Fistula diagnosis: MRI and EUA

At a glance

Diagnosing an anal fistula is not just about confirming that one exists — it is about understanding exactly where it goes. The tract’s path through or around the sphincter muscles determines which surgical approach will be safest and most effective. Getting this mapping right is essential.

The two main diagnostic tools are MRI scanning and examination under anaesthesia (EUA). They provide different but complementary information, and many people undergo both.

Why accurate diagnosis matters

A fistula is a tunnel between the inside of the anal canal and the skin nearby. The critical question is: how does this tunnel relate to the sphincter muscles?

• A tract that passes through a small amount of sphincter (intersphincteric) can often be treated straightforwardly
• A tract that passes through more sphincter (trans-sphincteric) requires more careful planning to avoid affecting continence
• Complex tracts with branches, horseshoe extensions, or connections to other structures need detailed mapping

Treating a fistula without understanding its full anatomy risks either incomplete treatment (and recurrence) or unnecessary damage to the sphincter. This is why the diagnostic phase, while it can feel frustratingly slow, is genuinely important.

MRI scanning

What it involves

An MRI (magnetic resonance imaging) scan of the pelvis produces detailed cross-sectional images of the soft tissues around the anus and rectum.

• You lie on a scanner table, typically on your back
• The scan takes 20 to 40 minutes
• You need to stay still during the scan
• The machine is noisy — you will be given earplugs or headphones
• Some centres use an endoanal coil (a small probe placed in the rectum) for higher-resolution images

What it shows

MRI provides excellent soft-tissue detail:

• The primary tract — its path, length, and relationship to the sphincter muscles
• Secondary tracts or branches — extensions that may not be obvious on examination
• Abscess collections — pockets of infection that need drainage
• The internal opening — where the tract begins inside the anal canal
• Surrounding anatomy — the sphincter muscles, levator plate, and other structures

Practical considerations

• Preparation — usually minimal. Some centres ask you to use an enema beforehand
• Claustrophobia — if this is a concern, let the team know in advance
• Metal implants — inform the team about any metallic devices in your body
• Timing — MRI is best done when active infection has been controlled, as acute inflammation can obscure the tract detail

Examination under anaesthesia (EUA)

What it involves

An EUA is performed in an operating theatre:

1. You receive general or regional anaesthesia
2. The surgeon examines the perianal area with you fully relaxed
3. They may probe the fistula tract with a fine instrument to determine its course
4. Hydrogen peroxide or dye may be injected to trace the tract
5. The relationship between the tract and the sphincter muscles is assessed
6. If appropriate, treatment (such as seton placement) may be performed at the same time

What it provides that MRI cannot

• Direct tactile assessment — the surgeon can physically feel the tract and surrounding tissues
• Dynamic evaluation — the examination can follow the tract in real time
• Immediate treatment opportunity — if the findings are straightforward, the surgeon can proceed with treatment during the same session
• Assessment of sphincter tone and function — relevant for treatment planning

The combined approach

Many specialists use both MRI and EUA:

• MRI first, to provide a roadmap before going to theatre
• EUA second, to confirm the MRI findings and proceed with treatment

This approach means the surgeon enters the operating theatre with the best possible understanding of the anatomy, reducing surprises and improving outcomes.

What to expect as a patient

Before the investigations

• Your surgeon will explain why these investigations are needed
• You may wait several weeks for an MRI appointment, particularly within the NHS
• It is reasonable to ask how the results will affect your treatment plan
• If you have an active abscess, this may be drained first before definitive investigations

Between investigations and treatment

• The waiting period can be frustrating — you know there is a fistula but treatment has not started
• Use this time to discuss the findings with your surgeon and understand the planned approach
• Ask questions about the type of fistula, the proposed treatment, and the expected outcomes

Questions to ask your surgeon

• What type of fistula do I have?
• How does the tract relate to my sphincter muscles?
• Are there any branches or complications?
• What treatment do you recommend based on these findings?
• What are the alternatives?
• What is the expected success rate for this type of fistula?

The diagnostic phase matters

It can be tempting to want to skip straight to treatment — the fistula is there, just fix it. But the diagnostic phase protects you. It ensures that the treatment chosen is appropriate for your specific anatomy, minimising the risk of recurrence and protecting your continence. The weeks spent on diagnosis are an investment in a better outcome.