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February 23, 2005

Cocaine and Phenylephrine Eye Drop Test for Parkinson Disease

Author Affiliations

Letters Section Editor: Robert M. Golub, MD, Senior Editor.

JAMA. 2005;293(8):931-934. doi:10.1001/jama.293.8.932-c

To the Editor: While there is no specific diagnostic test for Parkinson disease (PD), recent studies using iodine 123–metaiodobenzylguanidine (MIBG) have shown that cardiac sympathetic innervation is reduced in patients with PD.1,2 α-Synuclein aggregations are found in the sympathetic ganglions as well as in the midbrain dopaminergic neurons.3 It is possible that sympathetic innervation to the pupillary dilator muscle in PD is reduced because it is innervated by the cervical sympathetic ganglia. We studied sympathetic dysfunction in the pupils as a possible diagnostic marker of PD by comparing responses to cocaine eye drops and phenylephrine eye drops. Cocaine blocks norepinephrine uptake, and cocaine-induced mydriasis is dependent on the sympathetic nerve terminal density. In contrast, phenylephrine acts directly on the adrenergic receptor to cause mydriasis.

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