Goodsall's rule

This anatomical rule relates the external opening of an anal fistula to its internal opening. Fistulas can be described as anterior or posterior relating to a line drawn in the coronal plane across the anus. Anterior fistulas will have a direct track into the anal canal. Posterior fistulas will have a curved track with their internal opening lying in the posterior midline of the anal canal. An exception to the rule are anterior fistulas lying more than 3cm from the anus, which may open in the anterior midline of the anal canal.

The investigation of choice for anal fistulas is MRI.