How Are Anal Fistulas Mapped and Diagnosed for Surgery?

To successfully treat an anal fistula, your surgeon needs a detailed map of exactly where the fistula tunnel begins, where it ends, and how it navigates through your pelvic muscles. Because the anal sphincter muscles—which control your bowel movements—are in this same area, knowing the precise path of the tunnel is critical to preserving your bowel control while completely removing the infection ^[1][2].

Undergoing exams in a sensitive, currently painful area can be scary and embarrassing. However, your care team will use a combination of physical guidelines and advanced imaging to create this map as gently and accurately as possible.

Physical Examination and Goodsall’s Rule

During your initial exam, the doctor will look for the external opening of the fistula on the skin around your anus. Traditionally, surgeons have used a concept called Goodsall’s rule to predict where the tunnel goes inside ^[3].

Goodsall’s rule works like a clock face. It suggests that if the external hole is on the back half of the anus (toward the tailbone), the tunnel will curve toward the center back (midline) of the anal canal. If the hole is on the front half, the rule suggests the tunnel will go straight in ^[3].

While Goodsall’s rule is still taught, modern research shows it is often inaccurate, especially for fistulas in the front (anterior) half of the anus ^[4][3]. Because relying solely on this clinical rule can lack precision, doctors now rely heavily on advanced imaging to see the exact anatomy, which better safeguards your muscles during surgery ^[3].

Endoanal Ultrasound (EAUS)

Often used as a first step in mapping, an endoanal ultrasound (EAUS) involves inserting a small wand into the anal canal to take sound-wave pictures of the tissues ^[5].

• Highly accurate: It is excellent at pinpointing the exact location of the internal opening (where the fistula starts inside the rectum) ^[5][6].
• Practical and fast: It is generally faster and less expensive than an MRI ^[7]. Many clinics now use 3D endoanal ultrasounds, which give surgeons a detailed, three-dimensional view of the tunnel and muscles ^[8].
• What to expect: You may be asked to do a minor bowel prep (like a self-administered enema) at home before the test. Because the area is likely already painful from infection, a numbing gel is typically used. If inserting the wand is still too painful to tolerate awake, communicate this to your doctor immediately.

Pelvic MRI

A Pelvic Magnetic Resonance Imaging (MRI) scan is considered the “gold standard” for mapping anal fistulas ^[9][1].

• The big picture: An MRI provides highly detailed images of the entire pelvic region. It shows not just the main tunnel, but any hidden side-branches or deep pockets of infection (abscesses) ^[10][2].
• Protecting your muscles: The MRI clearly shows how much of the anal sphincter muscle is involved in the fistula ^[1]. This allows the surgeon to choose a surgical technique that maximizes healing while protecting your bowel control ^[2].
• Complex cases: For fistulas that have returned after previous surgeries, or those associated with conditions like Crohn’s disease, an MRI is considered essential ^[11][12].
• What to expect: You will need to lie relatively still on your back or stomach inside the MRI machine for about 30 to 45 minutes. An IV is usually placed in your arm to deliver a contrast dye (often gadolinium), which helps active inflammation and hidden abscesses light up clearly on the scan.

Exam Under Anesthesia (EUA)

Sometimes, the most accurate map is created in the operating room. An Exam Under Anesthesia (EUA) is when the surgeon physically traces the fistula tract while you are completely asleep and feeling no pain. This is often done if an awake ultrasound is too painful to tolerate, or it is performed immediately before the actual surgery begins to confirm what the imaging showed.

In many cases, surgeons use both an endoanal ultrasound and a pelvic MRI because the two tests complement each other, providing the safest and most accurate roadmap for your surgery ^[12].