Skip to content
How It Works
Explore
Resources Ask a Question
Who It's For
Patients Caregivers Providers
Log In Ask a Question
PubMed This is a summary of 23 peer-reviewed journal articles Updated
Colorectal Surgery

The Diagnostic Journey: Symptoms and Mapping

At a Glance

An anal fistula typically causes persistent perianal pain, swelling, and discharge of pus or blood. Proper diagnosis requires specialized imaging, primarily a pelvic MRI or endoanal ultrasound, to accurately map the fistula tract and its relationship to the anal sphincter muscles.

The journey to a diagnosis often starts with a combination of physical discomfort and confusing symptoms. Because the symptoms of an anal fistula can overlap with other conditions, a thorough diagnostic process is essential to ensure you receive the correct treatment [1].

Recognizing the Symptoms

Most patients first notice symptoms that signal an underlying infection or an abnormal connection in the tissue. Common signs include:

  • Perianal Discharge: This is the most common symptom, where pus, blood, or mucoid fluid leaks from a small opening near the anus [2][3].
  • Persistent Pain: You may feel a steady ache or a sharp, throbbing pain that often worsens when you sit, move, or have a bowel movement [3][4].
  • Fullness or Swelling: A feeling of pressure or a visible lump near the anal opening, which may be a sign of a coexisting abscess (a collection of pus) [3][5].
  • Skin Irritation: Constant drainage can irritate the skin around the anus, causing itching or redness [2].

Mapping the Path: Diagnostic Imaging

A physical exam is the first step, but it often cannot see the full “map” of the fistula tunnel. Specialized imaging is crucial for planning a safe surgery that protects your muscles [6].

Pelvic MRI: The Gold Standard

A Pelvic MRI is considered the most accurate tool for mapping a fistula [7][8]. It provides a detailed, 3D-like view of the area, allowing doctors to:

  • Identify secondary tracts (hidden side-branches) that might be missed during a physical exam [9].
  • See exactly where the tunnel passes through the anal sphincter muscles [10].
  • Locate the internal opening inside the anal canal, which is vital for a successful cure [8].

Endoanal Ultrasound (EAUS)

In some cases, your doctor may use Endoanal Ultrasound, where a small probe is inserted into the anal canal to use sound waves to see the tissues [10]. This is highly accurate for checking muscle involvement and can be used alongside an MRI for the most complete picture [11][12].

Understanding Goodsall’s Rule

When doctors perform an initial exam, they often use a guideline called Goodsall’s Rule to predict the path of the fistula [13].

  • The Theory: The rule suggests that if the external hole is behind the anus, the tunnel likely curves toward the back. If the hole is in front, the tunnel usually goes in a straight line [13][14].
  • The Reality: While helpful, this rule is not always accurate, especially for complex or long-standing fistulas [15][16]. This is why modern imaging like MRI has largely replaced reliance on this rule [14].

Avoiding Misdiagnosis

Several other conditions can mimic an anal fistula, and it is important to rule them out [1]:

  • Hidradenitis Suppurativa: A chronic skin condition that causes painful lumps and tunnels, often in the armpits or groin [17][18].
  • Pilonidal Disease: An infection near the tailbone that can sometimes track down toward the anal area [19].
  • Tuberculosis (TB): While rare in some regions, TB can cause chronic, non-healing fistulas that require specific medication rather than just surgery [20][21].
  • Crohn’s Disease: This inflammatory bowel disease often presents with complex fistulas and requires a different treatment approach to manage the underlying inflammation [22][23].

Common questions in this guide

What are the first signs of an anal fistula?
The most common early signs include persistent throbbing pain around the anus and drainage of pus, blood, or fluid. You may also notice a visible lump, a feeling of pressure, or skin irritation near the anal opening.
Why do I need a pelvic MRI for an anal fistula?
A pelvic MRI provides a highly detailed, three-dimensional view of the fistula tunnel and surrounding tissue. It helps your surgeon identify any hidden side branches and see exactly where the tunnel crosses the anal sphincter muscles, which is crucial for planning a safe surgery.
What is Goodsall's Rule?
Goodsall's Rule is a traditional guideline doctors use during a physical exam to predict the path a fistula tunnel takes. While it can be a helpful starting point, it is not always accurate for complex fistulas, which is why modern imaging is heavily relied upon.
Can other conditions mimic an anal fistula?
Yes, several conditions can cause similar symptoms, including hidradenitis suppurativa, pilonidal disease, and Crohn's disease. A thorough physical evaluation and proper imaging are essential to rule out these other conditions and ensure you get the right treatment.
What is an endoanal ultrasound?
An endoanal ultrasound is an imaging test that uses a small probe inserted into the anal canal to create pictures of the tissue using sound waves. It is highly accurate for evaluating the anal sphincter muscles and is often used alongside an MRI to get a complete picture of the fistula.

Questions to Ask Your Doctor

Curated prompts to bring to your next appointment.

  1. 1.Will you be using a pelvic MRI or an endoanal ultrasound to map my fistula before surgery?
  2. 2.Does my fistula follow Goodsall's Rule, or do the imaging results suggest a more complex path?
  3. 3.How does the location of my fistula relative to the sphincter muscles affect my treatment options?
  4. 4.Are there any signs that my condition could be related to something other than a simple infection, such as Crohn’s disease?
  5. 5.What specific features on my MRI (like secondary tracts) should I be aware of?

Questions For You

Tap a prompt to share your answer — we'll use it plus this page's context to start a tailored conversation.

References

References (23)
  1. 1

    MRI evaluation of anal and perianal diseases.

    Balcı S, Onur MR, Karaosmanoğlu AD, et al.

    Diagnostic and interventional radiology (Ankara, Turkey) 2019; (25(1)):21-27 doi:10.5152/dir.2018.17499.

    PMID: 30582572
  2. 2

    Complex Fistula-in-Ano Extending From the Anal Canal to Mid-Thigh Treated With Combine Fistulectomy and Video-Assisted Anal Fistula Treatment (VAAFT): A Case Report.

    Ullah Z, Sani U, Khan S, et al.

    Cureus 2023; (15(12)):e51373 doi:10.7759/cureus.51373.

    PMID: 38292978
  3. 3

    Chronic fistula in ano associated with adenocarcinoma: a case report with a review of the literature.

    Ghosh NK, Kumar A

    Annals of coloproctology 2024; (40(Suppl 1)):S1-S5 doi:10.3393/ac.2022.00752.0107.

    PMID: 38752339
  4. 4

    Mucinous adenocarcinoma arising from chronic perianal fistula mimicking horseshoe abscess.

    Prasad SN, Razik A, Siddiqui F, Lal H

    BMJ case reports 2018; (2018()) doi:10.1136/bcr-2017-223063.

    PMID: 29622704
  5. 5

    Infectious Diseases Perspective of Anorectal Abscess and Fistula-in-ano Disease.

    Wright WF

    The American journal of the medical sciences 2016; (351(4)):427-34.

    PMID: 27079352
  6. 6

    Pre-surgery assessment of anal fistula.

    Jain BK

    Techniques in coloproctology 2020; (24(7)):775-776 doi:10.1007/s10151-020-02220-x.

    PMID: 32303931
  7. 7

    Seeing Beyond the Tract: The Superiority of 3 Tesla (3T) MRI Over X-ray Fistulography in Perianal Fistula Evaluation.

    Khanduri S, Shahbaz M, Sana , et al.

    Cureus 2025; (17(10)):e93825 doi:10.7759/cureus.93825.

    PMID: 41209876
  8. 8

    The role of magnetic resonance imaging in the preoperative evaluation of anal fistulas.

    Vo D, Phan C, Nguyen L, et al.

    Scientific reports 2019; (9(1)):17947 doi:10.1038/s41598-019-54441-2.

    PMID: 31784600
  9. 9

    Magnetic Resonance Imaging (MRI): Operative Findings Correlation in 229 Fistula-in-Ano Patients.

    Garg P, Singh P, Kaur B

    World journal of surgery 2017; (41(6)):1618-1624 doi:10.1007/s00268-017-3886-x.

    PMID: 28097414
  10. 10

    Preoperative assessment of fistula-in-ano using SonoVue enhancement during three-dimensional transperineal ultrasound.

    Yang J, Li Q, Li H, et al.

    Gastroenterology report 2024; (12()):goae002 doi:10.1093/gastro/goae002.

    PMID: 38419722
  11. 11

    Preoperative Assessment of Perianal Fistulas with Combined Magnetic Resonance and Tridimensional Endoanal Ultrasound: A Prospective Study.

    Varsamis N, Kosmidis C, Chatzimavroudis G, et al.

    Diagnostics (Basel, Switzerland) 2023; (13(17)) doi:10.3390/diagnostics13172851.

    PMID: 37685389
  12. 12

    Diagnostic Accuracy of Three-Dimensional Endoanal Ultrasound for Anal Fistula: A Systematic Review and Meta-analysis.

    Li J, Chen SN, Lin YY, et al.

    The Turkish journal of gastroenterology : the official journal of Turkish Society of Gastroenterology 2021; (32(11)):913-922 doi:10.5152/tjg.2021.20750.

    PMID: 34872892
  13. 13

    Predictive accuracy of Goodsall’s rule for fistula-in-ano

    Jayarajah U, Samarasekera DN

    The Ceylon medical journal 2017; (62(2)):97-99 doi:10.4038/cmj.v62i2.8474.

    PMID: 28697591
  14. 14

    Rules are made to be broken.

    Cirocco WC

    Techniques in coloproctology 2022; (26(10)):841-842 doi:10.1007/s10151-022-02648-3.

    PMID: 35718844
  15. 15

    Three-dimensional endoanal ultrasound to assess the validity of Goodsall's Rule and the Midline Rule in predicting the path of perianal fistula-in-ano and the location of the internal opening.

    Garcia-Botello S, Martín-Arévalo J, Martí-Fernández R, et al.

    Techniques in coloproctology 2022; (26(5)):351-361 doi:10.1007/s10151-022-02592-2.

    PMID: 35217938
  16. 16

    Anal fistula, there is more than meets the eye!

    Stijns J, Zimmerman DDE

    Techniques in coloproctology 2022; (26(5)):331-332 doi:10.1007/s10151-022-02615-y.

    PMID: 35353253
  17. 17

    Lipocalin-2 is expressed by activated granulocytes and keratinocytes in affected skin and reflects disease activity in acne inversa/hidradenitis suppurativa.

    Wolk K, Wenzel J, Tsaousi A, et al.

    The British journal of dermatology 2017; (177(5)):1385-1393 doi:10.1111/bjd.15424.

    PMID: 28256718
  18. 18

    Pilonidal sinus disease: an intergluteal localization of hidradenitis suppurativa/acne inversa: a cross-sectional study among 2465 patients.

    Benhadou F, Van der Zee HH, Pascual JC, et al.

    The British journal of dermatology 2019; (181(6)):1198-1206 doi:10.1111/bjd.17927.

    PMID: 30919434
  19. 19

    Pilonidal sinus of perianal location. Difficult differential diagnosis with perianal fistula.

    Rubio-López L, Benito-Barbero S, Guillamot-Ruano P, Páramo-Zunzunegui J

    Cirugia y cirujanos 2025; (93(2)):231-233 doi:10.24875/CIRU.22000314.

    PMID: 40199281
  20. 20

    Comparison of histopathology and real-time polymerase chain reaction (RT-PCR) for detection of Mycobacterium tuberculosis in fistula-in-ano.

    Garg P

    International journal of colorectal disease 2017; (32(7)):1033-1035 doi:10.1007/s00384-017-2783-y.

    PMID: 28210854
  21. 21

    Diagnostic and therapeutic challenges in isolated perianal tuberculosis: A case report and comprehensive management overview.

    Belhaj A, Omry A, Touati MD, et al.

    International journal of surgery case reports 2024; (119()):109770 doi:10.1016/j.ijscr.2024.109770.

    PMID: 38761692
  22. 22

    Management of Perianal Crohn's Disease.

    Parian AM, Obi M, Fleshner P, Schwartz DA

    The American journal of gastroenterology 2023; (118(8)):1323-1331 doi:10.14309/ajg.0000000000002326.

    PMID: 37207318
  23. 23

    The anal fistula plug in Crohn's disease patients with fistula-in-ano: a systematic review.

    Nasseri Y, Cassella L, Berns M, et al.

    Colorectal disease : the official journal of the Association of Coloproctology of Great Britain and Ireland 2016; (18(4)):351-6 doi:10.1111/codi.13268.

    PMID: 26749385

This page provides educational information on anal fistula symptoms and diagnostic imaging. Always consult a colorectal surgeon or gastroenterologist for an accurate diagnosis and personalized treatment plan.

Get notified when new evidence is published on Anal fistula.

We monitor PubMed for new peer-reviewed studies on this topic and email a short summary when something meaningful changes.