neurologic disorders
Horner Syndrome
Diagnosis of Horner Syndrome
Cocaineorapraclonidineeye drop test MRI or CT to diagnose cause Instilling eyedrops can help confirm and characterize Horner syndrome. First,cocaine(4% or 10%) orapraclonidine(0.5%) drops are put in both eyes: Cocaine:Cocaineblocks the synaptic reuptake ofnorepinephrineand causes the pupil of the unaffected eye to dilate. If a postganglionic lesion (peripheral Horner syndrome) is present, the pupil of the affected eye does not dilate because the postganglionic nerve terminals have degenerated; the result is increased anisocoria. If the lesion is above the superior cervical ganglion (preganglionic or central Horner syndrome) and the postganglionic fibers are intact, the pupil of the affected eye also dilates, and anisocoria decreases. Apraclonidine:Apraclonidineis a weak alpha-adrenergic agonist that minimally dilates the pupil of a normal eye. If a postganglionic lesion is present (peripheral Horner syndrome), the pupil of the affected eye dilates much more than that of the unaffected eye because the iris dilator muscle of the affected eye has lost its sympathetic innervation and has developed adrenergic supersensitivity. As a result, anisocoria decreases. (However, results may be falsely normal if the causative lesion is acute.) If the lesion is preganglionic (or a central Horner syndrome), the pupil of the affected eye does not dilate because the iris dilator muscle does not develop adrenergic supersensitivity; as a result, anisocoria increases. If results suggest Horner syndrome, hydroxyamphetamine (1%) can be put in both eyes 48 hours later to help locate the lesion.Hydroxyamphetamineworks by causingnorepinephrineto be released from the presynaptic terminals. It has no effect if postganglionic lesions are present because these lesions cause postganglionic terminals to degenerate. Thus, whenhydroxyamphetamineis applied, the following occur: Postganglionic lesion:The pupil of the affected eye does not dilate, but the pupil of the unaffected eye dilates, resulting in increased anisocoria. Central or preganglionic lesion:The pupil of the affected eye dilates normally or more than it normally does, and the pupil of the unaffected eye dilates normally, resulting in decreased or unchanged anisocoria. (However, postganglionic lesions sometimes produce the same results.) Hydroxyamphetamine testing is done less frequently than testing withapraclonidine, partly becausehydroxyamphetaminetends to be available less often. For results to be valid,hydroxyamphetaminetesting must be delayed until at least 24 hours afterapraclonidineinstillation. Patients with Horner syndrome require MRI or CT of the brain, spinal cord, chest, or neck (depending on clinical suspicion) to localize the abnormality. 