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Goodsall's rule

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Goodsall's rule relates the external opening of an anal fistula to its internal opening.

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Goodsall's Rule states that the external opening situated behind the transverse anal line will open into the anal canal in the midline posteriorly. An anterior opening is usually associated with a radial tract.

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 internal opening lying in the posterior midline of the anal canal. An exception to the rule are anterior fistulas lying more than 3 cm. from the anus, which may open in the anterior midline of the anal canal.

The investigation of choice for anal fistulas is magnetic resonance imaging (MRI).