Perianal Abscess and Fistula-in-Ano - A Detailed Review

PART 1: PERIANAL ABSCESS

Definition and Pathophysiology

• Pfenninger & Fowler's Procedures for Primary Care, p. 781
• Mulholland & Greenfield's Surgery, p. 3557

Etiology

• Sabiston Textbook of Surgery, Box 97.1

Locations and Incidence

• Pfenninger & Fowler's Procedures for Primary Care, p. 781

Clinical Features

• Pain: Severe, throbbing perianal pain - the hallmark symptom. Aggravated by sitting, coughing, sneezing, and straining.
• Fever and marked leukocytosis depending on severity
• External signs: Swelling, erythema, induration, fluctuance in superficial abscesses
• Deeper abscesses may have no external signs - tenderness and fullness on digital rectal exam only
• Associated symptoms: tenesmus, difficulty voiding

Important clinical pitfall: A common error is diagnosing "cellulitis" when there is no fluctuance. Many of these patients simply have a deeper abscess - antibiotics alone will be insufficient.

Diagnosis

• Primarily clinical - history and examination
• Digital rectal exam reveals fluctuant mass (even for intersphincteric abscesses not visible externally)
• Thorough DRE and anoscopy are often deferred in the acute setting due to pain
• CT pelvis or pelvic MRI when physical findings are limited or diagnosis is uncertain
• Mulholland & Greenfield's Surgery, p. 3557
• Sabiston Textbook of Surgery, p. 2161

Treatment

• Antibiotics do not adequately penetrate abscess cavities
• Antibiotic therapy alone is inadequate and must never substitute for drainage
• Extension of a local infection can lead to sepsis and complex long-term problems

1. Drainage starting at the most fluctuant area, staying as close to the anus as possible (to shorten any subsequent fistula tract)
2. Incision size tailored to abscess size - large enough for adequate drainage
3. A cruciate incision with removal of corners ensures ongoing drainage and prevents premature skin closure
4. Loculations within the cavity are broken up carefully (avoid aggressive disruption that can injure the sphincter or pudendal nerve)
5. If abscess is >5 cm, consider counter-incisions bridged with Penrose drains or vessel loops
6. Deeper abscesses: Pezzar or Malecot drainage catheter left in place for days to weeks

• Large/deep abscesses (postanal, horseshoe): best managed in the operating room with sedation
• Intersphincteric abscesses without external fluctuation: drain through intersphincteric groove or into anal canal via internal sphincter incision (NOT externally, to avoid iatrogenic fistula)
• Supralevator abscesses from intersphincteric extension: drain internally through rectal wall
• Horseshoe abscess: modified Hanley technique - posterior midline drainage of the deep postanal space plus drainage of both ischiorectal spaces

• Immunocompromised patients (poorly controlled HIV, diabetes, transplant recipients, chemotherapy patients)
• Extensive cellulitis or severe systemic symptoms
• High-risk for endovascular infection (cardiac shunts, prosthetic valves)
• Goldman-Cecil Medicine, p. 1535
• Sabiston Textbook of Surgery, p. 2162

Complications of Untreated/Inadequately Treated Abscess

• Necrotizing fasciitis - life-threatening
• Perineal sepsis / Fournier's gangrene - medical emergency with classic triad of pain, fever, and inability to void
• Fistula formation - the chronic sequela
• Recurrence - most common complication after I&D; most often due to an unrecognized associated fistula

PART 2: FISTULA-IN-ANO

Definition and Epidemiology

• Occurs in 30-50% of patients following a perianal abscess (Goldman-Cecil) - though the figure of <20% after a single episode is cited by Pfenninger; recurrence risk rises to >50% with recurrent abscess at the same location
• Incidence: approximately 8.6 per 100,000
• 2-3 times more common in men than women
• After appropriate surgery, ~85% are healed at 6 years
• Goldman-Cecil Medicine, p. 1535

Pathogenesis

1. A source of recurrent infection
2. A tract from the internal gland to the abscess cavity/drainage site

Parks Classification (Relationship to Sphincter Complex)

• Goldman-Cecil Medicine, p. 1535
• Sabiston Textbook of Surgery, p. 2163

Goodsall's Rule

• Anterior external openings (anterior to the transverse anal line): track radially (straight) into the nearest crypt
• Posterior external openings (posterior to transverse anal line): curve in a horseshoe fashion to a posterior midline internal opening
• Exception: anterior openings >3 cm from the anal verge may also track to the posterior midline
• Sabiston Textbook of Surgery, p. 2163

Clinical Presentation

• Recurrent abscesses in the same perianal location
• Persistent purulent drainage from an abscess site that has not fully healed
• Intermittent cycle: area seems to heal for weeks, then a "boil" reforms at the same site, ruptures, and relieves symptoms
• External opening visible on perianal skin
• Fibrous tract may be palpable along the course toward the anal canal
• Internal opening occasionally visible on anoscopy (not essential for diagnosis)
• Goldman-Cecil Medicine, p. 1535

Investigation

• Clinical examination is the primary tool
• MRI pelvis (T2-weighted) is the gold standard imaging - accurately defines fistula anatomy, identifies secondary tracts, and assesses sphincter involvement
• Sabiston Textbook of Surgery, Fig. 97.15 - MRI showing intersphincteric and transsphincteric fistulae

• Fistula probe passed gently through the external opening (avoid creating false passages)
• Injection of dilute hydrogen peroxide, methylene blue, or milk into the external opening to identify the internal opening
• Anoscopy to inspect the dentate line and crypts

Treatment

1. Treat any undrained infection
2. Define fistula anatomy
3. Remove/ablate the epithelialized tract
4. Avoid or minimize fecal incontinence
5. Prevent recurrence

1. Fistulotomy (Lay-Open)

• Indication: Simple, low-lying fistulae (intersphincteric or low transsphincteric) involving <1/3 of the external anal sphincter
• The entire tract is laid open and allowed to heal by secondary intention
• Recurrence rate: 2-8%
• Incontinence risk: <5% in patients with normal preoperative sphincter function
• Caution: Anterior fistulae in women require particular care (shorter anterior sphincter, previous obstetric injury)

2. Seton Placement

• Indication: High or complex fistulae where fistulotomy would risk significant incontinence
• A suture, silastic vessel loop, or rubber band is passed through the fistula tract, out the anus, and secured to itself
• Draining seton: Keeps external opening patent, prevents recurrent abscess, allows the tract to fibrose over weeks
• Cutting seton: Progressively tightened at 2-4 week intervals to slowly erode through sphincter muscle with concurrent fibrosis - success up to 94%, but incontinence risk 23% or more

3. Endoanal/Endorectal Advancement Flap

• A flap of mucosa + underlying internal sphincter is raised; the internal fistula opening is sutured closed; the flap covers the internal opening
• Success rate: 66-87%
• Preferred for complex/high fistulae in sphincter-preserving surgery
• Failed flaps can be repeated after interval healing

4. LIFT Procedure (Ligation of Intersphincteric Fistula Tract)

• After seton-induced fibrosis, an incision is made in the intersphincteric groove, the fistula tract is isolated, both ends ligated, and the tract divided
• Success rate: 40-95% (variable)
• Sphincter-sparing technique

5. Fibrin Glue / Fistula Plug

• Bioresorbable plug or fibrin glue obliterates the tract; sphincter-sparing
• Generally lower long-term success rates but may be appropriate in selected complex cases

6. Primary Fistulotomy at Time of Abscess Drainage

• Controversial - inflammation and edema can make accurate assessment of sphincter involvement difficult, potentially underestimating muscle involvement
• May be considered by experienced surgeons in selected cases
• Sabiston Textbook of Surgery, pp. 2163-2165
• Mulholland & Greenfield's Surgery, pp. 3561-3563
• Goldman-Cecil Medicine, p. 1535

Special Situations

Crohn Disease-Associated Fistulae

• Anal fistulae occur in 13-54% of Crohn disease patients
• Asymptomatic fistulae do not need treatment
• Often multiple, complex, recurrent
• First step: unroof branching tracts to create a single track
• Anti-TNF therapy (e.g., infliximab, adalimumab) for medical management
• Fistulotomy is generally avoided except for most superficial fistulae
• Long-term draining setons + medical therapy is the preferred approach
• Temporary diversion may facilitate healing
• Proctectomy may be necessary for severe, refractory perianal disease
• Mulholland & Greenfield's Surgery, p. 3563

Horseshoe Abscesses/Fistulae

• Modified Hanley technique: posterior midline drainage of deep postanal space + drainage of both ischiorectal spaces

Preoperative Planning Considerations

• Preexisting continence status
• Stool consistency
• History of sphincter injury or prior surgery
• Amount of sphincter likely to be divided
• Anterior location in females
• Underlying Crohn disease or other predisposing factors

Summary Table

• Goldman-Cecil Medicine, International Edition, pp. 1535-1536
• Sabiston Textbook of Surgery, 21st ed., pp. 2161-2165
• Mulholland & Greenfield's Surgery, 7th ed., pp. 3557-3563
• Pfenninger & Fowler's Procedures for Primary Care, 3rd ed., pp. 781-783
• Bailey & Love's Short Practice of Surgery, 28th ed., Fig. 80.29