Hydroxyamphetamine test for Horner syndrome.
A. Before drops administered (suspected right Horner syndrome).
B. After drops administered. Note the dilation of both pupils. This indicates an intact 3rd-order, postganglionic neuron and localizes the lesion to the 1st-order (central) or 2nd-order (preganglionic) neuron.
Image credit: Modified from clinical images courtesy of Lanning B. Kline, M.D. American Academy of Ophthalmology. Used with permission for educational purposes.