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.

Both Midline and Goodsall's rules are highly predictive of the course of fistula tracts located in the posterior plane, and are lower for anterior located fistulae, female patients and when the external opening is located further from the anal verge.