SciCombinator

Discover the most talked about and latest scientific content & concepts.

Concept: Accommodation

27

PURPOSE. TO INVESTIGATE THE SHORT-TERM EFFECT OF ORTHOKERATOLOGY (ORTHO-K) LENS WEAR ON THE ANTERIOR SEGMENT LENGTH FOR VALIDATING THE USE OF AXIAL LENGTH FOR MONITORING MYOPIC PROGRESSION AFTER ORTHO-K TREATMENT. METHODS. THIRTY-SEVEN AND 39 SUBJECTS (AGED 7-10) WERE RANDOMLY ASSIGNED TO WEAR ORTHO-K AND SINGLE-VISION SPECTACLES, RESPECTIVELY. CENTRAL CORNEAL THICKNESS (CCT), ANTERIOR CHAMBER DEPTH (ACD), CRYSTALLINE LENS THICKNESS (LT), AND ANTERIOR SEGMENT LENGTH (ASL: summation of CCT, ACD and LT) were measured before and 6-month after the treatment under cycloplegia. Changes in these parameters were evaluated and compared between the two groups of subjects. Results. There were no significant between-group differences in the baseline data (p>0.37). After 6 months of lens wear, in the ortho-k group, CCT was significantly reduced by 0.009±0.009 mm (p<0.001) while ACD and LT remained unchanged (p>0.15). In the spectacle group, ACD was significantly increased by 0.01±0.03 mm (p=0.008) while CCT and LT remained unchanged (p>0.06). In both groups of subjects, ASL did not changed significantly but axial length was significantly increased by 0.10±0.10 mm and 0.20±0.11 mm in the ortho-k and the spectacle groups, respectively (p<0.001). Conclusions. Eyeball elongation occurred in children wearing both ortho-k and single-vision spectacles. Since, ASL was not affected by ortho-k treatment, axial length measured reflects the true growth of the eyeball and is a valid parameter for monitoring myopic progression in ortho-k treated eyes.

Concepts: Eye, Cornea, Myopia, Refractive surgery, Lens, Orthokeratology, Contact lens, Accommodation

25

A non-linear isotropic finite element (FE) model of a 29-year-old human crystalline lens was constructed to study the effects of various geometrical parameters on lens accommodation.

Concepts: Mathematics, Fundamental physics concepts, Carbon, Eye, Lens, Cardinality, Accommodation

14

Presbyopia, the loss of the eye’s accommodation capability, affects all humans aged above 45-50 years old. The two main reasons for this to happen are a hardening of the crystalline lens and a reduction of the ciliary muscle functionality with age. While there seems to be at least some partial accommodating functionality of the ciliary muscle at early presbyopic ages, it is not yet clear whether the muscle is still active at more advanced ages. Previous techniques used to visualize the accommodation mechanism of the ciliary muscle are complicated to apply in the older subjects, as they typically require fixation stability during long measurement times and/or to have an ultrasound probe directly in contact with the eye. Instead, we used our own developed method based on high-speed recording of lens wobbling to study the ciliary muscle activity in a small group of pseudophakic subjects (around 80 years old). There was a significant activity of the muscle, clearly able to contract under binocular stimulation of accommodation. This supports a purely lenticular-based theory of presbyopia and it might stimulate the search for new solutions to presbyopia by making use of the remaining contraction force still presented in the aging eye.

Concepts: Age, Eye, Ageing, Lens, Presbyopia, Intraocular lens, Ciliary muscle, Accommodation

3

Virtual Reality (VR) headsets create immersion by displaying images on screens placed very close to the eyes, which are viewed through high powered lenses. Here we investigate whether this viewing arrangement alters the binocular status of the eyes, and whether it is likely to provide a stimulus for myopia development. We compared binocular status after 40-minute trials in indoor and outdoor environments, in both real and virtual worlds. We also measured the change in thickness of the ocular choroid, to assess the likely presence of signals for ocular growth and myopia development. We found that changes in binocular posture at distance and near, gaze stability, amplitude of accommodation and stereopsis were not different after exposure to each of the 4 environments. Thus, we found no evidence that the VR optical arrangement had an adverse effect on the binocular status of the eyes in the short term. Choroidal thickness did not change after either real world trial, but there was a significant thickening (≈10 microns) after each VR trial (p < 0.001). The choroidal thickening which we observed suggest that a VR headset may not be a myopiagenic stimulus, despite the very close viewing distances involved.

Concepts: Eye, Vision, Depth perception, Simulated reality, Reality, Virtual reality, Distance, Accommodation

3

To investigate the presence of asymmetrical accommodation in hyperopic anisometropic amblyopia.

Concepts: Amblyopia, Hyperopia, Accommodation

3

Modern efforts to define and measure the refractive state of aberrated eyes have led to new insights about the nature of refractive error, the quality of the retinal image, the interplay of the crystalline lens, and the eye’s pupil during accommodation. Our change in mind-set engendered by wavefront concepts has the power to alter our way of thinking about many clinical issues that are fundamentally optical in nature, such as dynamic changes in optical quality of eyes caused by tear film deterioration, outcome assessment of refractive therapies, and myopia progression. The aim of this lecture is to help make advances in these areas of optometric science broadly accessible to educators, clinicians, and patients by explaining in simple terms the underlying optical concepts of wavefront aberrometry.

Concepts: Optics, Refraction, Eye, Visual perception, Cornea, Lens, Accommodation

3

Recent epidemiological evidence suggests that children who spend more time outdoors are less likely to be, or to become myopic, irrespective of how much near work they do, or whether their parents are myopic. It is currently uncertain if time outdoors also blocks progression of myopia. It has been suggested that the mechanism of the protective effect of time outdoors involves light-stimulated release of dopamine from the retina, since increased dopamine release appears to inhibit increased axial elongation, which is the structural basis of myopia. This hypothesis has been supported by animal experiments which have replicated the protective effects of bright light against the development of myopia under laboratory conditions, and have shown that the effect is, at least in part, mediated by dopamine, since the D2-dopamine antagonist spiperone reduces the protective effect. There are some inconsistencies in the evidence, most notably the limited inhibition by bright light under laboratory conditions of lens-induced myopia in monkeys, but other proposed mechanisms possibly associated with time outdoors such as relaxed accommodation, more uniform dioptric space, increased pupil constriction, exposure to UV light, changes in the spectral composition of visible light, or increased physical activity have little epidemiological or experimental support. Irrespective of the mechanisms involved, clinical trials are now underway to reduce the development of myopia in children by increasing the amount of time they spend outdoors. These trials would benefit from more precise definition of thresholds for protection in terms of intensity and duration of light exposures. These can be investigated in animal experiments in appropriate models, and can also be determined in epidemiological studies, although more precise measurement of exposures than those currently provided by questionnaires is desirable.

Concepts: Epidemiology, Light, Electromagnetic radiation, Experiment, Myopia, Presbyopia, Visible spectrum, Accommodation

2

To determine the rate of myopia progression in children fit with a commercially available extended depth of focus (center distance) multifocal soft contact lens with attributes theoretically expected to slow the progression of myopia.

Concepts: Myopia, Lens, Orthokeratology, Contact lens, Corrective lens, Focus, Accommodation, Glasses

2

In his Bakerian Lecture paper of 1801, Thomas Young provided the best account up to that time of the eye’s optical system, including refraction by the cornea and the surfaces of the lens. He built a device, an optometer, for determining the eye’s state of focus, making it possible to prescribe appropriate correction lenses. His main contribution, however, was to show that accommodation, the eye’s focusing mechanism, was not the result of changes to the curvature of the cornea, nor to the length of the eye, but was due entirely to changes in the shape of the lens, which he described with impressive accuracy. He was wrong, however, in believing that the reason the lens bulges when focusing on near objects was because it behaved as a contracting muscle. Half a century later, Helmholtz showed that the lens bulges not by its own contraction, but when it is relaxed as a result of contraction of newly discovered circular muscles in the ciliary body. This commentary was written to celebrate the 350th anniversary of the journal Philosophical Transactions of the Royal Society.

Concepts: Light, Eye, Visual perception, Cornea, Lens, Ciliary muscle, Accommodation, Royal Society

1

The effectiveness of multifocal contact lenses (MFCLs) at slowing myopia progression may hinge on the accommodative behavior of young eyes fit with these presbyopic style lenses. Can they remove hyperopic defocus? Convergence accommodation as well as pupil size and the zonal geometry are likely to contribute to the final accommodative responses.

Concepts: Eye, Cornea, Myopia, Presbyopia, Corrective lens, Hyperopia, Optometry, Accommodation