In anal fistula, goodsall’s rule predicts the type of track and location of internal opening.
An anterior external opening is more likely to have a straight track while a psoteriorly placed external opening is more likely to have a curved track.
Mnemonic: Think of a dog with a straight nose anteriorly and a curved tail posteriorly.
However, exception exists. With the patient lying supine, an imaginary transverse, or “trans-anal” line is drawn through the ischial tuberosities. The exception to Goodsall’s rule occurs when the external opening lies anterior to the line but >3 cm from the anal verge. In this case, the tract tends to follow the rule for posterior external openings with an indirect,curved path that ends at the posterior midline (similar to posterior fistula).