Retinal Abnormalities as Biomarker of Disease Progression and Early Diagnosis of Parkinson Disease

* To determine whether retinal abnormalities, as measured by high definition optical coherence tomography (HD-OCT) and visual electrophysiology techniques can be used as a clinical biomarker to monitor disease progression overtime in patients with Parkinson disease. * To establish whether these measures can be used to identify patients with PD in the premotor phase. * To define the rate of progression of retinal abnormalities in PD (both in the motor and premotor stages) for potential use as a clinical outcome measure