Grading of Fistula-in-Ano
Fistula-in-ano is classified using the Parks classification system, which categorizes fistulas based on their relationship to the anal sphincter complex into five main types: superficial, intersphincteric, transsphincteric, suprasphincteric, and extrasphincteric. 1
The Parks Classification System
The Parks classification uses the external sphincter complex (external sphincter and puborectal muscle) as the anatomical reference point for grading:
1. Superficial Fistulas (Grade 5)
- Do not involve the sphincter complex at all
- Track beneath the perianal skin without penetrating muscle
- Account for approximately 15-16% of cases 2, 3
2. Intersphincteric Fistulas (Grade 1)
- Most common type, representing 43-54% of all fistulas 2, 3
- Tract penetrates the internal sphincter only
- Runs through the intersphincteric space to reach perianal skin
- Generally considered "simple" fistulas with favorable prognosis
3. Transsphincteric Fistulas (Grade 2)
- Penetrate both internal and external sphincter or puborectal muscle
- Account for 21-35% of cases 2, 3
- Critical distinction: Low vs. High transsphincteric 1
- Low: Tract runs through lower one-third of external anal sphincter
- High: Involves middle or upper portions of sphincter
- More frequently complex than intersphincteric (32% vs 6% complexity rate) 3
4. Suprasphincteric Fistulas (Grade 3)
- Tract initially runs upward in intersphincteric space
- Then crosses the levator ani muscle downward
- Reaches perianal skin without penetrating external sphincter
- Rare, accounting for only 3% of cases 2
5. Extrasphincteric Fistulas (Grade 4)
- Originates from rectal wall
- Runs down through levator ani muscle
- Does not penetrate external sphincter or puborectal muscle
- Also rare at 3% of cases 2
Additional Classification Components
Beyond the basic Parks grades, comprehensive classification requires documenting 1:
Secondary Tract Extensions
- Infralevator: Below the levator plate
- Supralevator: Above the levator plate
- Horseshoe extensions: Horizontal spread in circumferential plane
Associated Abscess Location
- A: Perianal
- B: Intersphincteric
- C: Ischiorectal
- D: Suprasphincteric
Presence of Proctitis
- Any ulceration/stricture in rectum or anal canal
- Critical prognostic factor: Predicts persistent non-healing and higher proctectomy rates 1
Clinical Correlation
External opening location predicts complexity 3:
- Posterior midline (within 30° either side): 97% associated with simple superficial or intersphincteric fistulas
- Posterolateral openings: 47% complex, 56% transsphincteric or high
- Anterior openings: 16% complex
Diagnostic Approach
MRI is the gold standard imaging modality for classification, with 76-100% accuracy in visualizing sphincter anatomy, fistula tracts, and abscesses 1. The classification directly determines surgical approach and risk of postoperative incontinence—the higher the grade and more sphincter involvement, the greater the functional risk.