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Horner's syndrome describes a triad of pupillary miosis, partial eyelid ptosis and facial anhidrosis caused by disruption of the sympathetic pathway from the posterior hypothalamus to the eye. Diagnostic confirmation is crucial as causes can include life-threatening carotid dissection and apical lung tumours. Often the signs are subtle, with a difference of only 1-2 mm between affected and unaffected pupils and a similarly mild degree of ptosis. Anhidrosis is rarely obvious and generally present only in preganglionic lesions, as the sudomotor fibres diverge from the main pathway just after the superior cervical ganglion.
Historically pharmacological confirmation used 4% topical cocaine to inhibit norepinephrine re-uptake, dilating only the normal pupil. In modern practice, this …