Retinal Thickness in Essential Tremor and Early Parkinson Disease: Exploring Diagnostic Insights

Keeping an eye on Parkinson's disease: color vision and outer retinal thickness as simple and non-invasive biomarkers

Over the last two decades, visual symptoms and retinal changes in Parkinson's disease (PD) have emerged as important biomarkers. Color vision deficiency, which begins in the outer retina, has been increasingly investigated, but a focused review of these papers has not recently been conducted. Similarly, thinning of the outer retina as measured using optical coherence tomography (OCT) holds potential as a screening marker for PD, particularly as these devices are already commonplace in community and hospital settings. Moreover, outer retinal thinning may be more specific for Parkinson's disease as inner retinal changes also occur in more common neurodegenerative diseases like glaucoma and Alzheimer's disease. This review summarizes contemporary evidence on two outer retina focused measures, color vision and outer retinal thickness, which can be readily quantified using non-invasive approaches and thus examines their potential as biomarkers for screening, detection, and progression in PD.

Dynamics of retinal changes in early-stage Parkinson's disease

Parkinson's disease (PD) is a neurodegenerative disorder primarily characterized by motor symptoms, with emerging evidence suggesting retinal pathology, particularly in the ganglion cell-inner plexiform layer (GCIPL), detectable via optical coherence tomography (OCT). This study aimed to characterize early retinal dynamics in PD using OCT. We conducted a prospective one-year longitudinal multicenter study involving 53 early-stage PD patients with a disease duration of 5 years or less and 52 controls. The participants underwent retinal spectral-domain OCT, primary visual function and cognitive examinations. We examined baseline retinal measures and short-term longitudinal differences between groups via linear mixed effects models. In PD patients, the baseline GCIPL thickness in central regions was increased by up to 4 μm, and the rate of thinning in the parafoveal GCIPL was - 0.61 [0.29] µm/year faster over a one-year follow-up period than in controls in the 2- to 3-mm ring (p = 0.039). In PD patients, greater central GCIPL thickness was associated with poorer contrast sensitivity and reduced performance on the Farnsworth D15 color vision test. It also predicted subsequent thinning in both the GCIPL (2- to 3-mm ring) and the inner nuclear layer (2- to 5-mm rings). However, this increased thickness was not linked to prevalent or progressive motor or cognitive manifestations. In conclusion, this study provides the first detailed topographical description of early retinal dynamics in PD patients, revealing increased central GCIPL thickness and accelerated parafoveal GCIPL thinning in PD. However, the macular region shows complex and variable dynamics among PD patients, but these changes precede detectable progression in clinical scales.

Evaluation of retinal microvascular changes with OCT-A in Parkinson disease and essential tremor

This study aimed to evaluate the presence and comparison of microvascular abnormalities in essential tremor (ET) and Parkinson disease (PD) using optical coherence tomography angiography (OCT-A) and to show the relationship between retinal microvascular changes and disease stage in the patient group. A total of 176 eyes, including 26 PD diagnosed according to the United Kingdom PD Society Brain Bank criteria, 31 ET diagnosed according to the Washington heights-inwood genetic study of ET (WHIGET) criteria and 31 healthy controls, were included in the study. Unified PD assessment scale (UPDRS) motor scores, non-motor symptom scale (NMS), modified Hoehn&Yahr stages (mH&Y) and Fahn-Toloso-Marin grading scale were recorded. All patients were evaluated in terms of visual acuity, fundus examination, intraocular pressure measurements, and refractive errors in Ophthalmology department. Deep macular vascular density was significantly decreased in PD compared to both the controls and ET(P < .05). In the measurements in the inferior quadrant of the retinal nerve fiber layer (RNFL) of the optic disc (OD), the values of the controls were significantly higher than those of PD in both eyes (P = .014 and P = .010). Radial peripapillary capillarity density in the left eye was substantially lower in ET than in controls (P = .045). In both eyes, OD radial peripapillary capillarity inside the disc small values of PD were significantly lower than those of ET and controls (P < .05). In our study, deep macular vascular density, RNFL and radial peripapillary capillarity were significantly lower in PD compared with ET and control groups, and radial peripapillary capillarity was lower in ET compared with control group. This study provides valuable information regarding the potential of OCT-A as a diagnostic tool for PD and ET.

Fully automatic deep convolutional approaches for the screening of neurodegeneratives diseases using multi-view OCT images

The prevalence of neurodegenerative diseases (NDDs) such as Alzheimer's (AD), Parkinson's (PD), Essential tremor (ET), and Multiple Sclerosis (MS) is increasing alongside the aging population. Recent studies suggest that these disorders can be identified through retinal imaging, allowing for early detection and monitoring via Optical Coherence Tomography (OCT) scans. This study is at the forefront of research, pioneering the application of multi-view OCT and 3D information to the neurological diseases domain. Our methodology consists of two main steps. In the first one, we focus on the segmentation of the retinal nerve fiber layer (RNFL) and a class layer grouping between the ganglion cell layer and Bruch's membrane (GCL-BM) in both macular and optic disc OCT scans. These are the areas where changes in thickness serve as a potential indicator of NDDs. The second phase is to select patients based on information about the retinal layers. We explore how the integration of both views (macula and optic disc) improves each screening scenario: Healthy Controls (HC) vs. NDD, AD vs. NDD, ET vs. NDD, MS vs. NDD, PD vs. NDD, and a final multi-class approach considering all four NDDs. For the segmentation task, we obtained satisfactory results for both 2D and 3D approaches in macular segmentation, in which 3D performed better due to the inclusion of depth and cross-sectional information. As for the optic disc view, transfer learning did not improve the metrics over training from scratch, but it did provide a faster training. As for screening, 3D computational biomarkers provided better results than 2D ones, and multi-view methods were usually better than the single-view ones. Regarding separability among diseases, MS and PD were the ones that provided better results in their screening approaches, being also the most represented classes. In conclusion, our methodology has been successfully validated with an extensive experimentation of configurations, techniques and OCT views, becoming the first multi-view analysis that merges data from both macula-centered and optic disc-centered perspectives. Besides, it is also the first effort to examine key retinal layers across four major NDDs within the framework of pathological screening.

Distinguishing essential tremor from Parkinson's disease: clinical and experimental tools

INTRODUCTION

Essential tremor (ET) and Parkinson's disease (PD) are the most common causes of tremor and the most prevalent movement disorders, with overlapping clinical features that can lead to diagnostic challenges, especially in the early stages.

AREAS COVERED

In the present paper, the authors review the clinical and experimental studies and emphasized the major aspects to differentiate between ET and PD, with particular attention to cardinal phenomenological features of these two conditions. Ancillary and experimental techniques, including neurophysiology, neuroimaging, fluid biomarker evaluation, and innovative methods, are also discussed for their role in differential diagnosis between ET and PD. Special attention is given to investigations and tools applicable in the early stages of the diseases, when the differential diagnosis between the two conditions is more challenging. Furthermore, the authors discuss knowledge gaps and unsolved issues in the field.

EXPERT OPINION

Distinguishing ET and PD is crucial for prognostic purposes and appropriate treatment. Additionally, accurate diagnosis is critical for optimizing clinical and experimental research on pathophysiology and innovative therapies. In a few years, integrated technologies could enable accurate, reliable diagnosis from early disease stages or prodromal stages in at-risk populations, but further research combining different techniques is needed.