This KnowMed easy notes provides a comprehensive overview of Horner syndrome, which results from an interruption of the sympathetic nerve supply to the eye and is characterized by a triad of symptoms including constricted pupil, partial eyelid droop, and decreased facial sweating. The source details the anatomy of the three-neuron sympathetic arc essential for understanding the condition’s mechanism. A significant portion of the text is dedicated to enumerating the numerous potential causes of the syndrome, classifying them based on the location of the lesion (first, second, or third-order neurons), and discussing the importance of a thorough patient history and physical examination. Furthermore, the text covers the diagnostic process, highlighting the critical role of pharmacologic testing (such as the cocaine, apraclonidine, and hydroxyamphetamine tests) in both confirming and localizing the lesion, and concludes by addressing treatment options which are dependent on the underlying etiology.