Matching the LenStar optical biometer to A-Scan ultrasonography for use in small animal eyes with application to tree shrews

Purpose Exposure to narrow‐band red light, which stimulates only the long‐wavelength sensitive (LWS) cones, slows axial eye growth and produces hyperopia in tree shrews and macaque monkeys. We asked whether exposure to amber light, which also stimulates only the LWS cones but with a greater effective illuminance than red light, has a similar hyperopia‐inducing effect in tree shrews. Methods Starting at 24 ± 1 days of visual experience, 15 tree shrews (dichromatic mammals closely related to primates) received light treatment through amber filters (BPI 500/550 dyed acrylic) either atop the cage (Filter group, n = 8, 300–400 human lux) or fitted into goggles in front of both eyes (Goggle group, n = 7). Non‐cycloplegic refractive error and axial ocular dimensions were measured daily. Treatment groups were compared with age‐matched animals (Colony group, n = 7) raised in standard colony fluorescent lighting (100–300 lux). Results At the start of treatment, mean refractive errors were well‐matched across the three groups (p = 0.35). During treatment, the Filter group became progressively more hyperopic with age (p < 0.001). By contrast, the Goggle and Colony groups showed continued normal emmetropization. When the treatment ended, the Filter group exhibited significantly greater hyperopia (mean [SE] = 3.5 [0.6] D) compared with the Goggle (0.2 [0.8] D, p = 0.01) and Colony groups (1.0 [0.2] D, p = 0.01). However, the refractive error in the Goggle group was not different from that in the Colony group (p = 0.35). Changes in the vitreous chamber were consistent with the refractive error changes. Conclusions Exposure to ambient amber light produced substantial hyperopia in the Filter group but had no effect on refractive error in the Goggle group. The lack of effect in the Goggle group could be due to the simultaneous activation of the short‐wavelength sensitive (SWS) and LWS cones caused by the scattering of the broad‐band light from the periphery of the goggles.

Section: Methodsmentioning

confidence: 99%

Amber light treatment produces hyperopia in tree shrews

Khanal

Norton

Gawne

2021

“…Immediately after the refractive measurements, axial component dimensions were obtained with an optical biometer (LenStar LS‐900; Haag‐Streit, http://haag-streit.com). The raw data were analysed using a custom MATLAB program using tree shrew‐specific refractive indices 26 . This system gives comparable axial length values to A‐scan ultrasonography, but with better precision and repeatability 27 .…”

Section: Methodsmentioning

confidence: 99%

The effects of intensity, spectral purity and duty cycle on red light‐induced hyperopia in tree shrews

Gawne

Samal

She

2023

IntroductionThere have recently been several clinical studies suggesting that brief periods of exposure to red light (repeated low‐level red light, ‘RLRL’) may produce a dramatic anti‐myopia effect, calling for further investigations into its therapeutic parameters. Unfortunately, many experimental species used in refractive studies develop myopia in response to this wavelength. Tree shrews are the only animal model other than rhesus monkeys that consistently exhibit hyperopic responses to ambient red light. Here, tree shrews were used to study the influence of the spectral purity, duty cycle and intensity of red light on its anti‐myopic effect.MethodsJuvenile tree shrews (Tupaia belangeri) were raised from 24 to 35 days after eye opening under ambient lighting that was: standard white colony fluorescent light; pure narrow band red light of either 600, 50–100 or 5 lux; red light that was diluted with 10% white light (by lux) or 50% white and 2 s of pure red light that alternated with 2 s of pure white light (50% duty cycle). Refractive measures were taken with a NIDEK ARK‐700 autorefractor and axial dimensions with a LenStar LS‐900 Axial Biometer.ResultsThe pro‐hyperopia effect of ambient red light was greatly reduced by even small amounts of concurrent white light ‘contamination’, but remained robust if 2‐s periods of pure white light alternated with 2 s of red. Finally, the hyperopic effect of red light was maintained at reduced luminance levels in the 50–100 lux range and only failed at 5 lux.ConclusionsThese results have implications for understanding the mechanisms by which ambient red light affects refractive development, and possibly also for clinical therapies using RLRL. Nevertheless, it remains to be determined if the mechanism of the current clinical RLRL therapy is the same as that operating on tree shrews in ambient red light.

“…We exported the OPDs and transformed them into geometric distances (GDs) using the relationship:

GD = OPD / n_{g}

where

n_{g}

represents the refractive index of group

g

22 . We adopted values of

n_{g}

for the measurement wavelength of the Lenstar (818 nm), for the cornea, aqueous, lens and vitreous, using reference indices from the LeGrand model eye and the water scaling approach (see equations 4 and 7 in Suheimat et al 23 ), that is, 1.3845 for

n_{cornea}

, 1.3450 for

n_{aqueous}

, 1.4272 for

n_{lens}

, 1.3436 for

n_{vitreous}

and 1.4000 for

n_{retina}

.…”

Section: Methodsmentioning

confidence: 99%

“…We adopted values of

n_{g}

n_{cornea}

, 1.3450 for

n_{aqueous}

, 1.4272 for

n_{lens}

, 1.3436 for

n_{vitreous}

and 1.4000 for

n_{retina}

. We could not use the internal analysis of the Lenstar as it assumes an average group refractive index to calculate axial length 22,23 ; this average refractive index is only valid for on‐axis measurements and assumes relative eye tissue dimensions that are invalid for off‐axis measurements.…”

Section: Methodsmentioning

confidence: 99%

Modelling eye lengths and refractions in the periphery

Ramamirtham

Akula

Curran

et al. 2023

Self Cite

Purpose To create a simplified model of the eye by which we can specify a key optical characteristic of the crystalline lens, namely its power. Methods Cycloplegic refraction and axial length were obtained in 60 eyes of 30 healthy subjects at eccentricities spanning 40° nasal to 40° temporal and were fitted with a three‐dimensional parabolic model. Keratometric values and geometric distances to the cornea, lens and retina from 45 eyes supplied a numerical ray tracing model. Posterior lens curvature (PLC) was found by optimising the refractive data using a fixed lens equivalent refractive index (neq). Then, neq was found using a fixed PLC. Results Eccentric refractive errors were relatively hyperopic in eyes with central refractions ≤−1.44 D but relatively myopic in emmetropes and hyperopes. Posterior lens power, which cannot be measured directly, was derived from the optimised model lens. There was a weak, negative association between derived PLC and central spherical equivalent refraction. Regardless of refractive error, the posterior retinal curvature remained fixed. Conclusions By combining both on‐ and off‐axis refractions and eye length measurements, this simplified model enabled the specification of posterior lens power and captured off‐axis lenticular characteristics. The broad distribution in off‐axis lens power represents a notable contrast to the relative stability of retinal curvature.