Diagnosing an anal fistula usually starts with a physical exam in your doctor’s office. In most cases, a surgeon can identify the fistula by looking at and feeling the area around the anus, though imaging or an exam under anesthesia is sometimes needed to map the full path of the tunnel before treatment.
What Your Doctor Looks and Feels For
The first step is a visual inspection of the skin around the anus. An anal fistula typically has an external opening, a small hole in the skin near the anus that may be draining pus, blood, or fluid. The surrounding skin is often red, swollen, or irritated. If you’ve been dealing with recurring abscesses in the same spot, that pattern alone raises strong suspicion for a fistula.
Next comes a digital rectal exam, where the doctor inserts a gloved, lubricated finger into the anal canal. This allows them to feel for the internal opening of the fistula, which can present as a small dimple, a fibrous pit, or a soft bump of granulation tissue just inside the canal. They’ll also palpate along the fistula tract itself to assess how many tracks exist, how deep they run, and how much of the sphincter muscle is involved. A palpable cord-like track running between the two openings is a classic finding.
For many straightforward fistulas, this hands-on evaluation provides enough information to plan surgery. But when the tract is deep, branching, or difficult to trace, additional tools come into play.
How Fistulas Are Classified
Once a fistula is identified, your surgeon will classify it based on how the tunnel relates to the sphincter muscles that control bowel movements. The most widely used system, called Parks classification, divides fistulas into four types:
- Intersphincteric: The tunnel runs between the two layers of sphincter muscle. This is the most common type and generally the simplest to treat.
- Trans-sphincteric: The tunnel passes through the external sphincter. Treatment requires more care to avoid damaging the muscle.
- Suprasphincteric: The tunnel loops up and over the top of the external sphincter before opening into the skin. This is less common and more complex.
- Extrasphincteric: The tunnel bypasses the sphincter entirely, running from the rectum through surrounding tissue to the skin. This rare type often signals an underlying condition like Crohn’s disease.
Getting the classification right matters because it directly determines which surgical approach is safest. Cutting through too much sphincter muscle risks incontinence, so surgeons need a clear picture of the anatomy before operating.
Predicting the Fistula’s Path
Surgeons use a clinical guideline called Goodsall’s Rule to predict where the internal opening sits based on the location of the external opening. The idea is simple: draw an imaginary horizontal line across the anus. If the external opening is behind that line (toward the tailbone), the internal opening almost always sits at the back of the anal canal. If the external opening is in front of that line (toward the genitals) and within about 3 centimeters of the anus, the tunnel usually runs in a straight line to the nearest point inside the canal.
This rule is accurate about 75% of the time overall, but it works better for posterior fistulas (73% accuracy) than anterior ones (52% accuracy). It’s a useful starting point, not a guarantee, which is one reason imaging is valuable for fistulas that don’t follow the expected pattern.
When Imaging Is Needed
For complex or recurrent fistulas, your surgeon will likely order imaging to map the full anatomy before operating. The two main options are MRI and 3D endoanal ultrasound.
MRI is considered the gold standard for complex fistulas. It produces detailed cross-sectional images that show the fistula tract, any branching side tunnels, abscesses, and the relationship of the tract to the sphincter muscles. MRI is particularly good at detecting secondary extensions, the hidden branches that, if missed during surgery, lead to recurrence.
3D endoanal ultrasound uses a small probe inserted into the anal canal. It’s highly sensitive for finding the internal opening, detecting it correctly about 97% of the time. However, it’s less reliable at ruling out a fistula when one isn’t present (specificity around 61%), and it doesn’t map complex branching tracts as well as MRI does. For straightforward fistulas, ultrasound alone is often sufficient. For complex cases, combining both ultrasound and MRI gives the most complete picture.
Examination Under Anesthesia
In some cases, the most definitive way to diagnose and map a fistula is an examination under anesthesia. This is typically done in an operating room, sometimes as part of the same visit when surgery is planned. Under anesthesia, you’re completely relaxed, which lets the surgeon probe the fistula tract thoroughly without causing pain. They can pass a thin instrument through the tunnel to trace its full course, identify the internal opening with certainty, and check for complications like additional tracts or undrained abscesses.
This approach is especially useful when office exams and imaging haven’t provided a clear enough map, or when you’re in too much pain for a thorough exam while awake. Many surgeons consider examination under anesthesia the most reliable single diagnostic method, since it allows direct visualization of everything imaging can only approximate.
Conditions That Can Mimic a Fistula
Several other conditions produce similar symptoms, including drainage, pain, and recurring lumps near the anus. Your doctor will consider these possibilities during the evaluation:
- Hidradenitis suppurativa: A chronic skin condition that causes recurring abscesses and draining tunnels in areas where skin folds rub together, including the buttocks and perianal region. When it develops near the anus, it can look nearly identical to an anal fistula. Ultrasound can help distinguish the two by showing whether a tract actually connects to the inside of the anal canal.
- Pilonidal disease: Infected cysts or sinuses that form near the tailbone, usually in the cleft between the buttocks. These are located higher up than a typical anal fistula and don’t connect to the anal canal.
- Infected cysts: Sebaceous or other cysts near the anus can become infected and drain, mimicking a fistula’s external opening. The absence of a tract leading to the anal canal is the key difference.
- Specific infections: Tuberculosis, sexually transmitted infections like lymphogranuloma venereum, and other infections can create draining sinuses in the perianal area. If your doctor suspects an unusual infection, blood tests or tissue cultures help clarify the diagnosis.
- Crohn’s disease: Perianal fistulas are a well-known complication of Crohn’s. If you have multiple fistulas, fistulas that keep coming back, or other digestive symptoms, your doctor may recommend further evaluation for inflammatory bowel disease.
What to Expect During the Process
For most people, the diagnostic path starts with a visit to a colorectal surgeon (or a referral to one from your primary care doctor). The initial office exam takes only a few minutes and, while uncomfortable, gives enough information to either plan treatment or decide whether imaging is needed. If you’re sent for an MRI, the scan itself is painless and typically takes 20 to 40 minutes. Endoanal ultrasound is a brief in-office procedure with mild discomfort similar to a digital rectal exam.
The whole diagnostic process, from first appointment through imaging results, usually wraps up within a few weeks. If an examination under anesthesia is needed, that’s often scheduled as a combined diagnostic and surgical session, so you may go from diagnosis to treatment in a single visit to the operating room.