Goodsall's rule
Goodsall's rule relates the external opening (in the perianal skin) of an anal fistula to its internal opening (in the anal canal). It states that if the perianal skin opening is posterior to the transverse anal line, the fistulous tract will open into the anal canal in the midline posteriorly, sometimes taking a curvilinear course. A perianal skin opening anterior to the transverse anal line is usually associated with a radial fistulous tract. Or in more direct terms, it means that anterior-opening fistulas tend to follow a simple, direct course while posterior-opening fistulas may  follow a devious, curving path with some even being horseshoe-shaped before opening in the posterior midline. Fistulas can be described as anterior or posterior relating to a line drawn in the coronal plane across the anus, the so-called transverse anal line. Anterior fistulas will have a direct track into the anal canal. Posterior fistulas will have a curved track with their external opening lying in the posterior midline of the anal canal. An exception to the rule are anterior fistulas lying more than 3.75 cm from the anus, which may have a curved track (similar to posterior fistulas ) that opens into the posterior midline of the anal canal. Goodsall's rule may not be applicable when the fistula is more than 3.75 cm from the anal verge. In such situations, the fistula is almost always indirect. In situations where there are multiple anal fistulae, the course would be similar to that of posterior-opening fistulae because of branching and communication between these openings. The investigation of choice for anal fistulas is magnetic resonance imaging (MRI). Fistulograms can be used to demonstrate the track of the fistula. Goodsall's rule is named after David Henry Goodsall who described it  in 1900.