Comparison of central corneal thickness measurement by scanning slit topography, infrared, and ultrasound pachymetry in normal and post-LASIK eyes.

OBJECTIVES: To compare central corneal thickness (CCT) measurements by scanning slit topography (SST), infrared pachymetry (IRP), and ultrasound pachymetry (USP), and their agreement in normal and post-laser in situ keratomileusis (LASIK) eyes. METHODS: Sixty normal and 35 post-LASIK subjects were recruited. Only one eye from each subject was analyzed. Non-contact pachymetry was performed first, and the order for SST (Orbscan IIz) and IRP (Tonoref III) was randomized for each patient, to be followed by contact USP (Echoscan US-4000). Pearson's correlation, paired t test, and Bland-Altman plots were used to investigate association, difference, and agreement among different instruments respectively. RESULTS: The measurements obtained with the instruments were highly correlated. Compared to CCT determined by USP (CCTUSP), CCT determined by SST (after correction with acoustic factor) (CCTSSTC) was thicker by 7 µm in normal eyes (P < 0.001). There was no significant difference between CCTSSTC and CCTUSP (P = 0.128), but a thickness-dependent deviation in post-LASIK eyes (P = 0.003). The CCT determined by IRP (CCTIRP) was thicker than CCTUSP in normal (P < 0.001) and post-LASIK eyes (P < 0.001) and demonstrated proportional overestimation with thinner corneas, with less predictable ultrasonic equivalent corneal thickness in normal eyes. Conversely, CCTIRP significantly underestimated CCT compared to CCTSST and showed increasing underestimation with thinner corneas in both normal and post-LASIK eyes (both P < 0.001). CONCLUSION: Central corneal thickness determined by SST, IRP and USP were not interchangeable or interconvertible, probably attributed to difference in methodologies. Compensation with algorithms may improve agreements amongst instruments.

Relationship between Vision and Visual Perception in Hong Kong Preschoolers.

PURPOSE: Although superior performance in visual motor and visual perceptual skills of preschool children has been documented in the Chinese population, a normative database is only available for the US population. This study aimed to determine the normative values for these visuomotor and visual perceptual tests for preschool children in the Hong Kong Chinese population and to investigate the effect of fundamental visual functions on visuomotor and visual perceptual skills. METHODS: One hundred seventy-four children from six different kindergartens in Hong Kong were recruited. Distance visual acuity, near visual acuity, and stereopsis were tested, along with two measures of visual perception (VP): Visual-Motor Integration (VMI) and Test of Visual-Perceptual Skills (TVPS). Raw VMI and TVPS scores were converted into standard/scaled scores. The impact of basic visual functions on VP (VMI and TVPS) was examined using multiple regression. RESULTS: Visual functions were generally good: only 9.2 and 4.6% of subjects had unilateral and bilateral reduced habitual vision, respectively (distance visual acuity in the better eye >0.3 logMAR [logarithm of the minimum angle of resolution]). Performance in the VMI and in the visual memory and spatial relationships subtests of the TVPS exceeded that reported for age-matched children from the United States. Multiple regression analysis provided evidence that age had the strongest predictive value for the VMI and VP skills. In addition, near visual acuity was weakly associated with performance in the VMI and the visual discrimination and spatial relationships subtests of the TVPS, accounting for a limited proportion of the intersubject variability (R < 3%; p < 0.001). CONCLUSIONS: Hong Kong preschoolers outperformed their US peers in the VMI and visual memory/spatial relationships of TVPS subtests, perhaps attributed to greater exposure to such material during their preschool home education. This study provided normality data for VMI and four subtests of the TVPS for Hong Kong Chinese preschool children as a reference for future studies.

Myopia progression in children is linked with reduced foveal mfERG response.

PURPOSE: To study the changes in retinal electrophysiology in children during myopia progression during a 1-year period. METHODS: Twenty-six children aged from 9 to 13 years were recruited for the global flash multifocal electroretinogram (mfERG) measured at 49% and 96% contrast, in two visits 1 year apart. The amplitudes and implicit times of both direct component (DC) and induced component (IC) measured at these two visits were analyzed and compared. Pearson's correlation was used to study the association between the changes of mfERG response and myopia progression during the test period. RESULTS: Myopia increased by -0.48 ± 0.32 diopter (D) (P < 0.001) during the year, with 24 of 26 children becoming more myopic (range = 0.00 to ∼ -1.38 D); axial length increased by 0.25 ± 0.11 mm (P < 0.001) during the year. The increased myopia was highly correlated with increase in axial length (r = -0.70; P < 0.001). The central DC and IC amplitudes at 49% contrast reduced significantly as myopia progressed and the paracentral implicit times of these two components were reduced considerably. However, the high-contrast responses were virtually unaffected. CONCLUSIONS: Our findings suggested that the inner retinal functions in the central retina, with some involvement of the paracentral region, were decreased as myopia progressed in children.

Myopic children have central reduction in high contrast multifocal ERG response, while adults have paracentral reduction in low contrast response.

PURPOSE: To compare the retinal function of myopic children and young adults using the multifocal electroretinogram (mfERG). METHODS: Fifty-two children (aged 9-14 years) and 19 young adults (aged 21-28 years) with spherical equivalent refractive errors ranging from plano to -5.50 diopter (D) were recruited. They were examined using the global flash mfERG at 49% and 96% contrasts. Each local mfERG response was pooled into five concentric rings for analysis. The amplitudes and implicit times of direct components (DC) and induced components (IC) from the global flash response were analyzed. Hierarchical multiple regressions were used to evaluate the influence of refractive error and axial length on the DC and IC responses. RESULTS: Compared with the emmetropes of the same age group, myopic children had a significant reduction in central macular DC response at 96% contrast while the IC response was unaffected, but myopic adults showed significant reductions in paracentral IC amplitudes at 49% contrast. Implicit times for DC and IC responses were unaffected for either group. CONCLUSIONS: Retinal function was unaffected in myopic children, except for the outer retina in the central macular region. In contrast, the inner retinal function was substantially reduced in myopic adults, especially in the paracentral region. This study provides further evidence for different retinal, physiological characteristics in myopic children and myopic adults.

Temporal interactive response is resistant to cloudy ocular media in the slow double-stimulation multifocal electroretinogram.

AIM: To examine the influence of cloudy media on the slow double-stimulation multifocal electroretinogram (mfERG). METHODS: Slow double-stimulation mfERG responses were measured from 26 subjects with normal ocular health under normal and light scattering conditions (induced using acrylic sheets) (Experiment 1) and another nine cataract patients before and after cataract surgery (Experiment 2). The amplitudes and implicit times of the first (M(1)) and second (M(2)) stimulation were compared under normal and light scattering conditions in Experiment 1 and they were compared under precataract and postcataract surgery in Experiment 2. RESULTS: Compared with control conditions (normal and postcataract surgery), the M(1) amplitude in the central region was significantly reduced in light scattering conditions (acrylic sheets and precataract surgery); the M(2) amplitude and both M(1) and M(2) implicit times of all regions examined were moderately affected in precataract surgery. The M(1):M(2) amplitude ratio and implicit time ratio were virtually unaffected in cloudy media for either central or mid-peripheral regions. CONCLUSION: Cloudy media affects the mfERG amplitude and implicit time in the slow double-stimulation, but does not affect the response ratio (ie, M(1):M(2) amplitude ratio and implicit time ratio) between the two stimulations. This suggests that the ratio analysis can be applied in patients with mild to moderately cloudy ocular media to evaluate the functional integrity of the retina.

Sign-dependent changes in retinal electrical activity with positive and negative defocus in the human eye.

The purpose of this study was to investigate the effect of optical defocus on changes of electrical response as a function of retinal region. Twenty-three subjects (aged 19-25 year) with normal ocular health were recruited for global flash multifocal electroretinogram (mfERG) recordings under control (fully corrected) condition, and short-term positive defocus (+2D and +4D) and negative defocus (-2D and -4D) conditions. The amplitudes and implicit times of direct (DC) and induced (IC) components of mfERG responses were pooled into six concentric rings for analyses. The mfERG responses demonstrated more significant changes in amplitude in paracentral retinal regions than in the central regions under defocused conditions. The paracentral DC amplitude showed a significant reduction under negative defocus conditions. In contrast, the paracentral IC amplitude showed a significant increment under positive defocus conditions. Interestingly, the central IC response showed significant reduction in amplitude only to negative defocus, while increasing its amplitude to positive defocus. However, the DC and IC implicit times were virtually unaffected under defocused conditions. Our findings suggest that human retina is able to differentiate defocused signals and to identify positive and negative defocus. It shows that paracentral retina reacts more vigorously to optical defocus than does central retina.

Detection of early functional changes in diabetic retina using slow double-stimulation mfERG paradigm.

Aim Diabetes mellitus (DM) is a systemic disease with insufficient secretion of insulin or poor response to insulin. This typically causes poor control of blood glucose level leading to a range of complications. Early detection of the retinal function alteration in DM is needed. Methods A newly modified paradigm-slow double-stimulation multifocal electroretinogram (mfERG)-was introduced to measure early changes of retinal function in DM and to investigate changes in the adaptation mechanisms in the diabetic retina. The mfERG was measured by using a slow double-stimulation mfERG paradigm (M(1)M(2)OOO). Results The m1 amplitude of M(1) stimulation from diabetic subjects was significantly reduced in ring 1 in contrast to that of a control group. The m2 amplitude of M(2) stimulation from diabetic subjects was also significantly reduced in ring 1 and 2 as compared with those of the control group. The m1/m2 ratio which minimises intersubject variation shows a reasonable differentiation between the control and diabetic groups. There was a significant increase in the amplitude ratio from diabetic subjects in ring 2 and 3 as compared with those of the control group. Conclusions The present findings suggest that the new mfERG paradigm is a fast and sensitive test for the detection of early functional changes in the diabetic retina.

Impairment of retinal adaptive circuitry in the myopic eye.

Previous studies have proposed that the inner retina is affected in myopes. This study aimed to investigate the changes in adaptive circuitry of the inner retina in myopia, using the global flash multifocal electroretinogram (global flash mfERG) with different levels of contrast (luminance modulation). Fifty-four myopes had global flash mfERG recorded with different contrasts. The direct component (DC) and the induced component (IC) of the mfERG response were pooled into six regions for analysis. The response amplitudes and implicit times at different contrasts were also analysed. Results showed that myopes had significant reduction in the paracentral DC amplitude for the 29% and 49% contrasts and in the paracentral IC amplitude at all contrasts measured. The peripheral IC amplitude for the 49% contrast was also reduced. No significant change was found in implicit time for either DC or IC response. Refractive error explained about 14% of the variance in DC and 16% of the variance in IC amplitude respectively; axial length could not account for additional variance in either paracentral DC or IC amplitudes in the hierarchical regression models used. We concluded that the paracentral retinal region in myopes showed signs of impaired retinal adaptation, suggesting a functional loss at the inner retinal layer. In addition, functions attributed to the outer retinal layer showed only small changes due to myopia.

Effect of inner retinal dysfunction on slow double-stimulation multifocal electroretinogram.

PURPOSE: This study investigated the retinal adaptive mechanism in inner retinal dysfunction using the slow double-stimulation multifocal electroretinogram (mfERG) paradigm. METHODS: Slow double-stimulation mfERG responses were recorded from 15 eyes of 15 4-month-old Mongolian gerbils in control conditions and after suppression of inner retinal responses with injections of tetrodotoxin (TTX) and N-methyl-d-aspartic acid (NMDA). The stimulation consisted of five video frames: the two initial frames with multifocal flashes were triggered by two independent m-sequences, followed by three dark video frames. The results were compared with findings in humans: 7 subjects with glaucoma and 31 age-matched normal subjects were measured using the same mfERG protocol. RESULTS: The stimulation generates two responses (M(1) and M(2)) from the two independent multifocal frames. The M(1):M(2) ratio showed a significant reduction after administration of TTX+NMDA in the animal study. This matched with the human glaucoma findings. Glaucoma subjects generally have a reduced M(1):M(2) ratio; this ratio showed a sensitivity of 86%, with a specificity of 84% for differentiating normal eyes from glaucomatous eyes. CONCLUSION: This stimulation paradigm provides a method of measuring temporal visual characteristics. The M(1):M(2) ratio acts as an indirect functional indicator of retinal adaptation, which may be abnormal in the diseased retina. Further development of this method may help to describe the functional variation in the diseased retina and to predict the occurrence of a range of retinopathies.