How to Assess a Fistula: Methods for an Accurate Diagnosis

Fistula assessment relies on matching the right diagnostic tool to the type and location of the abnormal connection, and in most cases more than one method is needed to get a complete picture. For perianal fistulas, MRI has become the reference standard, but physical examination, ultrasound, and surgical exploration each contribute information the others can miss. For fistulas elsewhere in the body, the toolkit shifts entirely: duplex ultrasound for hemodialysis access, CT angiography for aortoenteric emergencies, dye testing for vesicovaginal fistulas, and bronchoscopy for congenital airway connections. The common thread is that no single test is perfect, and the consequences of missed anatomy range from surgical failure to life-threatening bleeding.

Starting With the Physical Exam

A hands-on clinical assessment is the first step for most fistulas, and for perianal fistulas it still provides the baseline that all imaging builds on. The examiner looks for external openings on the skin, feels for cord-like tracts beneath the surface, and checks for tenderness or swelling that might suggest an undrained abscess. For perianal fistulas specifically, Goodsall’s rule is a classic clinical guideline that predicts where the internal opening sits based on where the external opening appears relative to an imaginary line drawn across the anus. If the external opening is behind that line, the tract is expected to curve toward the midline at the back; if it is in front, the tract should run in a straight radial line.

In practice, Goodsall’s rule is a useful starting point but far from infallible. A study of 365 patients found the rule was accurate about 73% of the time for posterior fistulas and only about 52% for anterior ones, with an overall accuracy rate near 48% when both categories were combined. The rule performed better for longer posterior tracts (around 86% accuracy) but worse for longer, deeper anterior tracts (roughly 29%).¹ That inconsistency is exactly why imaging almost always follows the physical exam rather than replacing it. The clinical assessment tells you where to look; imaging tells you what is actually there.

MRI for Perianal Fistulas

MRI has earned its status as the go-to imaging modality for perianal fistulas because it excels at showing soft-tissue detail without radiation. It maps the origin of the tract, its course through and around the sphincter muscles, and any secondary branches or hidden abscesses that could sabotage a surgical repair if left undetected.² The relationship between the fistula tract and the sphincter complex is particularly critical because the surgeon needs to know how much muscle is at risk during the procedure; damaging too much sphincter muscle can lead to incontinence.

MRI also provides a framework for classifying the fistula. Under the widely used Parks classification, fistulas are grouped by how they pass through the sphincter muscles. In one study of 150 primary fistulas classified on MRI, about 55% were intersphincteric (running between the two sphincter rings), roughly 39% were trans-sphincteric (passing through the external sphincter), and smaller fractions were suprasphincteric or extrasphincteric.³ Getting this classification right before surgery changes the operative plan, because a simple intersphincteric fistula can often be laid open directly, while a high trans-sphincteric tract demands a sphincter-preserving technique.

Even so, MRI is not the last word. Correlation with what the surgeon actually finds in the operating room remains essential, because internal openings, secondary tracts, and abscesses identified on MRI occasionally turn out to look different once the tissue is directly visualized.⁴ That reality check is why many surgeons treat MRI as a road map to be confirmed, not a verdict to be followed blindly.

Endoanal Ultrasound

Endoanal ultrasound (EAUS) offers a cheaper, more widely available alternative to MRI for perianal fistulas. A small ultrasound probe is inserted into the anal canal, producing detailed cross-sectional images of the sphincters and surrounding tissue. A systematic review and meta-analysis of three-dimensional endoanal ultrasound found that its ability to detect internal openings was good, with an area under the curve of 0.86.⁵ That is solid but leaves room for missed openings, particularly in complex or high fistulas.

Several enhancements have been developed to push ultrasound accuracy higher. One well-known technique involves injecting hydrogen peroxide through the external opening during the scan. The peroxide creates air bubbles that light up the tract on the ultrasound image, making it easier to follow. Results with this approach have been mixed, though. In one study, hydrogen peroxide-enhanced ultrasound correctly identified the internal opening in only about 61% of patients when compared with what surgeons found during the operation, and it was particularly unreliable for suprasphincteric and extrasphincteric fistulas.⁶ A more advanced combination using contrast-enhanced ultrasound with three-dimensional transrectal imaging pushed diagnostic accuracy above 90% for the main tract and internal opening in complex fistulas, significantly outperforming conventional transrectal ultrasound.⁷

The practical takeaway is that basic endoanal ultrasound works well for straightforward fistulas, but complex cases with multiple tracts or high extensions tend to push beyond its reliable range unless enhanced techniques are used. In a head-to-head prospective comparison of endoanal ultrasound, MRI, and examination under anesthesia for Crohn’s disease perianal fistulas, all three methods achieved accuracy above 85%.⁸ That study underlines something clinicians have long suspected: when you combine any two of these three modalities, the chance of missing important anatomy drops substantially.

Examination Under Anesthesia

Examination under anesthesia (EUA) is both a diagnostic and sometimes a therapeutic step. With the patient fully relaxed, the surgeon can probe the tract, pass instruments from external to internal opening, and inspect areas that would be too painful to evaluate in a clinic. The same prospective study mentioned above found EUA to be about 91% accurate for mapping Crohn’s perianal fistulas, essentially on par with MRI and endoanal ultrasound.⁸ EUA has the added advantage of allowing the surgeon to drain any abscess encountered and place a draining seton (a thin loop of material threaded through the tract to keep it open) in the same session.

The downside is that EUA is invasive and requires a general or regional anesthetic. It also depends heavily on the surgeon’s experience and tactile skill. In practice, many centers now perform EUA with a preoperative MRI already in hand, so the surgeon enters the operating room with a map and uses direct examination to confirm or update it.

Endoscopic and Fistuloscopic Approaches

Beyond traditional probing, endoscopic tools give a direct view inside the fistula tract itself. Video-assisted anal fistula treatment, known as VAAFT, uses a tiny fistuloscope inserted through the external opening. The scope is advanced along the tract, letting the surgeon see both the primary path and any secondary branches, identify the internal opening, and debride infected tissue under direct vision.⁹ This is particularly useful when the internal opening has been difficult to locate on imaging or clinical exam. VAAFT blurs the line between diagnosis and treatment: the same procedure that identifies the tract anatomy can also treat it, reducing the number of separate interventions the patient needs.

CT Imaging and Contrast Studies

While MRI dominates perianal fistula assessment, CT scanning fills a different niche. For enterocutaneous fistulas, which connect the intestine to the skin surface, CT with intravenous contrast can reveal abscesses forming along the tract, identify which loop of bowel is involved, and detect complications like obstruction or malnutrition-related organ changes. Radiologists look specifically for a peripherally enhancing abscess with fluid or gas content along the tract, because finding one changes the treatment plan from conservative management to drainage.¹⁰

Older fluoroscopic contrast studies, such as fistulography (injecting contrast dye directly into the tract and taking X-rays), remain part of the diagnostic toolkit. These studies are well documented in the earliest investigations of fistulas and continue to provide useful anatomic detail, particularly for identifying which part of the bowel the fistula originates from.¹¹ However, conventional X-ray fistulography has real limitations in complex cases. In a comparison study, it missed the correct definition of fistulous branches in half of patients and failed to identify connections to intestinal loops in over a third, whereas hydrogen peroxide-enhanced ultrasound-fistulography visualized the extent of the fistula in every case.¹²

Assessing Fistulas in Crohn’s Disease

Crohn’s disease deserves separate discussion because perianal fistulas occur in a large fraction of patients with this condition, and the disease’s chronic, relapsing nature demands not just diagnosis but ongoing monitoring. MRI plays a dual role here: it maps the fistula before treatment and then tracks the response afterward. One particularly useful finding on follow-up MRI is the degree of fibrosis within the tract. A completely fibrotic tract, meaning the active inflammation has been replaced by scar tissue, has been shown to be a robust predictor of long-term clinical closure.¹³ This is a meaningful advance because external healing can be deceptive in Crohn’s: a fistula can look closed on the surface while active inflammation persists underneath, setting the stage for recurrence.

The MAGNIFI-CD index is one of the scoring tools developed to standardize MRI assessment of Crohn’s perianal disease. By scoring features like tract signal intensity, abscess size, and involvement of surrounding structures, it gives clinicians a quantifiable measure of disease activity rather than relying on subjective interpretation. Having a score to track over time makes it easier to judge whether a biologic medication is working or whether escalation is needed.

Vesicovaginal Fistula Diagnosis

Vesicovaginal fistulas, abnormal connections between the bladder and vagina, present a completely different diagnostic challenge. The hallmark symptom is continuous involuntary leakage of urine through the vagina, which can be confused with stress incontinence if the fistula is small. The traditional diagnostic approach relies on clinical methods and dye testing.¹⁴ In a typical dye test, a colored solution like methylene blue is instilled into the bladder through a catheter while a tampon or gauze sits in the vagina. If the dye appears on the gauze, the fistula is confirmed, and the location can sometimes be inferred from which part of the gauze is stained.

Cystoscopy, looking into the bladder with a camera, adds anatomic precision by showing the fistula opening on the bladder wall and its relationship to the ureters and bladder neck. A combined vaginoscopy-cystoscopy technique takes this further: using a picture-in-picture display that simultaneously shows a camera view inside the bladder and another inside the vagina, the examiner can see the fistula from both sides at once. Passing a guide wire through the cystoscope and out the vaginal wall pinpoints the fistula’s exact position and aids surgical planning.¹⁵

Hemodialysis Access Fistulas

Arteriovenous fistulas created surgically for hemodialysis access are assessed using an entirely different toolkit, centered on duplex ultrasound. Here the question is not whether an abnormal connection exists, but whether the intentional one is working properly. Duplex ultrasound combines traditional anatomic imaging with Doppler flow measurements, allowing the sonographer to assess vessel diameter, blood flow volume, and the presence of narrowing or clots that could threaten the access.¹⁶

Ultrasound mapping is also used before the fistula is even created. Preoperative scanning of the arm’s arteries and veins helps the surgeon choose vessels that are large enough and free of disease, which improves the odds that the fistula will mature properly. After surgery, follow-up ultrasound checks maturation: is the vein dilating enough and carrying enough flow to support dialysis needles? Clinical signs like a weak thrill (the vibration you feel over a working fistula) or difficulty getting adequate flow during dialysis sessions can suggest developing stenosis, and ultrasound can confirm and localize the problem before total failure occurs.¹⁷

Congenital and Pediatric Fistulas

Congenital fistulas, particularly those connecting the airway and the esophagus, present unique diagnostic challenges because the anatomy is often subtle and the patient is a newborn. H-type tracheoesophageal fistulas lack the more obvious features of the common type (which involves a gap in the esophagus), so they can go undetected for days or longer. Clues include excessive oral secretions, repeated lung infections, and stomach distension during mechanical ventilation. One diagnostic approach involves injecting methylene blue through an esophagoscope while simultaneously performing bronchoscopy: when the dye appears in the airway, the fistula is confirmed and localized.¹⁸ The concept is the same as vesicovaginal dye testing — introduce a visible tracer on one side and watch for it to appear on the other — but adapted for the airway with real-time endoscopic visualization.

Aortoenteric Fistulas and Emergency Diagnosis

Aortoenteric fistulas represent the most acutely dangerous end of the fistula spectrum. These connections between the aorta and the intestine can cause massive gastrointestinal bleeding, and mortality is high without rapid surgical intervention. The clinical presentation can include vomiting blood, dark tarry stools, sepsis, or abdominal pain, but the condition can also be clinically silent until a catastrophic bleed occurs.¹⁹ Because the clinical signs are neither always present nor specific enough to pin down the diagnosis, imaging is critical.

CT angiography is the frontline imaging study in this scenario. Its speed and widespread availability make it the practical choice in emergency departments. In a retrospective series of ten patients, CT angiography diagnosed the fistula in every case, whereas upper endoscopy, performed in three of those patients, was diagnostic in only one.²⁰ Endoscopy sometimes shows a pulsating mass or ulcer in the intestine, but a normal-looking endoscopy does not rule the diagnosis out. The lesson is that when aortoenteric fistula is suspected, CT angiography should not be delayed by other tests.

When Lab Tests and Tissue Samples Are Part of the Workup

Not all fistula assessment involves imaging. After pancreatic surgery, clinicians watch for postoperative pancreatic fistula, a feared complication where digestive enzymes leak from the pancreatic stump. The diagnosis rests partly on measuring the amylase level in the surgical drain fluid: a level three or more times the upper limit of normal serum amylase on postoperative day three is the standard threshold. A combined scoring approach found that drain fluid amylase, a positive bacterial culture from the drain, and an elevated white blood cell count were independent predictors of a clinically significant pancreatic fistula. Having two or more of these risk factors simultaneously gave the maximum overall accuracy for predicting trouble.²¹

Tissue biopsy plays a different role. Most fistulas are benign inflammatory tracts, but in rare cases a chronic fistula can harbor cancer. Recurrent or long-standing anal fistulas that resist standard treatment should raise suspicion, and histopathological evaluation of tissue from the tract can reveal malignancy that would otherwise be missed.²² This is an uncommon scenario, but it underscores why a fistula that keeps coming back despite appropriate surgery warrants a biopsy rather than just another attempt at repair.

Emerging Technologies

Two innovations are pushing the boundaries of fistula diagnosis. The first is intraoperative fluorescence imaging using indocyanine green (ICG), a dye that glows under near-infrared light. When injected into the rectum during laparoscopic surgery for rectourethral fistulas, ICG helps the surgeon locate the fistula tract in real time, providing greater accuracy for dissection and reducing the risk of urological complications.²³

The second is artificial intelligence applied to diagnostic imaging. Deep learning models trained on MRI and CT data have shown high performance in feasibility studies for detecting and segmenting anal fistula tracts.²⁴ One research approach integrated two types of neural network architectures to automatically segment fistula tracts and sphincter muscles on pelvic MRI, achieving strong performance for delineating the internal sphincter in particular.²⁵ These tools could eventually standardize MRI interpretation, reduce reader variability, and speed up the reporting process. For now, however, the evidence is constrained by small datasets and limited testing outside the institutions that developed the algorithms. AI for fistula assessment remains firmly in the research phase, with real clinical deployment still some distance away.