Hema Radhakrishnan

Repeatability and validity of the PowerRefractor and the Nidek AR600-A in an adult population with healthy eyes.

We assessed the repeatability and validity of the PowerRefractor and the Nidek AR-600A autorefractor. This is the first independent study conducted on adults to evaluate the performance of these instruments in a laboratory setting. Fifty subjects (23 males and 27 females) aged 16 to 61 years (mean, 37 ± 12) participated in the study. The validity of the PowerRefractor and the Nidek autorefractor readings were determined by comparing them to subjective refraction. Measurements of refractive error were obtained from the two instruments on two separate occasions to assess their repeatability. The measured refractive error was converted into a dioptric power matrix for data analysis. No significant difference was found between the measurements obtained with the two instruments and the subjective refraction. The estimate of refractive error given by the two instruments was also found to be repeatable. In addition to measuring the refractive error, the PowerRefractor also offers the facility to measure eye position, pupil size, and dynamics of accommodation. We suggest some improvements to the PowerRefractor measurement technique to standardize its clinical use and to improve accuracy.

Peripheral refraction and the development of refractive error: a review.

It has been suggested that emmetropic and low‐hyperopic eyes in which the refractive error in the periphery of the visual field is relatively hyperopic with respect to the axial refraction may be at greater risk of developing myopia than eyes with similar refractions but relatively myopic peripheral refractive errors. The animal and human evidence to support this hypothesis is reviewed. The most persuasive studies are those in which emmetropization has been shown to occur in infant rhesus monkeys with ablated foveas but intact peripheral fields, and the demonstration that, in similar animals, lens‐induced relative peripheral hyperopia produces central axial myopia. Evidence for emmetropization in animals with severed optic nerves suggests that emmetropization is primarily controlled at the retinal level but that the higher levels of the visual system play a significant role in refining the process: there appear to be no directly equivalent human studies. Since any contribution of the higher centres to the control of refractive development must depend upon the sensitivity to defocus, the results of human studies of the changes in depth‐of‐focus across the field and of the contribution of the retinal periphery to the accommodation response are discussed. Although peripheral resolution is relatively insensitive to focus, this is not the case for detection. Moreover accommodation occurs to peripheral stimuli out to a field angle of at least 10 deg, and the presence of a peripheral stimulus can influence the accommodation to a central target. Although the basic hypothesis that a relatively hyperopic peripheral refractive error can drive the development of human myopia remains unproven, the available data support the possibility of an interaction between the states of focus on axis and in the periphery.

Age-Related changes in ocular aberrations with accommodation

This study investigates the changes in aberrations with monocular accommodation as a function of age. Second-order and higher order wavefront aberrations and pupil size were measured as a function of accommodation demand over the range of 0-4 D in the right eyes of 47 normal subjects with ages between 17 and 56 years. Higher order ocular Zernike aberrations were analyzed for the natural pupil size in terms of their equivalent defocus and were also determined for fixed pupil diameters of 4.5 mm in the unaccommodated eyes and 2.5 mm in the accommodating eyes. With relaxed accommodation (0 D accommodation stimulus), the major change with age was in the value of C4(0), which increased in positive value over the age range studied, although the total higher order RMS wavefront aberration did not increase. When the data were analyzed for natural pupils, spherical aberration was again found to change systematically in the positive direction with age. The equivalent defocus of total higher order RMS error for natural pupils showed no significant correlation with age (p > .05). With active accommodation, spherical aberration was found to decrease and become negative as the accommodative response increased in the younger subjects (<40 years). Near-zero spherical aberration was found at accommodation levels of about 0.50 D in the youngest subjects (<20 years) and at around 2-3 D in subjects between 20 and 39 years. In the older subjects (>40 years), the spherical aberration showed only small changes, some of which were positive, within the limited amplitude of accommodation available. Other higher order aberrations and the RMS of higher order aberrations did not appear to change systematically with accommodation, except in the oldest subjects. The change with age in the relationship between aberration and accommodation is interpreted in terms of the changing gradients of refractive index and surface curvatures of the crystalline lens.

Peripheral refraction for distance and near vision in emmetropes and myopes

We investigated the relationship between myopia and peripheral refraction for distance and near vision by measuring peripheral refractive errors in 10 myopic and 10 emmetropic participants at viewing distances of 2.5 and 0.4 m. Measurements were made at the fovea, and at eccentricities of 10°, 20° and 30° in the temporal and nasal hemispheres of the horizontal visual field. Our results showed that peripheral astigmatism increased with increasing eccentricity, but there was no significant difference between refractive error groups except at 30° eccentricity in the temporal retina. Considering the Mean Spherical Equivalent errors, emmetropes became relatively myopic at peripheral eccentricities, but there was little change in myopes. The effect of viewing distance on astigmatism or Mean Spherical Equivalent error was not significant. Our results do not support the view that myopia is associated with changes in peripheral refraction during distance or near vision.

Effect of positive and negative defocus on contrast sensitivity in myopes and non-myopes

This study investigated the effect of lens induced defocus on the contrast sensitivity function in myopes and non-myopes. Contrast sensitivity for up to 20 spatial frequencies ranging from 1 to 20 c/deg was measured with vertical sine wave gratings under cycloplegia at different levels of positive and negative defocus in myopes and non-myopes. In non-myopes the reduction in contrast sensitivity increased in a systematic fashion as the amount of defocus increased. This reduction was similar for positive and negative lenses of the same power (p = 0.474). Myopes showed a contrast sensitivity loss that was significantly greater with positive defocus compared to negative defocus (p = 0.001). The magnitude of the contrast sensitivity loss was also dependent on the spatial frequency tested for both positive and negative defocus. There was significantly greater contrast sensitivity loss in non-myopes than in myopes at low-medium spatial frequencies (1-8 c/deg) with negative defocus. Latent accommodation was ruled out as a contributor to this difference in myopes and non-myopes. In another experiment, ocular aberrations were measured under cycloplegia using a Shack-Hartmann aberrometer. Modulation transfer functions were calculated using the second order term for defocus as well as the fourth order Zernike term for spherical aberration. The theoretical maximal contrast sensitivity based on aberration data predicted the measured asymmetry in contrast sensitivity to positive and negative defocus that was observed in myopic subjects. The observed asymmetry in contrast sensitivity with positive and negative defocus in myopes may be linked to the altered accommodative response observed in this group.

Repeatability of corneal thickness measured using an oculus pentacam

Purpose. To evaluate the repeatability of corneal thickness measurements over time, obtained with an Oculus Pentacam. Methods. Measurements of corneal thickness (apical and peripheral) were performed at three different sessions in one eye of 23 healthy subjects, using an Oculus Pentacam instrument. In each session, three consecutive measurements were taken (without realignment) by the same observer. Measurements were performed, immediately after a blink. Results. Mean values obtained were in agreement with published literature values for central corneal thickness (mean ± SEM: 546.23 ± 5.24 μm for first session over the first 1 min, 545.13 ± 5.49 μm for second session 1 h later, and 543.23 ± 4.96 μm for the third session measured 1 week later). The data showed an increase in corneal thickness towards the periphery with a decrease in intraobserver reproducibility. The confidence limits in the Bland and Altman charts for pachymetry at the corneal apex ranged between −10.91 μm and 13.00 μm within the first session measured over the first 1 min, −13.43 μm to 11.23 μm between the first and second sessions after 1 h and −11.99 μm to 10.02 μm between the first and third sessions after 1 week. The confidence limits were found to increase towards the peripheral cornea. There was no significant difference in within-subjects variance between the three time scales (p > 0.05 for all parameters measured). Conclusion. Our data showed that the Oculus Pentacam provides repeatable measures of corneal thickness when measured within a few seconds, after 1 h, and 1 week.

Peripheral refraction measurement: Does it matter if one turns the eye or the head?

It has been suggested that, following eye movements, the changing pressures exerted by the extraocular muscles and the lids might distort the shape of the eyeball and alter refraction across the visual field. To confirm or refute this hypothesis, the pattern of monocular peripheral refraction in the right eyes of 10 healthy young adults was measured either by turning the eye to fixate a series of horizontally spaced targets or by turning the head to view the same targets while maintaining central fixation and the eye in its primary position. In each case a fixed Shin‐Nippon autorefractor was used to measure peripheral refraction at 5° intervals over the central ±30° of the visual field. The duration of any eye turn was ≤1 min. Repeated‐measures analysis showed no significant differences between the spherical equivalents of peripheral refraction measured under the two conditions (p = 0.223). A further study of five subjects involving 2.5‐min periods of fixation with an eye or head turn of 25° also showed no significant refractive differences. Thus, within the conditions of the study (eye‐turn durations and field angles ≤2.5 min and 30° respectively), the results fail to confirm the occurrence of large differences in peripheral refraction when measurements are made with eye turn rather than head turn.

Biomechanical properties of corneal tissue after ultraviolet-A-riboflavin crosslinking

Photodynamic collagen crosslinking (CXL) using ultraviolet-A (UVA) irradiation combined with the photosensitizer riboflavin has been introduced as a new treatment for progressive keratoconus. The results of clinical studies are promising, but the efficacy of the treatment in halting the progression depends on the stability of the induced biomechanical effects. The effects of corneal CXL on corneal rigidity; collagen fiber diameter; and resistance to heat degradation, enzymatic digestion, and swelling due to hydration are reviewed in this paper. The collective results indicate that CXL using UVA and riboflavin enhances the biomechanical properties of the corneal tissue, which remain stable over time. Therefore, this treatment could become the future standard therapy for keratoconus or used to halt the progression of keratoconus and postpone the need for corneal transplantation. The increase in availability and popularity of the CXL technique accentuates the requirement for reliable and accurate techniques for measuring corneal biomechanical properties before and after treatment.

Pellucid corneal marginal degeneration: A review

Pellucid marginal corneal degeneration (PMD) is a rare ectatic disorder which typically affects the inferior peripheral cornea in a crescentic fashion. The condition is most commonly found in males and usually appears between the 2nd and 5th decades of life affecting all ethnicities. The prevalence and aetiology of this disorder remain unknown. Ocular signs and symptoms of patients with PMD differ depending on the severity of the condition. Unless corneal topography is evaluated, early forms of PMD may often be undetected however, in the later stages PMD can often be misdiagnosed as keratoconus. Visual signs and symptoms include longstanding reduced visual acuity or increasing against-the-rule irregular astigmatism leading to a slow reduction in visual acuity. In rare cases, patients may present with a sudden loss of vision and excruciating ocular pain due to corneal hydrops or spontaneous perforation. The vast majority of PMD patients are managed using spectacles and contact lenses. Several surgical procedures have been used in an attempt to improve visual acuity when spectacles and contact lenses do not provide adequate vision correction. Since patients with PMD make poor candidates for laser vision correction, an awareness of the topographical and slit-lamp features of PMD will be useful to clinicians screening for signs of corneal abnormality before corneal refractive surgery. This review describes the clinical features of PMD, its differential diagnosis and various management strategies presently available.

Repeatability of corneal and ocular aberration measurements and changes in aberrations over one week

Background:  Both Shack‐Hartmann aberrometry (IRX3, Imagine Eyes, Orsay, France) and Scheimpflug photography (Oculus Pentacam, Oculus Inc Wetzlar, Germany) are known to provide repeatable measurements. The variability in measurements of corneal and ocular aberrations obtained with these instruments over one week had not been assessed. The aim was to study the variability in corneal and ocular aberrations in the human eye over one week and to determine the impact of age on corneal and crystalline lens aberrations and on the variability of these measurements.