What is the role of Goodsall's rule in predicting the internal opening of an anal fistula?

Goodsall's Rule for Predicting Internal Fistula Openings

Goodsall's rule has limited clinical utility and should not be relied upon as the primary method for predicting internal fistula openings, particularly for anterior fistulas and in women, where modern imaging with MRI or endoanal ultrasound achieves 91-97% accuracy compared to Goodsall's rule's overall 60-75% accuracy. 1, 2, 3

What Goodsall's Rule Actually States

Goodsall's rule predicts that:

• Posterior external openings (behind a transverse line through the anal center) track in a curved path to a midline posterior internal opening at 6 o'clock 2, 4
• Anterior external openings (in front of the transverse line) track in a straight radial path to the nearest point on the anal canal 2, 4

Actual Clinical Accuracy

The rule's predictive accuracy varies dramatically by location:

Posterior Fistulas

• 73-91% accuracy for posterior external openings 2, 3, 5
• More reliable when the external opening is 4.5-7.5 mm from the anal verge 2
• Works best for intersphincteric fistulas (93% accuracy) 5

Anterior Fistulas

• Only 52-69% accuracy for anterior external openings 3, 5
• Fails most dramatically in women (31-90% variable accuracy) 4
• Long anterior fistulas (>3 cm) frequently defy the rule by tracking to a midline anterior origin rather than radially 3

Overall Performance

• Overall accuracy: 60-75% across all studies 2, 3
• Positive predictive value: 49-77% 3, 4
• Less accurate for transsphincteric fistulas (68% accuracy) compared to intersphincteric (93%) 5

The Superior Alternative: Midline Rule

The Midline Rule states that up to 95% of all anal fistulas—regardless of external opening location—originate from a midline internal opening (either anterior or posterior). 4

This approach increases overall positive predictive value from 49% (Goodsall's) to 71% (Midline Rule) 4. The midline represents the dominant primary opening site because most cryptoglandular fistulas originate from infected anal glands at the dentate line, which are concentrated in the midline positions 6.

Modern Imaging Supersedes Clinical Rules

MRI with contrast and endoanal ultrasound should be the standard for identifying internal openings, not anatomical rules. 1

MRI Performance

• 74-97% accuracy for internal opening identification 1
• Sensitivity: 81-100% 1
• Particularly superior for complex, transsphincteric, and suprasphincteric fistulas 1
• Gadolinium contrast enables differentiation of active inflammation from fibrotic tracts 1

Endoanal Ultrasound Performance

• 91-95% accuracy for internal opening identification 1, 7
• More sensitive for intersphincteric and low transsphincteric fistulas 1
• Hydrogen peroxide enhancement creates brightly hyperechoic tracts, improving visualization 1, 7
• Limited by depth penetration for high fistulas and gas obscuration 1, 7

Critical Clinical Pitfalls

When Goodsall's Rule Fails Most Dangerously

• Anterior fistulas in women: The rule's poor performance (31-90% variable accuracy) in this population creates risk of surgical misadventure with "disastrous consequences" including fecal incontinence 4
• Scrotal extension fistulas: 82-88% have anterior internal openings, directly contradicting the expectation that high posterior fistulas would extend into the scrotum 8
• Complex transsphincteric fistulas: Only 68% accuracy, with frequent secondary tracts and horseshoe extensions not predicted by the rule 5

Examination Under Anesthesia Cautions

• Never probe aggressively to find a fistula based on Goodsall's predictions during acute abscess drainage—this creates iatrogenic tracts 1, 6
• If no obvious fistula exists, drain the abscess adequately and defer fistula assessment to elective setting with imaging 1

Practical Clinical Algorithm

For any patient with suspected anal fistula:

1. Perform digital rectal examination to assess for palpable cord-like tract and internal opening 6

2. Order preoperative imaging before any definitive surgery:

   • MRI pelvis with contrast (preferred for complex fistulas, Crohn's disease, recurrent disease) 1
   • Endoanal ultrasound (excellent for simple intersphincteric/low transsphincteric fistulas) 1, 7

3. Do not rely on Goodsall's rule to guide surgical planning, especially for:

   • Anterior fistulas 3, 4
   • Female patients 4
   • External openings >7.5 mm from anal verge 2
   • Any complex or recurrent fistula 5

4. Screen for Crohn's disease in all patients with recurrent perianal fistulas (occurs in ~33% of perianal disease) 1, 6

The era of relying on 19th-century anatomical rules has passed—modern imaging provides objective, accurate identification of internal openings and should guide all surgical decision-making. 1, 4