At a glance
When an anal fistula is diagnosed, one of the first things the surgeon determines is its relationship to the sphincter muscles. This classification — of which intersphincteric and trans-sphincteric are the two most common types — directly affects which treatments are safe to offer.
Understanding the difference helps you make sense of your surgical team’s recommendations and ask informed questions about your treatment plan.
The sphincter muscles
To understand fistula classification, you need to know about the two sphincter muscles:
Internal sphincter — an involuntary muscle (you cannot consciously control it). It provides most of the resting tone that keeps the anus closed. It lines the inside of the anal canal.
External sphincter — a voluntary muscle (you can squeeze it). It provides the ability to defer a bowel movement — the conscious squeeze. It wraps around the outside of the internal sphincter.
Between these two muscles is the intersphincteric space — a potential space that is relevant to fistula classification and treatment.
Intersphincteric fistula
The anatomy
An intersphincteric fistula runs within the intersphincteric space — the gap between the internal and external sphincter. The tract originates from an infected anal gland and tracks downward (or sometimes upward) through this space, emerging at the skin near the anus.
The critical point: the tract does not cross through the external sphincter.
What it means for treatment
Because the external sphincter is not involved, treatment can be relatively straightforward:
- Fistulotomy (laying the tract open) is often appropriate — dividing the internal sphincter and the tissue of the tract without cutting through the external sphincter
- The risk to continence is low, because the external sphincter — the muscle responsible for voluntary control — is preserved
- Healing is typically quicker and more predictable
Prevalence
Intersphincteric fistulas are the most common type, accounting for roughly 45 to 70 percent of all anal fistulas in most surgical series.
Trans-sphincteric fistula
The anatomy
A trans-sphincteric fistula passes through both the internal sphincter and some or all of the external sphincter. The tract crosses the sphincter complex, emerging on the skin surface after traversing through the muscle.
The key variable is how much of the external sphincter the tract passes through:
- Low trans-sphincteric — passes through a small amount of the lower external sphincter. May still be treatable with fistulotomy if the surgeon judges that the amount of muscle involved is small enough
- High trans-sphincteric — passes through a significant portion of the external sphincter. Fistulotomy would divide too much muscle and risk continence
What it means for treatment
Trans-sphincteric fistulas require more careful consideration:
- Simple fistulotomy may be appropriate for low tracts but is generally avoided for high tracts
- Seton placement — a loose seton may be used to drain the tract and allow it to mature before definitive treatment
- Sphincter-preserving procedures — LIFT, advancement flap, VAAFT, and other techniques that treat the fistula without dividing the sphincter
- Staged treatment — addressing infection first, then the tract, in a planned sequence
Prevalence
Trans-sphincteric fistulas account for roughly 20 to 35 percent of anal fistulas. They are the most common type to be classified as complex.
Other types
For completeness, the Parks classification also includes:
- Suprasphincteric — the tract passes upward above the external sphincter and then down through the levator muscle. Rare and complex
- Extrasphincteric — the tract passes entirely outside the sphincter complex, from the rectum through the levators to the skin. Very rare and usually from a different cause (such as Crohn’s disease or previous surgery)
Why the classification matters to you
Understanding whether your fistula is intersphincteric or trans-sphincteric helps you understand:
- Why your surgeon recommends a particular approach — the classification drives the treatment choice
- What the risks are — specifically the continence risk, which is the main concern in fistula surgery
- What the timeline might look like — simple intersphincteric fistulas often have shorter treatment journeys; trans-sphincteric fistulas may need staged treatment
- What questions to ask — knowing your fistula type helps you have a more specific and useful conversation with your surgeon
Questions to ask your surgeon
- Which type of fistula do I have?
- How much of the sphincter is involved?
- What treatment do you recommend and why?
- What are the continence risks with this approach?
- Are there alternatives?
- What is the expected success rate for my type of fistula?