Goodsall's Rule in Anal Fistulae
Goodsall's rule has limited clinical utility with only 74.75% overall accuracy and should not be relied upon as the primary method for predicting internal fistula openings, particularly in anterior fistulae where accuracy drops to 52.4%. 1
Understanding Goodsall's Rule
Goodsall's rule traditionally predicts that:
- Posterior external openings track in a curved path to a midline posterior internal opening 1
- Anterior external openings track radially (straight) to the internal opening 1
However, this rule has significant limitations that impact surgical planning and patient outcomes.
Accuracy and Clinical Performance
The rule performs poorly in several key scenarios:
- Anterior fistulae: Only 52.4% accuracy, meaning nearly half are misidentified 1
- Posterior fistulae: Better at 73% accuracy but still fails in over one-quarter of cases 1
- Long anterior fistulae (>3 cm): Frequently defy Goodsall's rule by tracking to a midline anterior origin rather than radially 1
- Short posterior fistulae: Often open in a direct radial course rather than curving to the posterior midline 1
The positive predictive value is only 49% when using Goodsall's rule, compared to 71% when using the "Midline Rule" (which assumes all fistulae originate at the midline) 2
The Midline Rule: A Superior Alternative
Recent evidence demonstrates that the midline is the dominant primary (internal) opening site for up to 95% of all anal fistulae, regardless of external opening location. 2
- The Midline Rule increases overall predictive accuracy from 49% to 71% 2
- This is particularly important for anterior fistulae in women, where Goodsall's rule has disastrous predictive failure rates (31-90% variability) 2
- Anterior off-midline external openings tend to mirror posterior fistulae and curve to a midline origin, contradicting Goodsall's prediction of a straight radial course 2
Clinical Implications and Pitfalls
The most critical pitfall is relying on Goodsall's rule for anterior-based fistulae, especially in women, where surgical misidentification increases the risk of fecal incontinence. 2
Mandatory Preoperative Assessment:
- MRI or endoanal ultrasound should be obtained for surgical planning rather than relying on anatomical rules alone 3, 4
- Digital rectal examination to assess for internal opening, but avoid probing to search for occult fistulae as this creates iatrogenic tracts 3
- Rule out Crohn's disease in all patients with recurrent fistulae, as this occurs in 13-27% of Crohn's patients and markedly reduces surgical success 5, 6
Special Anatomical Considerations:
Fistulae with scrotal extension predominantly have anterior internal openings (87.5% in primary cases), contradicting the expectation that high posterior fistulae would extend into the scrotum 7. The relative risk of scrotal extension is 18.67 times higher with anterior versus posterior internal openings 7.
Practical Surgical Approach
Rather than applying Goodsall's rule, use preoperative MRI to identify the internal opening directly, combined with intraoperative assessment. 3, 1
- For simple intersphincteric fistulae: Fistulotomy is the procedure of choice with >95% healing rates 4
- For complex fistulae: Use sphincter-saving techniques (LIFT, advancement flaps, TROPIS) with 60-90% healing rates 4, 8
- The overall success rate using algorithm-based approaches (fistulotomy for simple, TROPIS for complex) reaches 93.5% 8
The key takeaway: Goodsall's rule should be retired from clinical practice in favor of direct imaging-based identification of internal openings, particularly given the high risk of incontinence from surgical misadventure in anterior fistulae. 2