Andreas Hartwig

Analysis of Higher-Order Aberrations in a Large Clinical Population

PURPOSE To use a large wave-front database of a clinical population to investigate relationships between refractions and higher-order aberrations and between aberrations of right and left eyes. METHODS Third- and fourth-order aberration coefficients and higher-order root-mean-squared aberrations (HO RMS), scaled to a pupil size of 4.5-mm diameter, were analyzed in a population of approximately 24,000 patients from Carl Zeiss Visions European wave-front database. Correlations were determined between the aberrations and the variables of refraction, near addition, and cylinder. RESULTS Most aberration coefficients were significantly dependent upon these variables, but the proportion of aberrations that could be explained by these factors was less than 2% except for spherical aberration (12%), horizontal coma (9%), and HO RMS (7%). Near addition was the major contributor for horizontal coma (8.5% out of 9.5%) and spherical equivalent was the major contributor for spherical aberration (7.7% out of 11.6%). Interocular correlations were highly significant for all aberration coefficients, varying between 0.16 and 0.81. Anisometropia was a variable of significance for three aberrations (vertical coma, secondary astigmatism, and tetrafoil), but little importance can be placed on this finding because of the small proportion of aberrations that can be explained by refraction (all <1.0%). CONCLUSIONS Most third- and fourth-order aberration coefficients were significantly dependent upon spherical equivalent, near addition, and cylinder, but only horizontal coma (9%) and spherical aberration (12%) showed dependencies greater than 2%. Interocular correlations were highly significant for all aberration coefficients, but anisometropia had little influence on aberration coefficients.

Comparison of central corneal thickness and anterior chamber depth measured using LenStar LS900, Pentacam, and Visante AS-OCT.

Purpose: To compare central corneal thickness (CCT) and anterior chamber depth (ACD) measured using 3 different techniques. Methods: CCT and ACD were measured in one eye of 27 healthy subjects (age, mean ± SD: 34 ± 7 years), using the LenStar LS900, Pentacam, and Visante AS-OCT. The agreement between the measurement techniques was assessed using the coefficient of agreement (CoA) and the 95% limits of agreement (LoA). Results: Mean ± SD values for CCT using the LenStar, Pentacam, and Visante were 550.7 ± 37.4 &mgr;m, 542.7 ± 37.9 &mgr;m, and 556.7 ± 44.4 &mgr;m, respectively (P = 0.005). Plots of differences against means displayed relatively good agreement between the Pentacam and LenStar (CoA, 19.97 &mgr;m; LoA, 15.53 &mgr;m to −24.40 &mgr;m), poorer agreement between the LenStar and Visante (CoA, 31.26 &mgr;m; LoA, 40.78 &mgr;m to −21.74 &mgr;m), and agreement was poor between the Pentacam and Visante (CoA, 37.36 &mgr;m; LoA, 25.61 &mgr;m to −49.11 &mgr;m). Mean ± SD values for ACD using the LenStar, Pentacam, and Visante were 2.93 ± 0.30 mm, 2.96 ± 0.32 mm, and 3.03 ± 0.29 mm, respectively (P < 0.001). Relatively good agreement for ACD measures was observed between the Pentacam and LenStar (CoA, 0.08 mm; LoA, 0.12 to −0.04 mm), poor agreement was obtained between the Visante and LenStar (CoA, 0.13 mm; LoA, 0.21 to −0.06 mm), and agreement was also poor between the Pentacam and Visante (CoA, 0.14 mm; LoA, 0.11 to −0.17 mm). Conclusions: Our data show that the LenStar, Pentacam, and Visante provide measurements that are in agreement with published values for CCT and ACD in human subjects. Although reasonable agreement for CCT and ACD was found between the Pentacam and LenStar, agreement was poorer between the Visante and LenStar and between the Visante and Pentacam. Thus, CCT and ACD measures from these instruments should not be used interchangeably.

Correlations between refractive error and biometric parameters in human eyes using the LenStar 900

PURPOSE To investigate the relationship between refractive error and ocular biometry in healthy subjects using a new optical low coherence reflectometry device. METHODS Biometric measurements were obtained with a LenStar LS 900 (Haag Streit, Switzerland) on one eye of 70 phakic subjects (mean ± SD age; 29 ± 9 years). Forty myopes and 30 non-myopes (best sphere range -9.63 D to +0.63 D) were included. Outcome measures were compared for the two groups using one way between groups ANOVA. These included; keratometry, central corneal thickness, iris width, anterior chamber depth, pupil diameter, lens thickness, axial length and retinal thickness. No mydriatic or cycloplegic agents were used. RESULTS There were significant differences between groups for keratometry readings (p = 0.021 and p = 0.038 for steep and flat k readings respectively), anterior chamber depth (p = 0.001), lens thickness (p = 0.026) and axial length (p<0.001). As expected significant correlations were found between spherical equivalent power and axial length (Pearson product-moment correlation r = -0.75, p<0.001) and between spherical equivalent power and anterior chamber depth (r = -0.29, p = 0.018). Anterior chamber depth and pupil diameter decreased with age (r = -0.429, p<0.001 and r = -0.386, p = 0.001 respectively) whereas lens thickness increased with age (r = 0.618, p < 0.001). CONCLUSIONS Our data showed significant differences between myopes and non-myopes for the key biometric parameters assessed and provides information about the relationships between these biometric parameters and age. The results, coupled with a unique ability to image and analyse the ocular structures non-invasively make the LenStar a promising new instrument for ocular evaluation in research and clinical practice.

Accommodative response to peripheral stimuli in myopes and emmetropes

Citation information: Hartwig A, Charman WN & Radhakrishnan H. Accommodative response to peripheral stimuli in myopes and emmetropes. Ophthalmic Physiol Opt 2011, 31, 91–99. doi: 10.1111/j.1475‐1313.2010.00796.x

Analysis of head position used by myopes and emmetropes when performing a near-vision reading task.

The aim of the study was to compare head posture in young, adult emmetropes and corrected myopes during a reading task. Thirty-two (32) myopes (mean spherical equivalent: -3.46±2.35 D) and 22 emmetropes (mean spherical equivalent: -0.03±0.36 D) participated in the study. Of the myopes, 16 were progressing (rate of progression ⩾-0.5D over the previous 2 years), 12 were stable (changes of -0.25 D or less over 2 years) and four could not be classified. Seated subjects were asked to read a text binocularly in their habitual posture. To measure head posture, two simultaneous images were recorded from different directions. In a separate study with the same subjects and conditions, a motion monitor was used to track head posture for 1 min. The habitual reading distance was measured in both studies, together with the stereoscopic acuity and fixation disparity for each subject. The results of the photographic study showed no significant differences in head posture or reading distance between the myopic and emmetropic groups (p>0.05) but there was some evidence that downward pitch angles were greater in progressing myopes than in non-progressing myopes (p=0.03). No correlations were observed between the binocular parameters and head posture. Reading distances were systematically shorter with the helmet-mounted eye tracker and it was concluded that posture was affected by the weight of the equipment. With this reservation, it appeared that the rate of change of downward pitch angle over the 1-min recording session increased with the subjects rate of myopia progression (correlation between myopia progression and slope of pitch: r(2)=-0.69, p=0.001), implying a greater reliance on head movements when reading down a page. Overall, while no differences in mean head posture were found between myopes and emmetropes, there was some evidence that head posture and movement during reading may differ in progressing myopes.

Higher-order aberrations and anisometropia.

Purpose/aim: Myopia incidence is increasing around the world. Myopization is considered to be caused by a variety of factors. One consideration is whether higher-order aberrations (HOA) influence myopization. More knowledge of optics in anisometropic eyes might give further insight into the development of refractive error. Materials and methods: To analyze the possible influence of HOA on refractive error development, we compared HOA between anisometropes and isometropes. We analyzed HOA up to the 4th order for both eyes of 20 anisometropes (mean age: 43 ± 17 years) and 20 isometropes (mean age: 33 ± 17 years). HOA were measured with the Shack-Hartman i.Profiler (Carl Zeiss, Germany) and were recalculated for a 4 mm pupil. Mean spherical equivalent (MSE) was based on the subjective refraction. Anisometropia was defined as ≥1 D interocular difference in MSE. The mean absolute differences between right and left eyes in spherical equivalent were 0.28 ± 0.21 D in the isometropic group and 2.81 ± 2.04 D in the anisometropic group. Interocular differences in HOA were compared with the interocular difference in MSE using correlations. Results: For isometropes oblique trefoil, vertical coma, horizontal coma and spherical aberration showed significant correlations between the two eyes. In anisometropes, all analyzed higher-order aberrations correlated significantly between the two eyes except oblique secondary astigmatism and secondary astigmatism. When analyzing anisometropes and isometropes separately, no significant correlations were found between interocular differences of higher-order aberrations and MSE. For isometropes and anisometropes combined, tetrafoil correlated significantly with MSE in left eyes. Conclusions: The present study could not show that interocular differences of higher-order aberrations increase with increasing interocular difference in MSE.

Working distance and eye and head movements during near work in myopes and non-myopes.

Purpose:  Reasons for the development and progression of myopia remain unclear. Some studies show a high prevalence of myopia in certain occupational groups. This might imply that certain head and eye movements lead to ocular elongation, perhaps as a result of forces from the extraocular muscles, lids or other structures. The present study aims to analyse head and eye movements in myopes and non‐myopes for near‐vision tasks.

Baseline peripheral refractive error and changes in axial refraction during one year in a young adult population

Purpose To determine whether the initial characteristics of individual patterns of peripheral refraction relate to subsequent changes in refraction over a one-year period. Methods 54 myopic and emmetropic subjects (mean age: 24.9 ± 5.1 years; median 24 years) with normal vision were recruited and underwent conventional non-cycloplegic subjective refraction. Peripheral refraction was also measured at 5° intervals over the central 60° of horizontal visual field, together with axial length. After one year, measurements of subjective refraction and axial length were repeated on the 43 subjects who were still available for examination. Results In agreement with earlier studies, higher myopes tended to show greater relative peripheral hyperopia. There was, however, considerable inter-subject variation in the pattern of relative peripheral refractive error (RPRE) at any level of axial refraction. Across the group, mean one-year changes in axial refraction and axial length did not differ significantly from zero. There was no correlation between changes in these parameters for individual subjects and any characteristic of their RPRE. Conclusion No evidence was found to support the hypothesis that the pattern of RPRE is predictive of subsequent refractive change in this age group.

Accommodation response to Chinese and Latin characters in Chinese-illiterate young adults.

Differences in accommodation when reading Chinese, as compared to Latin, characters have been suggested to have a role in the higher prevalence of myopia in some Asian countries. Yeo and colleagues (Optom Vis Sci 2013; 90: 156‐163) found that, in Chinese‐literate children, accommodation was marginally more accurate (by less than 0.05 D), when reading Chinese text. This was attributed to the additional cognitive demand associated with interpreting the more complex Chinese symbols. The present study compared responses to single Chinese and Latin characters, while controlling for cognitive demand.

Binocular saccades in myopes and emmetropes

Purpose To compare saccadic eye movements in groups of myopes and emmetropes, as eye movements could have an influence on refractive error development. Individual saccadic eye movement parameters were also compared with subjective refraction and axial length data. Methods Horizontal eye movements of 28 participants (14 myopes and 14 emmetropes; mean age [SD], 27.0 [4.7] years) were recorded using a head-mounted eye tracker. To reduce the influence of head movements, a chin rest was used. Two fixation stimuli lying symmetrically at ±10 degrees on either side of the median line were presented on a computer monitor and were alternately displayed for durations of 2 seconds each. The participants alternated their fixation between the target positions immediately after they became aware that the target had changed. Only right eye data were considered for analysis. Results Durations, amplitudes, and peak velocities of the main saccades and the numbers of overshoots, undershoots, and exact fixations were analyzed. For all analyzed parameters, no significant differences were found between myopes and emmetropes. When analyzing the whole study population or the emmetropic group alone, none of the saccadic eye movement parameters were correlated with axial length or refractive error. In myopes, only the peak velocity showed a weak correlation with refractive error and axial length, but this failed to reach statistical significance when allowance was made for multiple testing. Conclusions Because saccadic eye movements seem to be similar in myopes and emmetropes, there is no evidence that saccadic eye movements are involved in myopia development.