A fistula is an abnormal, tube-like connection that forms between two organs, vessels, or other body structures. While some fistulas can be created deliberately for medical purposes, such as an arteriovenous fistula for dialysis, most are the result of injury, infection, or chronic inflammation. Because they can cause significant discomfort and other complications, accurate diagnosis is the first step toward finding a successful treatment plan.
The Initial Consultation and Physical Examination
The diagnostic process begins with a detailed medical history and a physical examination. Your doctor will ask about your symptoms, which for an anal fistula might include persistent pain, swelling, or a foul-smelling discharge around the anus. For other types of fistulas, symptoms vary widely. The physical exam is a critical component, where the doctor looks for external signs of the fistula tract.
- Patient History: A history of a previously drained abscess is a significant risk factor for an anal fistula. Your doctor may also inquire about inflammatory bowel diseases like Crohn's, which can cause fistulas.
- Visual Inspection: The external opening of an anal fistula often appears as an open sore or a small mound of granulation tissue on the skin near the anus.
- Digital Rectal Examination (DRE): During a DRE, the doctor uses a gloved finger to feel for a fibrous tract or hard cord beneath the skin, which can indicate the fistula's path.
- Anoscopy: A small endoscope called an anoscope may be used to examine the anal canal to help identify the internal opening of the fistula.
Advanced Imaging and Diagnostic Procedures
When a physical exam is not enough to determine the fistula's complete path, location, and relationship to surrounding muscles, advanced imaging is necessary.
Magnetic Resonance Imaging (MRI)
An MRI is a powerful tool for diagnosing fistulas, particularly complex ones. It can provide detailed, 3D images of the fistula tract, map its full course, and show its relationship to important structures like the anal sphincter muscles. This information is crucial for planning an effective surgical approach that preserves continence.
Endoscopic Ultrasound
An endoscopic ultrasound uses high-frequency sound waves from a probe inserted into the rectum or vagina to produce images of the fistula tract and surrounding tissues. It is particularly effective for evaluating the sphincter muscles. For high-lying anal fistulas, 3D-endoscopic ultrasound is as effective as MRI in some cases.
Computed Tomography (CT) Scan
CT scans are more frequently used for fistulas involving the abdomen, such as those connecting the colon to the bladder (enterovesical). A CT angiogram, which uses an injected contrast dye, can highlight the vessels and structures involved. For suspected enterovesical fistulas, a CT scan of the abdomen and pelvis is a sensitive and effective tool.
Fistulography
This is a traditional radiographic technique where a contrast dye is injected into the external opening of the fistula. X-rays are then taken to visualize the path of the fistula tract, allowing the doctor to understand its anatomy.
Examination Under Anesthesia
For some complex cases, a surgeon may recommend a thorough examination of the fistula tract under anesthesia. This allows for a more detailed and pain-free evaluation, including the use of a fistula probe to confirm the tract. It can also help identify complications and other openings.
The Poppy Seed Test
This simple, low-cost test can be used to detect enterovesical fistulas. The patient ingests poppy seeds, and their urine is collected over 48 hours and examined for the presence of the seeds. The poppy seed test has shown high accuracy in some studies.
Comparison of Fistula Diagnostic Procedures
Different diagnostic tools serve distinct purposes depending on the fistula's location and complexity. The following table provides a comparison of common methods:
| Diagnostic Method | Primary Use | Advantages | Disadvantages |
|---|---|---|---|
| Physical Exam | Initial assessment, anal fistulas | Non-invasive, quick, can identify external opening | Limited view of internal tract, painful for patient |
| MRI | Anal, vaginal, and complex fistulas | High-resolution images, excellent for mapping tracts and sphincter muscles, no radiation | Higher cost, can be claustrophobic for some |
| Endoscopic Ultrasound | Anal and vaginal fistulas | Effective for evaluating sphincter muscles and internal opening | Less effective than MRI for detecting deep extensions |
| CT Scan | Abdominal fistulas (e.g., enterovesical) | Sensitive for detecting air or contrast in other organs | Uses radiation, may not visualize fine details as well as MRI |
| Fistulography | Various fistulas | Inexpensive, defines tract anatomy | Can be painful, sometimes inaccurate, requires dye injection |
| Exam Under Anesthesia | Complex or recurrent fistulas | Allows thorough, pain-free evaluation and exploration | Requires sedation and a surgical setting |
The Role of Underlying Conditions in Diagnosis
Beyond just locating the fistula, a doctor also needs to identify any underlying causes. Inflammatory bowel diseases (IBD), such as Crohn's disease, are a common cause of fistulas. In such cases, the diagnostic process may involve additional tests like a colonoscopy to examine the large intestine and look for signs of IBD. A definitive diagnosis guides the treatment plan, as management for a fistula caused by Crohn's may differ from one resulting from a simple abscess.
Conclusion
In summary, the process of diagnosing a fistula is a comprehensive, multi-step journey that relies on a combination of medical history, physical examination, and advanced imaging technologies. The journey begins with the initial consultation, where a doctor's examination may reveal the external opening of a fistula. From there, diagnostic procedures like MRI and endoscopic ultrasound are used to map the tract's full complexity, ensuring a complete picture for a proper treatment plan. By using these various tools, doctors can accurately confirm the presence of a fistula and identify its underlying cause, which is the cornerstone of effective management and long-term healing.
