Anterior openings track radially; posterior openings curve to the midline
Mnemonic
“Anterior straight, posterior curved”, for the internal opening of an anal fistula:
- An external opening anterior to the transverse anal line tracks in a straight, radial line to the internal opening
- An external opening posterior to it tracks in a curved path to the posterior midline
The exception is a long anterior track, over about 3 cm from the anal verge, which also tends to curve posteriorly.
The rule reflects the anatomy of the intersphincteric anal glands, most of which lie posteriorly, so posterior sepsis spreads circumferentially in a horseshoe pattern.
It is a guide, not a guarantee; MRI has largely superseded it for complex and recurrent fistulae, particularly in Crohn’s disease.
Expansion
Draw an imaginary transverse line across the anus:
- Anterior external openings - the tract runs radially and directly to the nearest point in the anal canal
- Posterior external openings - the tract curves and opens into the posterior midline
The exception is an anterior opening more than 3 cm from the anal verge, which also tends to curve posteriorly.
The rule reflects the underlying anatomy: fistulas originate in the anal glands at the dentate line, and the posterior glands drain into a curved horseshoe-shaped intersphincteric space.
Fistulas are then classified by their relation to the sphincters, intersphincteric, transsphincteric, suprasphincteric and extrasphincteric. The higher the tract crosses the external sphincter, the greater the risk that laying it open will cause incontinence, which is why high tracts are managed with a seton rather than immediate fistulotomy.