Lateral thinking – Interocular symmetry and asymmetry in neurovascular patterning, in health and disease

Abstract

No biological system or structure is likely to be perfectly symmetrical, or have identical right and left forms. This review explores the evidence for eye and visual pathway asymmetry, in health and in disease, and attempts to provide guidance for those studying the structure and function of the visual system, where recognition of symmetry or asymmetry may be essential. The principal question with regards to asymmetry is not 'are the eyes the same?', for some degree of asymmetry is pervasive, but 'when are they importantly different?'. Knowing if right and left eyes are 'importantly different' could have significant consequences for deciding whether right or left eyes are included in an analysis or for examining the association between a phenotype and ocular parameter. The presence of significant asymmetry would also have important implications for the design of normative databases of retinal and optic nerve metrics. In this review, we highlight not only the universal presence of asymmetry, but provide evidence that some elements of the visual system are inherently more asymmetric than others, pointing to the need for improved normative data to explain sources of asymmetry and their impact on determining associations with genetic, environmental or health-related factors and ultimately in clinical practice.

Towards Standardization of Retinal Vascular Measurements: On the Effect of Image Centering

“…Symmetry. The right-left symmetry of morphometric measurements of the vascular network remains an object of study [20]. Our pilot strengthens the hypothesis that good symmetry levels must not be taken for granted.…”

Quantitative Measurements of Enlarged Perivascular Spaces in the Brain are Associated with Retinal Microvascular Parameters in Older Community-Dwelling Subjects

“…We decided to analyse the associations with both eyes separately and found asymmetric results. This supports some of the conclusions of the laterality study by Cameron et al 34 . Future work could investigate the reasons for this, potentially looking at the carotid Doppler data 48 .…”

“…We used all subjects with suitable PVS and retinal data from both eyes. Following recommendations from previous studies and considering that symmetry of any retinal or ocular structure, or the brain and major vessels to which it is connected, cannot be assumed 34 , we analysed associations with each eye separately.…”

Retinal Biomarker Discovery for Dementia in an Elderly Diabetic Population

“…A single operator (LB), trained with an established protocol, used the semi-automatic VAMPIRE 3.0 (Vascular Assessment and Measurement Platform for Images of the Retina) [14,15] to measure optic disc (OD) radius, central retinal arteriolar equivalent (CRAE) and central retinal venular equivalent (CRVE), retinal arterio-venule-ratio (AVR), tortuosity of arteries (tortA) and veins (tortV), as well as maximum tortuosity (tortAmax and tortVmax) in right-eye images, after verifying sufficient correlation (r > ~0.6) of AVR, CRAE, CRVE with the left eye as done by others, e.g. [20]. Clinically non-semantic features.…”