What is Goodsall's rule in the management of anal fistulas?

Goodsall's Rule for Anal Fistulas

Goodsall's rule has limited accuracy (approximately 75% overall) and should be used cautiously, particularly for anterior fistulas where it fails in the majority of cases—the "Midline Rule" (which predicts that most fistulas track to a midline internal opening regardless of external opening location) is more accurate, especially in women. 1, 2

What is Goodsall's Rule?

Goodsall's rule, described in 1887, attempts to predict the location of the internal (primary) opening of an anal fistula based on the position of the external (secondary) opening 2:

• Posterior external openings (behind a transverse anal line): The rule predicts these track in a curved path to a midline posterior internal opening 1, 2
• Anterior external openings (in front of the transverse anal line): The rule predicts these track in a straight radial path to an anterior internal opening directly opposite the external opening 2, 3

Accuracy and Limitations

Overall Performance

• The overall accuracy of Goodsall's rule is only 74.75%, with a positive predictive value of 77.1% 1
• Accuracy varies dramatically by location: 73% for posterior fistulas but only 52.4% for anterior fistulas 1

Where Goodsall's Rule Works

• Posterior external openings: The rule is accurate in 90% of cases, correctly predicting midline posterior tracking 3
• Short, superficial fistulas: Better accuracy than long, complex tracts 1

Where Goodsall's Rule Fails Critically

Anterior fistulas defy Goodsall's rule in the majority of cases 2, 3:

• Only 49% of anterior external openings track radially as predicted by Goodsall 3
• Instead, 71% of anterior fistulas actually track to the midline (either anterior or posterior midline), not radially 3
• This failure is particularly pronounced in women, where 90% of anterior fistulas track to midline anterior (not radially), versus 62% in men 3
• Long anterior fistulas (>3 cm) commonly track to midline anterior origin, directly contradicting Goodsall's rule 1

The Superior Alternative: Midline Rule

The Midline Rule states that the internal opening of most anal fistulas (up to 95%) is located at the midline, regardless of where the external opening is positioned 2:

• Increases overall positive predictive value from 49% (Goodsall) to 71% (Midline Rule) 2
• For men: 57-62% accuracy 2
• For women: 31-90% accuracy (though variable, still superior to Goodsall for anterior fistulas) 2
• Overall, 81% of complete submuscular anal fistulas course to the midline (51% midline posterior, 30% midline anterior) 3

Clinical Implications and Pitfalls

Critical Safety Concern

Reliance on Goodsall's rule for anterior fistulas, especially in women, can lead to "disastrous consequences" including unnecessary sphincter damage and fecal incontinence 2:

• Anterior-based fistulas carry increased risk of fecal incontinence with surgical intervention 2
• Misidentifying the internal opening based on Goodsall's rule may result in inappropriate sphincter division 2

Special Anatomical Considerations

• Scrotal extension fistulas: These are predominantly (87.5%) associated with anterior internal openings, not posterior as might be expected from high transsphincteric fistulas 4
• High transsphincteric or suprasphincteric fistulas rarely extend into the scrotum except in recurrent cases 4

Practical Surgical Approach

• Always confirm internal opening location with MRI and intraoperative findings rather than relying solely on Goodsall's rule 1
• For simple intersphincteric fistulas, fistulotomy remains the procedure of choice with >95% healing rates 5
• For complex fistulas, sphincter-saving techniques (LIFT procedure, rectal advancement flaps) achieve 60-90% healing rates 5
• Loose setons should be placed for drainage before definitive surgery, particularly with concomitant proctitis 6