Horner syndrome may indicate a serious disorder such as carotid dissection, Pancoast tumour or medullary infarction. Autonomic headache syndromes such as cluster headache and hemicrania continua may also cause Horner syndrome.1 Absence of ptosis and pupillary miosis may delay the diagnosis and lead to grave consequences. We present a patient with painful Horner syndrome initially masked by Visine (tetrahydrozoline) use. A 46-year-old man developed sudden-onset, severe right periorbital aching for 8 weeks. The pain was constant with intermittent exacerbations. He noted ptosis of the right upper lid and red eye for 1 week. He subsequently used Visine (tetrahydrozoline HCl 0.05%) several times daily in …