Edward A. H. Mallen

Refractive error and ocular biometry in Jordanian adults

The aim of this study was to establish the prevalence of refractive errors in Jordanian adults of working age, and to study the ocular biometric correlates of refractive error in this population. Refractive error and ocular biometry were measured in 1093 Jordanian adult subjects aged 17–40 years to determine the prevalence of refractive error, and explore structural correlations of ametropia. Refractive error was measured using a Grand‐Seiko GR‐3100K closed‐view infrared autorefractor. Ocular component measurements were made using A‐scan ultrasonography and autokeratometry. The prevalence of myopia [spherical equivalent refraction (SER) less than −0.50 DS] and hyperopia (SER greater than +0.50 DS) was 53.71 and 5.67% respectively; 40.62% of the sample was emmetropic (refraction between +0.50 D and −0.50 D inclusive in both principal meridians). The distribution of SER was found to show marked leptokurtosis, exhibiting a peak between plano and 1 D of myopia. Corneal radius, anterior chamber depth, crystalline lens thickness, vitreous chamber depth and axial length (AL) parameters were normally distributed in the population studied. AL to corneal curvature ratio was not normally distributed, and showed marked leptokurtosis. Linear regression analysis showed that AL correlated most closely with spherical equivalent refractive error. This study has established a database of refractive error prevalence and ocular biometric correlates of ametropia in a Middle Eastern population of working age.

Refractive group differences in accommodation microfluctuations with changing accommodation stimulus

Purpose:  Microfluctuations of accommodation are known to increase in magnitude with increasing accommodation stimulus. Reduced sensitivity to blur in myopic subjects could also lead to increases in the magnitude of the microfluctuations. The aim of this study is to examine the effect of variations in accommodation stimulus upon the microfluctuations in different refractive groups.

Eye shape and retinal shape, and their relation to peripheral refraction

Citation information: Verkicharla PK, Mathur A, Mallen EAH, Pope JM & Atchison DA. Eye shape and retinal shape, and their relation to peripheral refraction. Ophthalmic Physiol Opt 2012, 32, 184–199. doi: 10.1111/j.1475‐1313.2012.00906.x

Role of ocular aberrations in dynamic accommodation control

Background:  Accommodation control is mediated by a number of cues, including blur, chromatic aberration and target proximity. Data from wavefront measurements have shown clear shifts in ocular aberrations during increasing accommodative demand, most notably a negative shift in spherical aberration. Work in adaptive optics, where aberrations have been corrected, has suggested a role for aberrations in the control of accommodation for some individuals. This study aimed to determine the relative effects of aberration correction and inversion on closed‐loop stepwise accommodation responses to small increases and decreases in stimulus vergence.

Technical note: measurement of retinal contour and supine axial length using the Zeiss IOLMaster.

Accurate and repeatable assessments of ocular biometric factors are of vital importance to research laboratories conducting studies of ametropia. This is of particular importance to studies assessing the efficacy of myopia control strategies, and when investigating the effect of accommodation and posture on ocular biometry. In this technical note, two novel adaptations of the Zeiss IOLMaster are described: an adaptation to enable measurement of the axial length of the eye of a subject in a supine position, and an attachment to enable measurement of peripheral ocular dimensions and plotting of the central 80° of the retina relative to the anterior corneal surface. Pilot data obtained using these modifications to the instrument are presented.

Sympathetic innervation of ciliary muscle and oculomotor function in emmetropic and myopic young adults

PURPOSE Evidence exists for an additional inhibitory accommodative control system mediated by the sympathetic branch of the autonomic nervous system (ANS). This work aims to show the relative prevalence of sympathetic inhibition in young emmetropic and myopic adults, and to evaluate the effect of sympathetic facility on accommodative and oculomotor function. METHODS Profiling of ciliary muscle innervation was carried out in 58 young adult subjects (30 emmetropes, 14 early onset myopes, 14 late onset myopes) by examining post-task open-loop accommodation responses, recorded continuously by a modified open-view infrared optometer. Measurements of amplitude of accommodation, tonic accommodation, accommodative lag at near, AC/A ratio, and heterophoria at distance and near were made to establish a profile of oculomotor function. RESULTS Evidence of sympathetic inhibitory facility in ciliary smooth muscle was observed in 27% of emmetropes, 21% of early-onset myopes and 29% of late-onset myopes. Twenty-six percent of all subjects demonstrated access to sympathetic facility. Closed-loop oculomotor function did not differ significantly between subjects with sympathetic facility, and those with sympathetic deficit. CONCLUSIONS Emmetropic and myopic groups cannot be distinguished in terms of the relative proportions having access to sympathetic inhibition. Presence of sympathetic innervation does not have a significant effect on accommodative function under closed-loop viewing conditions.

Cross-sectional sample of peripheral refraction in four meridians in myopes and emmetropes.

PURPOSE The study of peripheral refractive error is of growing interest as degradation of the retinal image quality in the periphery is known to affect central refractive error development and ocular shape in animal models. The purpose of this study was to measure peripheral refraction across the horizontal, vertical, and two oblique meridians in a group of myopic and emmetropic adults and to investigate retinal asymmetry in the human retina. METHODS Thirty-one myopes (spherical equivalent between -2.00 and -9.62 D) and 20 emmetropes (spherical equivalent between -0.50 and +0.50 D) with astigmatism less than -0.75 D, participated in the project. Noncycloplegic peripheral refraction measurements were captured with an autorefractor (NVision K-5001; Shin-Nippon, Tokyo, Japan) at the fovea and up to 30° eccentricity in the horizontal, vertical, and two oblique meridians in 10° steps. RPR was calculated by subtracting the foveal spherical equivalent refraction from that obtained at each eccentric location. RESULTS Along all measured meridians, myopic eyes showed a relative hyperopic shift in the periphery, with the superior-temporal portion of the retina exhibiting the smallest shift. Emmetropic eyes, however, exhibited a relatively consistent refractive profile across all meridians and eccentricities, confirming a spherical retinal shape for this group. In addition, off-axis astigmatism increased with eccentricity in all meridians. CONCLUSIONS These results suggest that the myopic eye tends toward an ellipsoid shape, rather than the globular shape of an emmetropic eye.

Effect of correction of ocular aberration dynamics on the accommodation response to a sinusoidally moving stimulus

We used an adaptive optics system to correct the aberration dynamics of five subjects while they fixated on a monochromatic stimulus undergoing sinusoidal vergence changes between 1.5 and 2.5 D, at a temporal frequency of 0.2 Hz. The aberrations were measured at 20 Hz using a Shack-Hartmann sensor and corrected using a 37-actuator deformable mirror. The accommodation response (AR) was analyzed in terms of the gain and phase lag. Manipulation of aberrations significantly affected the gain of the AR for only one subject when the odd-order aberrations were corrected. The predictability of the sinusoidal stimulus could account for the lack of an effect in the remaining subjects and conditions.

Non-invasive phakometric measurement of corneal and crystalline lens alignment in human eyes

We describe a non‐invasive phakometric method for determining corneal axis rotation relative to the visual axis (β) together with crystalline lens axis tilt (α) and decentration (d) relative to the corneal axis. This does not require corneal contact A‐scan ultrasonography for the measurement of intraocular surface separations. Theoretical inherent errors of the method, evaluated by ray tracing through schematic eyes incorporating the full range of human ocular component variations, were found to be larger than the measurement errors (β < 0.67°, α < 0.72° and d < 0.08 mm) observed in nine human eyes with known ocular component dimensions. Intersubject variations (mean ± S.D.: β = 6.2 ± 3.4° temporal, α = 0.2 ± 1.8° temporal and d = 0.1 ± 0.1 mm temporal) and repeatability (1.96 × S.D. of difference between repeat readings: β ± 2.0°, α ± 1.8° and d ± 0.2 mm) were studied by measuring the left eyes of 45 subjects (aged 18–42 years, 29 females and 16 males, 15 Caucasians, 29 Indian Asians, one African, refractive error range −7.25 to +1.25 D mean spherical equivalent) on two occasions.

Binocular Shack–Hartmann sensor for the human eye

During steady-state fixation the aberrations of the human eye display dynamic behaviour. It has been suggested that the fluctuations in focus are correlated between both eyes. However, nothing is known about the dynamic correlation between the aberrations other than focus. We have developed an open-view binocular Shack–Hartmann sensor which measures the ocular wavefront aberrations simultaneously in both eyes at 25 Hz. A single sensor and laser source are used to reduce system cost and complexity. Speckle is reduced using a rotating diffuser in a plane conjugate to the retinae. We measured the wavefront dynamics of two subjects and decomposed the data into Zernike aberration terms up to and including fifth radial order. Coherence function analysis was used to determine the correlation between aberrations in the frequency domain. The correlations were dependent upon subject, frequency and aberration type.