Exceptions that Prove the Rules in Neuro-Ophthalmology

Although often misinterpreted in today's vernacular, the idiom 'the exception that proves the rule' should never be disregarded in medicine as it results in a correct, albeit unexpected, diagnosis. From the Latin 'exceptio firmat regulam in casibus non exceptis,' the legal interpretation of the idiom emphasizes that the exception to a rule proves the existence of that rule (1). As clinical practice has become increasingly complex, this idiom is ever more important as it compels us to question our decisions and diagnoses when clinical evidence is not always straightforward. We present 3 guiding principles in neuro-ophthalmology with an illustrative case that violates each principle. These exceptions prove that indeed these rules do exist.