitting, swelling and redness in the perianal area, fever and general malaise, and sometimes difficulty urinating. Pain typically increases over 2-5 days before treatment is sought.

Perianal abscess symptoms develop rapidly, often over just a few days. The pain is characteristically severe, constant, and throbbing in nature. Unlike hemorrhoid pain, which tends to come and go, abscess pain is persistent and progressively worsening. The pain is typically aggravated by sitting, walking, coughing, or having a bowel movement. Many patients describe being unable to find a comfortable position.

The external signs of a perianal abscess depend on its depth. Superficial perianal abscesses typically cause visible swelling, redness, and warmth near the anus. You may be able to see or feel a tender lump. Deeper abscesses (ischiorectal or intersphincteric) may have fewer external signs but cause more severe pain, fever, and systemic symptoms. These deeper abscesses can be more dangerous because they may not be recognized early.

Common Symptoms of Perianal Abscess

• Severe throbbing pain: Constant, intense pain near the anus that worsens over days
• Swelling and tenderness: A painful lump or area of swelling near the anal opening
• Redness and warmth: The skin over the abscess may appear red and feel warm to touch
• Fever and chills: Systemic infection symptoms, especially with larger abscesses
• Difficulty sitting: Pain makes sitting extremely uncomfortable
• Painful bowel movements: Defecation intensifies the pain
• Pus discharge: If the abscess begins to drain spontaneously
• Urinary symptoms: Some patients have difficulty urinating due to pain and swelling

Symptoms of Anal Fistula

Anal fistula symptoms are quite different from abscess symptoms. While an abscess causes acute, severe pain, a fistula typically causes chronic, intermittent symptoms. The most characteristic symptom is persistent drainage from a small opening in the skin near the anus. This drainage may be purulent (pus), bloody, or fecal in nature. The external opening may heal temporarily, only to reopen and drain again - this cycle of healing and reopening is highly suggestive of a fistula.

Other fistula symptoms include:

• Intermittent pain: Milder than abscess pain, often worse when the external opening is closed
• Skin irritation: Chronic drainage can cause perianal skin breakdown and itching
• Recurrent abscesses: The fistula tract can become blocked, causing new abscess formation
• Soiling of underwear: Due to pus or fecal discharge through the fistula

What Causes Perianal Abscess and Anal Fistula?

Most perianal abscesses (approximately 90%) are caused by infection of the anal glands, following the cryptoglandular theory. Other causes include Crohn's disease, trauma, sexually transmitted infections, tuberculosis, and cancer. Risk factors include diabetes, immunosuppression, and inflammatory bowel disease.

The cryptoglandular theory explains the origin of most perianal abscesses and fistulas. The anal canal contains 6-10 small glands that open into the anal crypts at the dentate line. These glands normally secrete mucus to aid in defecation. When a gland becomes blocked (often by fecal matter, foreign material, or trauma), bacteria that normally reside in the anal canal can become trapped and multiply, leading to infection and abscess formation.

The bacteria involved are typically normal intestinal flora, including Escherichia coli, Bacteroides, Enterococcus, and other anaerobic organisms. Because multiple bacterial species are usually involved, these infections are termed "polymicrobial." Understanding the bacterial cause is important because it explains why antibiotics alone cannot cure an established abscess - the pus must be physically drained.

Risk Factors for Perianal Abscess

While anyone can develop a perianal abscess, certain conditions significantly increase the risk:

• Diabetes mellitus: Impaired immune function and poor blood circulation increase infection risk and slow healing
• Inflammatory bowel disease: Crohn's disease, in particular, causes perianal disease in 25-50% of patients
• Immunosuppression: HIV/AIDS, chemotherapy, organ transplant medications, or chronic steroid use
• Anorectal surgery: Previous hemorrhoid surgery or other anal procedures
• Anal fissures: Chronic tears in the anal lining can become infected
• Sexually transmitted infections: Especially in men who have sex with men
• Pregnancy and recent childbirth: Trauma during delivery can predispose to infection

Less Common Causes

While cryptoglandular infection accounts for most cases, other causes should be considered, especially in atypical presentations:

Crohn's disease deserves special mention because it causes perianal complications in 25-50% of patients. Crohn's-related fistulas tend to be complex, with multiple tracts and higher recurrence rates. They often require different treatment approaches, including medical therapy with biologics.

Other causes include:

• Trauma: Foreign body insertion, impalement injuries, or surgical complications
• Infections: Tuberculosis, actinomycosis, lymphogranuloma venereum
• Malignancy: Anal or rectal cancer can present as or cause abscess formation
• Radiation: Pelvic radiation therapy can damage tissue and lead to fistula formation
• Hidradenitis suppurativa: Chronic skin condition affecting the perianal region

When Should You Seek Medical Care?

Seek medical care if you have persistent perianal pain lasting more than 1-2 days, a visible or palpable lump near the anus, fever with perianal symptoms, or any discharge from the perianal area. Seek emergency care immediately for high fever, inability to urinate, rapidly spreading redness, or severe worsening symptoms.

Many people delay seeking care for perianal problems due to embarrassment, but early treatment significantly improves outcomes and reduces complications. A perianal abscess will not resolve on its own - it requires surgical drainage. The longer treatment is delayed, the larger the abscess grows, and the greater the risk of complications including fistula formation, systemic infection, and tissue destruction.

See a healthcare provider promptly if you have:

• Persistent or worsening pain near the anus lasting more than 1-2 days
• A visible or palpable lump or swelling in the perianal area
• Fever along with perianal pain or swelling
• Any discharge of pus, blood, or foul-smelling fluid from near the anus
• Pain that prevents normal activities, sitting, or sleeping
• Recurrent episodes of perianal swelling and drainage (suggests fistula)

How Are Perianal Abscess and Anal Fistula Diagnosed?

Perianal abscess is usually diagnosed by physical examination alone, showing a tender, fluctuant swelling near the anus. Anal fistulas require more detailed evaluation, often including examination under anesthesia (EUA), MRI of the pelvis, or endoanal ultrasound to map the fistula tract and identify the internal opening.

The diagnosis of a perianal abscess is primarily clinical - meaning it's based on the patient's history and physical examination rather than laboratory tests or imaging. The classic presentation of severe, constant perianal pain combined with a tender, fluctuant (soft and compressible) swelling is usually sufficient for diagnosis. However, deeper abscesses may not have obvious external signs and can be more challenging to diagnose.

Physical Examination

The examination for perianal disease includes visual inspection of the perianal area, digital rectal examination (DRE), and sometimes anoscopy (looking inside the anal canal with a small scope). During the examination, the doctor looks for:

• External signs of abscess: redness, swelling, warmth, tenderness
• Fluctuance: a soft, compressible feel indicating pus collection
• External fistula openings: small holes in the skin that may discharge pus
• Internal fistula openings: felt during digital rectal exam or seen on anoscopy
• Associated conditions: hemorrhoids, fissures, skin tags

Imaging Studies

For straightforward superficial abscesses, imaging is usually unnecessary. However, imaging becomes important for:

MRI of the pelvis is considered the gold standard for evaluating anal fistulas. It provides excellent soft tissue detail and can accurately map the fistula tract, identify secondary extensions, and show the relationship to the sphincter muscles. MRI has a sensitivity of 90-97% for detecting fistulas.

Endoanal ultrasound is another excellent option, using a small ultrasound probe inserted into the anal canal. It's particularly good for identifying the internal opening of fistulas and assessing the anal sphincter muscles. It's less expensive than MRI and provides real-time imaging.

CT scan may be used for deep abscesses or when there's concern about extension into the pelvis, but it provides less detail of the anal canal than MRI or endoanal ultrasound.

How Are Perianal Abscess and Anal Fistula Treated?

Perianal abscess requires surgical incision and drainage - antibiotics alone are not sufficient. Anal fistulas are treated surgically, with the approach depending on the fistula's relationship to the sphincter muscles. Options include fistulotomy (opening the tract), seton placement, advancement flaps, or LIFT procedure. The goal is to eliminate the fistula while preserving continence.

Treatment of Perianal Abscess

The definitive treatment for a perianal abscess is incision and drainage (I&D). This involves making an incision over the abscess to release the pus. For small, superficial abscesses, this can sometimes be done in the office or emergency department under local anesthesia. Larger or deeper abscesses typically require drainage in the operating room under general or regional anesthesia.

Key principles of abscess drainage include:

• Adequate drainage: The incision must be large enough to allow complete drainage and prevent premature closure
• Breaking up loculations: Finger exploration breaks up any compartments within the abscess cavity
• Open wound healing: The wound is left open to heal from the inside out (secondary intention)
• Fistula assessment: The surgeon looks for an obvious fistula tract that might be treated simultaneously

Antibiotics are generally not needed for healthy patients with simple abscesses. However, antibiotics may be prescribed for:

• Significant surrounding cellulitis (skin infection)
• Systemic symptoms (fever, elevated white blood cell count)
• Immunocompromised patients (diabetes, HIV, chemotherapy)
• Prosthetic heart valves or joint replacements (infection prevention)

Treatment of Anal Fistula

Treating anal fistulas is more complex than treating abscesses because the surgeon must balance two goals: eliminating the fistula and preserving anal sphincter function. The choice of procedure depends on the fistula's classification - specifically, how much sphincter muscle is involved.

Fistulotomy is the simplest and most effective treatment for simple, low fistulas that involve minimal sphincter muscle (intersphincteric or low transsphincteric fistulas). The surgeon opens the entire fistula tract, converting the tunnel into an open groove that heals from the bottom up. Success rates exceed 90%, but the procedure is not suitable for complex fistulas due to the risk of incontinence.

Seton placement involves threading a silicone loop or suture through the fistula tract. Setons serve several purposes:

• Draining seton: Allows continuous drainage to control infection and inflammation
• Cutting seton: Gradually cuts through the sphincter muscle, allowing it to heal behind the cutting line
• Staging: Allows inflammation to settle before definitive repair

Advancement flaps involve covering the internal fistula opening with a flap of healthy tissue, typically rectal mucosa. This is used for complex fistulas where fistulotomy would risk incontinence. Success rates range from 60-80%.

LIFT procedure (Ligation of Intersphincteric Fistula Tract) is a relatively newer technique that identifies and ligates the fistula tract in the intersphincteric space. It preserves the sphincter muscles and has success rates of 60-80% with minimal risk of incontinence.

Other options include fibrin glue injection, anal fistula plugs, VAAFT (video-assisted anal fistula treatment), and laser treatment, though these generally have lower success rates than traditional surgery.

What Is the Recovery Like After Surgery?

Recovery after perianal abscess drainage typically takes 2-6 weeks, depending on abscess size. Wound care includes sitz baths 3-4 times daily, keeping the wound clean, and taking pain medication as needed. Most patients can return to work within 1-2 weeks. Complete healing from fistula surgery may take longer, especially for complex cases requiring multiple procedures.

After surgical drainage of a perianal abscess, the wound is left open to heal from the inside out. This approach, called healing by secondary intention, prevents the skin from closing over before the deeper tissue has healed, which could trap infection and cause recurrence. While healing by secondary intention takes longer than primary closure, it's safer for infected wounds.

Post-Operative Care Instructions

The following care measures help promote healing and reduce complications after perianal surgery:

Sitz baths are warm water soaks that cleanse the wound, increase blood flow, and reduce discomfort. Fill a basin or bathtub with 3-4 inches of warm (not hot) water and sit for 15-20 minutes. Perform sitz baths:

• After each bowel movement
• 3-4 times daily, or as directed by your surgeon
• For 2-4 weeks or until the wound has healed

Pain management: Some discomfort is normal, especially during the first few days. Pain medications (typically over-the-counter acetaminophen or ibuprofen, or prescription medications for severe pain) help control discomfort. Pain should gradually improve each day - worsening pain may indicate a problem.

Wound care: Keep the wound clean and dry between sitz baths. A gauze pad may be placed over the wound to absorb drainage. Change the gauze pad whenever it becomes soiled. Your surgeon may prescribe specific dressings or ointments.

Bowel function: Maintaining soft stools is important to prevent straining. Eat a high-fiber diet, drink plenty of fluids, and take stool softeners if needed. Some surgeons recommend fiber supplements (psyllium, methylcellulose) during the recovery period.

Activity: Rest as needed for the first few days. Most patients can return to normal activities within 1-2 weeks, though this varies based on the extent of surgery and type of work. Avoid heavy lifting, strenuous exercise, and sexual activity until cleared by your surgeon.

Can Perianal Abscess and Fistula Be Prevented?

While not all perianal abscesses can be prevented, you can reduce your risk by maintaining good anal hygiene, treating constipation and diarrhea promptly, managing underlying conditions like diabetes and inflammatory bowel disease, avoiding anal trauma, and seeking early treatment for any perianal symptoms.

Complete prevention of perianal abscess is not always possible, since the underlying cause (infection of an anal gland) can occur even in healthy individuals. However, certain lifestyle measures and health management strategies can reduce the risk or help prevent recurrence:

Preventive Measures

• Good anal hygiene: Keep the perianal area clean and dry. Avoid harsh soaps and excessive wiping that can irritate the skin. Consider using moist wipes or a bidet after bowel movements.
• Prevent constipation: Straining during bowel movements may contribute to anal gland blockage. Eat a high-fiber diet (25-30g daily), drink adequate fluids, and exercise regularly.
• Treat diarrhea promptly: Frequent loose stools can irritate the anal canal and increase infection risk.
• Manage diabetes: Good blood sugar control improves immune function and wound healing.
• Control inflammatory bowel disease: Work with your gastroenterologist to maintain remission, especially with Crohn's disease.
• Seek early treatment: Don't ignore perianal symptoms. Early treatment of small abscesses may prevent larger complications.

Preventing Fistula After Abscess

The best way to prevent fistula formation is prompt, adequate drainage of the initial abscess. Some surgeons advocate for primary fistulotomy (treating the underlying fistula tract) at the time of abscess drainage if a fistula is clearly identified. However, this approach must be balanced against the risk of sphincter damage in the acute setting when tissue planes may be obscured by infection.

Follow-up after abscess drainage is important. If you notice continued drainage, recurrent swelling, or other symptoms suggestive of fistula formation, see your surgeon for evaluation. Early detection and treatment of a fistula may allow for simpler surgical repair.

What Complications Can Occur?

The most common complication of perianal abscess is anal fistula formation, occurring in 30-50% of cases. Other complications include recurrent abscess, chronic pain, fecal incontinence (after surgery involving sphincter muscles), and rarely, serious spreading infections like Fournier's gangrene. Complications are more common in patients with diabetes or immunosuppression.

Understanding potential complications helps emphasize the importance of prompt treatment and careful follow-up:

Fistula Formation

As discussed, 30-50% of perianal abscesses lead to anal fistula formation. The fistula may become apparent weeks to months after the initial abscess treatment. Signs include persistent drainage from the healed abscess site, recurrent episodes of swelling and pain, or a new opening near the anus. Fistulas require additional surgical treatment.

Recurrent Abscess

Abscesses can recur, especially if the initial drainage was inadequate or if an underlying fistula tract was not addressed. Recurrence is also more common in patients with Crohn's disease or immunosuppression. Each recurrent abscess should be thoroughly evaluated for underlying fistula.

Fecal Incontinence

Surgery for perianal disease carries a risk of damage to the anal sphincter muscles, which can result in fecal incontinence (loss of bowel control). The risk depends on the extent of surgery - simple drainage has minimal risk, while fistulotomy for high fistulas carries greater risk. Surgeons carefully assess fistula anatomy to choose procedures that minimize this risk.

Fournier's Gangrene

This is a rare but life-threatening necrotizing infection of the perineum and genital area. It can develop from an untreated or inadequately treated perianal abscess, especially in patients with diabetes, immunosuppression, or other serious illnesses. Fournier's gangrene requires emergency surgery and carries significant mortality. Warning signs include rapidly spreading redness, severe systemic illness, and crepitus (crackling sensation in the skin).