Letters Section Editor: Robert M. Golub,
MD, Senior Editor.
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.
Sawada H, Yamakawa K, Yamakado H, Hosokawa R, Ohba M, Miyamoto K, Kawamura T, Shimohama S. Cocaine and Phenylephrine Eye Drop Test for Parkinson Disease. JAMA. 2005;293(8):931-934. doi:10.1001/jama.293.8.932-c