Light conditions and the evolution of the visual system in birds

Despite vision is an essential sense for many animals, the intuitively appealing notion that the visual system has been shaped by environmental light conditions is backed by insufficient evidence. Based on a comprehensive phylogenetic comparative analysis of birds, we investigate if exposure to different light conditions might have triggered evolutionary divergence in the visual system through pressures on light sensitivity, visual acuity, and neural processing capacity. Our analyses suggest that birds that have adopted nocturnal habits evolved eyes with larger corneal diameters and, to a lesser extent, longer axial length than diurnal species. However, we found no evidence that sensing and processing organs were selected together, as observed in diurnal birds. Rather than enlarging the processing centers, we found a tendency among nocturnal species to either reduce or maintain the size of the two main brain centers involved in vision -the optic tectum and the wulst. These results suggest a mosaic pattern of evolution, wherein optimization of the eye optics for efficient light capture in nocturnal species may have compromised visual acuity and central processing capacity.

Using three-dimensional modelling of the anterior sclera to investigate the scleral profile in myopic eyes

PURPOSE

This study used three-dimensional (3D) modelling to investigate scleral profiles in myopic eyes and compare them with emmetropic eyes.

METHODS

In this prospective observational study, the eyes of 151 participants were analysed using the corneoscleral profile module (CSP) of the Pentacam HR. Non-rotationally symmetrical ellipsoids were fitted to the anterior scleral sagittal height. Three radii were analysed, namely the nasal-temporal (Rx), superior-inferior (Ry) and anterior-posterior (Rz) orientations. Additionally, the area index (AI) and aspherical parameters (Qxy, Qxz and Qyz) of the anterior sclera-fitted ellipsoid (ASFE) were quantified.

RESULTS

The findings showed an increase in Rx (-0.349 mm/D), Ry (-0.373 mm/D), Rz (-1.232 mm/D) and AI (-36.165 mm2 /D) with increasing myopia. From emmetropia to high myopia, the vertical and horizontal planes of the anterior sclera became increasingly prolate (emmetropia, Qxz: 0.02, Qyz: 0.01; low myopia, Qxz: -0.28, Qyz: -0.28; high myopia, Qxz: -0.41, Qyz: -0.43). There were no significant differences in the coronal plane across the three groups (H = 2.65, p = 0.27). The anterior scleral shape of high myopes in the horizontal and vertical planes was more prolate than that of emmetropes and low myopes (Qxz, high myopes vs. low myopes: p = 0.03, high myopes vs. emmetropes: p < 0.001; Qyz, high myopes vs. low myopes: p = 0.04, high myopes vs. emmetropes: p < 0.001).

CONCLUSIONS

As the degree of myopia increased, non-uniform anterior scleral enlargement was observed. These findings provide a better understanding of the anterior segment with varying degrees of myopia.

Eye Size and Shape in Relation to Refractive Error in Children: A Magnetic Resonance Imaging Study

Purpose

The purpose of this study was to determine the association between eye shape and volume measured with magnetic resonance imaging (MRI) and optical biometry and with spherical equivalent (SE) in children.

Methods

For this study, there were 3637 10-year-old children from a population-based birth-cohort study that underwent optical biometry (IOL-master 500) and T2-weighted MRI scanning (height, width, and volume). Cycloplegic refractive error was determined by automated refraction. The MRI images of the eyes were segmented using an automated algorithm combining atlas registration with voxel classification. Associations among optical biometry, anthropometry, MRI measurements, and RE were tested using Pearson correlation. Differences between refractive error groups were tested using ANOVA.

Results

The mean volume of the posterior segment was 6350 (±680) mm3. Myopic eyes (SE ≤ -0.5 diopters [D]) had 470 mm3 (P < 0.001) and 970 mm3 (P < 0.001) larger posterior segment volume than emmetropic and hyperopic eyes (SE ≥ +2.0D), respectively. The majority of eyes (77.1%) had an oblate shape, but 47.4% of myopic eyes had a prolate shape versus 3.9% of hyperopic eyes. The correlation between SE and MRI-derived posterior segment length (r -0.51, P < 0.001) was stronger than the correlation with height (r -0.30, P < 0.001) or width of the eye (r -0.10, P < 0.001).

Conclusions

In this study, eye shape at 10 years of age was predominantly oblate, even in eyes with myopia. Of all MRI measurements, posterior segment length was most prominently associated with SE. Whether eye shape predicts future myopia development or progression should be investigated in longitudinal studies.

The effect of sustained eye rotation upon eye length in healthy myopic adults

AIM

The aim of the study was to determine whether ocular movement can affect the shape of the globe and lead to measurable change in axial and peripheral eye length.

METHODS

Ten subjects aged 18-30 years (6 M/4 F) participated with informed consent. The mean spherical equivalent refractive error was ≤-1.00 DS with cylindrical refraction <-1.25 DC. One drop of tropicamide hydrochloride 1% was instilled 20 min before measurement to induce mydriasis and mild cycloplegia. Using IOLMaster, eye length was measured centrally and temporally (25° off-axis) in four different positions. Subjects then rotated their eyes 25° in the temporal direction to fixate on a target for 10 min. After that, the same measurements were repeated.

RESULTS

Before rotation, the group mean peripheral eye length was significantly shorter than the central eye length (P < 0.05). There was no significant variation in central or peripheral eye length due to off-axis fixation, either after the initial eye rotation or after fixation for 10 min at the off-axis point. The difference between central and peripheral eye lengths was maintained after 10 min of temporal fixation (P < 0.05).

CONCLUSION

Peripheral eye length was shorter than central eye length showing the prolate shape associated with myopia. The action of the extraocular muscles on the globe has no significant effect upon the retinal shape assessed by off-axis eye length measurement in myopic subjects.

Rapid Change in Mammalian Eye Shape Is Explained by Activity Pattern

The rate of morphological evolution along the branches of a phylogeny varies widely [1-6]. Although such rate variation is often assumed to reflect the strength of historical natural selection resulting in adaptation [7-14], this lacks empirical and analytical evidence. One way to demonstrate a relationship between branchwise rates and adaptation would be to show that rapid rates of evolution are linked with ecological shifts or key innovations. Here, we test for this link by determining whether activity pattern, the time of day at which species are active, explains rapid bursts of evolutionary change in eye shape. Using modern approaches to identify shifts in the rate of morphological evolution [7, 13], we find that over 74% of rapid eye-shape change during mammalian evolutionary history is directly explained by distinct selection pressures acting on nocturnal, cathemeral, and diurnal species. Our results reveal how ecological changes occurring along the branches of a phylogeny can manifest in subsequent changes in the rate of morphological evolution. Although selective pressures exerted by different activity patterns have acted uniformly across all mammals, we find differences in the rate of eye-shape evolution among orders. The key to understanding this is in how ecology itself has evolved. We find heterogeneity in how activity pattern has evolved among mammals that ultimately led to differences in the rate of eye-shape evolution among species. Our approach represents an exciting new way to pinpoint factors driving adaptation, enabling a clearer understanding of the factors that drive the evolution of biological diversity.

Current and predicted demographics of high myopia and an update of its associated pathological changes

PURPOSE

Excessive axial elongation of the eye in high myopia can cause biomechanical stretching leading to various ocular complications. The purpose of this review is to provide an update on various pathologic changes, especially in the chorio-retina and sclera that have been reported recently using advanced ophthalmic bio-imaging modalities such as optical coherence tomography, magnetic resonance imaging and fundus photography.

RECENT FINDINGS

The prevalence rates of pathologic myopia (myopic retinopathy and maculopathy) mirror the prevalence rates of high myopia as the risks of pathologic myopia increase with high myopia. Peripapillary and sub-foveal choroidal thinning, scleral thinning, and deformed/irregular eye shapes were found to be strongly associated with various pathologic myopic lesions, especially with posterior staphyloma and chorio-retinal atrophy. Considering the increasing prevalence rate of myopia and pathologic myopia, these degenerative changes are likely to increase dramatically over the next few decades due to the rapid growth in the number of individuals with high myopia and the ageing population.

SUMMARY

The current prevalence rates of pathologic myopia in older adults might have significantly underestimated the future prevalence rates and warrants age of onset of myopia being considered a major risk factor for pathologic myopia. Using advanced technology, identification of novel quantifiable chorio-retinal and scleral parameters in pathologic myopia would help to identify people at risk of developing pathologic myopia and potentially may revolutionise the management of myopia especially in risk prognostication and monitoring of disease progression.