A 58-year-old patient with anisocoria has the following pupil measurements: Light  room    OD:  3.5mm     OS: 5.5mm Dark room     OD:  4.0mm        OS: 8.0mm You administer 4% cocaine and no dilation occurs in the right eye.  After 48 hours, you re-test  the patient with 1% hydroxyamphetamine (Paradrine) and the pupil does not dilate.

The most likely diagnosis is:

c -- Horner’s syndrome: third order neuron When evaluating a patient with anisocoria it is important to look at the asymmetry in pupil size both in light and dark conditions. This patient has a Horner’s syndrome- a third-order (post-ganglionic) lesion. Topical cocaine helps diagnose a Horner’s syndrome and paradrine is used to help localize the lesion. Paradrine releases norepinephrine (NE) from the nerve terminal and is used to distinguish pre- and post-ganglionic lesions.