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PubMed This is a summary of 17 peer-reviewed journal articles Updated
Colorectal Surgery

Anatomy and Classification: Understanding Your Report

At a Glance

An anal fistula MRI or surgical report maps the abnormal tunnel using the Parks classification system based on its path through the sphincter muscles. Identifying whether the fistula is simple or complex, along with its exact location and branches, is crucial for planning safe, effective surgery.

Understanding the anatomy of an anal fistula is like reading a map of a tunnel. The 3D anatomy of the anal sphincter (the internal and external muscles) can be notoriously difficult to visualize, so you may find it helpful to ask your doctor to draw a diagram or show you a medical illustration. To provide the best care, your medical team uses specific classification systems to describe exactly where that tunnel goes and which muscles it crosses.

The Parks Classification System

The most common way doctors categorize these tunnels is the Parks classification [1]. It identifies four main types based on their relationship to the sphincter muscles (the muscles that control bowel movements) [2][3]:

  • Intersphincteric: The most common type. The tunnel stays in the space between the internal and external sphincter muscles [1].
  • Transsphincteric: The tunnel passes through both the internal and external sphincter muscles to reach the skin [1].
  • Suprasphincteric: The tunnel loops over the top of the sphincter muscles before heading down to the skin [1].
  • Extrasphincteric: The rarest type. The tunnel starts high up in the rectum and bypasses the sphincter muscles entirely, often caused by trauma or severe inflammation [1].

Simple vs. Complex Fistulas

Your doctor will likely tell you if your fistula is simple or complex. This distinction is critical for deciding which surgery is safest for you [4].

  • Simple Fistulas: These are usually low-lying and involve only a small amount of muscle (like a low intersphincteric fistula). They are often cured with a single, straightforward procedure [5][6].
  • Complex Fistulas: These involve more muscle or have “extra” features. A fistula is generally called complex if it is high-transsphincteric, suprasphincteric, or extrasphincteric [4][7]. It is also considered complex if it has multiple branches, is a recurrence of a previous fistula, or is associated with conditions like Crohn’s disease [8][9].

Completeness Checklist: Your MRI or Surgical Report

A high-quality Pelvic MRI or operative report is the foundation of a successful treatment plan. When you review your records, look for these specific details to ensure your “map” is complete [10][11]:

  • [ ] The Internal Opening: Does the report specify where the tunnel starts inside the anal canal? This is often described using a “clock position” (e.g., 6 o’clock is toward the tailbone) [9][12].
  • [ ] The Primary Tract: Does it clearly state the Parks classification (e.g., “transsphincteric”)? [3][2].
  • [ ] Secondary Tracts: Does the report mention any side-branches or “hidden” tunnels? [13][14].
  • [ ] Horseshoe Extensions: Does the fistula curve around to both sides of the anus? This is a specific type of complex extension [13][9].
  • [ ] Sphincter Involvement: Does it estimate how much of the external sphincter muscle is involved (e.g., “lower third”)? [15][12].
  • [ ] Abscess Presence: Does the report mention any active collections of pus that need to be drained? [16].

Having this level of detail helps your surgeon choose a procedure that maximizes your chance of a cure while minimizing the risk to your bowel control [12][17].

Common questions in this guide

What is the Parks classification system for an anal fistula?
The Parks classification system is how doctors categorize anal fistulas based on how they pass through or around your sphincter muscles. The four main types are intersphincteric, transsphincteric, suprasphincteric, and extrasphincteric.
What makes an anal fistula 'complex' instead of 'simple'?
A fistula is considered complex if it involves a significant amount of the sphincter muscle, has multiple branching tunnels, or is linked to conditions like Crohn's disease. These require more careful surgical planning to preserve bowel control.
Why does my MRI report use a clock position to describe my fistula?
Doctors use a 'clock face' system to describe exactly where the internal opening of the fistula tunnel starts inside the anal canal. For example, a 6 o'clock position refers to the direction pointing toward your tailbone.
What is a horseshoe fistula extension?
A horseshoe extension is a specific type of complex anal fistula that curves around to both sides of the anus. It is important for your surgeon to identify this on an MRI so that all parts of the tunnel are treated.

Questions to Ask Your Doctor

Curated prompts to bring to your next appointment.

  1. 1.Which of the four Parks classifications describes my fistula?
  2. 2.Is my fistula considered 'simple' or 'complex' based on the muscle involvement?
  3. 3.Does the MRI show any secondary tracts or 'horseshoe' extensions?
  4. 4.Where is the internal opening located using the 'clock position' (e.g., 6 o'clock)?
  5. 5.How much of my external sphincter muscle is involved in the tract?
  6. 6.Does my report indicate any supralevator extension, and how does that change the surgery?

Questions For You

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References

References (17)
  1. 1

    [Classification of anal fistulas based on magnetic resonance imaging].

    Liu D, Li W, Wang X, et al.

    Zhonghua wei chang wai ke za zhi = Chinese journal of gastrointestinal surgery 2018; (21(12)):1391-1395.

    PMID: 30588591
  2. 2

    External validation of the modified Parks classification of cryptoglandular anal fistula to predict failure of healing: the protocol for a retrospective analysis.

    Teymouri A, Keshvari A, Khorasanizadeh F, et al.

    International journal of surgery protocols 2025; (29(3)):118-121 doi:10.1097/SP9.0000000000000055.

    PMID: 40860206
  3. 3

    The development of a minimum dataset for MRI reporting of anorectal fistula: a multi-disciplinary, expert consensus process.

    Iqbal N, Sackitey C, Gupta A, et al.

    European radiology 2022; (32(12)):8306-8316 doi:10.1007/s00330-022-08931-z.

    PMID: 35732929
  4. 4

    [Diagnosis and treatment of complex anal fistula:current status and prospects].

    Zhu J, Ding JH

    Zhonghua wei chang wai ke za zhi = Chinese journal of gastrointestinal surgery 2024; (27(12)):1221-1226 doi:10.3760/cma.j.cn441530-20240912-00315.

    PMID: 39710447
  5. 5

    [Transformation and evidence-based progress of diagnosis and treatment mode for complex anal fistula].

    Zhu J, Ding JH

    Zhonghua wei chang wai ke za zhi = Chinese journal of gastrointestinal surgery 2025; (28(12)):1404-1410 doi:10.3760/cma.j.cn441530-20250928-00366.

    PMID: 41397822
  6. 6

    Fistulotomy versus Fistulectomy for Fistula-in-Ano: A Randomized Prospective Study.

    Hiremath SCS, Patil R

    Surgery journal (New York, N.Y.) 2022; (8(4)):e336-e340 doi:10.1055/s-0042-1758633.

    PMID: 36425406
  7. 7

    Risk Factors for Recurrence after anal fistula surgery: A meta-analysis.

    Mei Z, Wang Q, Zhang Y, et al.

    International journal of surgery (London, England) 2019; (69()):153-164 doi:10.1016/j.ijsu.2019.08.003.

    PMID: 31400504
  8. 8

    Predictive factors for recurrence of high transsphincteric anal fistula after placement of seton.

    Emile SH, Elfeki H, Thabet W, et al.

    The Journal of surgical research 2017; (213()):261-268 doi:10.1016/j.jss.2017.02.053.

    PMID: 28601324
  9. 9

    MRI of Recurrent Fistula-in-Ano: Is it Different from Treatment-Naïve Fistula-in-Ano and How Does it Correlate with Anal Sphincter Morphology?

    Augustine A, Patel PG, Augustine A, et al.

    The Indian journal of radiology & imaging 2023; (33(1)):19-27 doi:10.1055/s-0042-1758202.

    PMID: 36855724
  10. 10

    Structured magnetic resonance imaging and endoanal ultrasound anal fistulas reporting template (SMART): An interdisciplinary Delphi consensus.

    Sudoł-Szopińska I, Garg P, Mellgren A, et al.

    World journal of gastrointestinal surgery 2024; (16(10)):3288-3300 doi:10.4240/wjgs.v16.i10.3288.

    PMID: 39575264
  11. 11

    ESGAR consensus statement on the imaging of fistula-in-ano and other causes of anal sepsis.

    Halligan S, Tolan D, Amitai MM, et al.

    European radiology 2020; (30(9)):4734-4740 doi:10.1007/s00330-020-06826-5.

    PMID: 32307564
  12. 12

    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
  13. 13

    A case report of primary complex anal fistula with 7 external openings treated with combined preoperative 3D MRI model.

    Hong Y, Qiu Y, Li G

    Medicine 2023; (102(11)):e33264 doi:10.1097/MD.0000000000033264.

    PMID: 36930087
  14. 14

    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
  15. 15

    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
  16. 16

    Is modern management of fistula-in-ano acceptable?

    Oldfield F, Gilbert T, Skaife P

    British journal of hospital medicine (London, England : 2005) 2016; (77(7)):388-93 doi:10.12968/hmed.2016.77.7.388.

    PMID: 27388377
  17. 17

    [Clinical characteristics and risk factors for recurrence of anal fistula patients].

    Li J, Yang W, Huang Z, et al.

    Zhonghua wei chang wai ke za zhi = Chinese journal of gastrointestinal surgery 2016; (19(12)):1370-1374.

    PMID: 28000193

This page explains anal fistula classification and MRI reports for educational purposes. Your colorectal surgeon or radiologist is the best source for interpreting your specific imaging and surgical reports.

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