Classification and Management of Fistula-in-Ano

Anatomical Classification

The Parks anatomical classification system describes five main types of fistula-in-ano based on their relationship to the anal sphincter complex: superficial, intersphincteric, transsphincteric, suprasphincteric, and extrasphincteric 1
Intersphincteric fistulas, which represent approximately 54% of all anal fistulas, have a tract that runs between the internal and external anal sphincters 2
Transsphincteric fistulas, accounting for approximately 21% of cases, have a tract that crosses through the external anal sphincter muscle into the ischioanal fossa 2
Suprasphincteric fistulas, representing approximately 3% of cases, have a tract that passes upward in the intersphincteric plane above the puborectalis muscle, then tracks laterally and downward into the ischioanal fossa 2
Extrasphincteric fistulas, accounting for approximately 3% of cases, have a tract that originates outside the anal canal and sphincter complex, passing directly from the rectum to the perineal skin through the ischioanal fossa 2
Superficial fistulas, representing approximately 16% of cases, have a tract that involves only the distal anal canal without involving the anal sphincters 2

A simple fistula has characteristics including low anatomical origin, single external opening, no pain or fluctuation suggesting perianal abscess, no rectovaginal involvement, and no anorectal stricture 1
A complex fistula has one or more of the following characteristics: high anatomical origin, multiple external openings, associated pain or fluctuation suggesting abscess, rectovaginal involvement, or active rectal disease on endoscopy 1, 2

Horseshoe fistulas are characterized by a semilunar region of sepsis spreading horizontally on either side of an internal opening, involving two or more adjacent quadrants in the ischioanal, intersphincteric, or supralevator compartments 2
Rectovaginal/anovaginal fistulas are tracts connecting the rectum or anus to the vagina, appearing as a hypoechoic tract on ultrasound or high-signal/enhancing structure on MRI 2

Examination under anesthesia (EUA) performed by an experienced colorectal surgeon is approximately 90% accurate in detecting and classifying perianal fistulas 1
Additional diagnostic evaluation with endoscopic ultrasound or pelvic MRI may be required to accurately classify fistulas, as clinical examination alone may be insufficient for complex cases 1

In patients with perianal fistula, pelvic MRI with gadolinium contrast demonstrates a sensitivity of 92‑97 % and specificity of 85‑96 % for distinguishing complex from simple fistulas, outperforming endoanal ultrasound (≈ 75 % sensitivity, 64 % specificity)【3】.

The American College of Radiology recommends pelvic MRI performed with a multichannel phased‑array body coil as the standard imaging modality for perianal fistula evaluation, particularly for complex cases with clinically occult tracts【4】【3】.

Gadolinium‑based intravenous contrast is preferred for perianal MRI because active inflammatory tracts show avid enhancement, abscesses display rim‑like enhancement, and contrast helps differentiate inactive (diffuse enhancement) from active (ring enhancement) fistulous tracts【4】【3】.