Optical Coherence Tomography in Parkinson's Disease: Is the Retina a Biomarker?

Visual symptoms are a common feature in patients with Parkinson's disease (PD), and retinal dopamine loss and dysfunctions in foveal vision have been described in PD patients. Because visual hallucination (VH) is a specific feature of PD which is differentiated from other parkinsonian disorders, defective visual information processing from both the central and peripheral pathways is suggested to be the pathophysiological mechanisms of VH in PD. Decreased visual acuity as well as impaired contrast sensitivity and color vision is known to be related to the appearance of VH in PD. However, these functional studies were also affected by cognitive or cortical dysfunctions; thus, structural imaging can more reliably reflect visual afferent dysfunction. Recently, optical coherence tomography (OCT) scans have been used to investigate the structural changes in the retina in vivo. Segmental measures of the vertical retinal layers by OCT provide structural evidence for retinal dopamine loss and foveal dysfunction in PD. Retinal nerve fiber layer (RNFL) thinning in PD was shown to be significantly associated with VH and correlated with PD duration and severity. Thus, there is a possibility that the retina could be a biomarker for disease progression and risk of VH in PD. A standard protocol for OCT studies in PD with more accurate measures of the retinal layers needs to be developed in the future.