Optometry and Vision Science

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Browsing Optometry and Vision Science by Subject "Accommodation"

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    Bifocals in children with Down syndrome (BiDS)
    (University of Waterloo, 2010-07-28T14:05:20Z) Nandakumar, Krithika
    Down syndrome (DS) is the most common genetic cause of mental challenge in individuals and is associated with many ocular disorders. One of these anomalies which is frequently present in this population is reduced accommodation and many studies have reported this. Accommodation is the ability of the crystalline lens in the eye to focus for objects at different distances. Prescribing bifocals could potentially help in correcting the resultant inaccurate focus, although this modality of treatment is not very commonly practiced. The impact of bifocals on reading and literacy skills (academic skills) as well as visual-perceptual skills in individuals with DS has not been studied previously. The aim of this study was to investigate the impact of bifocals on the educational attainment of children and young adults with DS who have reduced accommodation and monitor their performance longitudinally. This is the first time that the impact of bifocal provision on the functional performance of children and young adults with DS has been studied. Also for the first time in children with DS, frequent measures of performance have been used to control for progression with time before and after bifocal prescription. A battery of tests comprising early literacy and visual-perceptual skills was administered before and after bifocal prescription. Accommodation and printing skills were also measured periodically. It was expected that the prescription of bifocals would help to improve near visual acuity and that the improved near acuity would result in educational achievements at school. Compliance with spectacle wear and school reports were also considered. A longitudinal observational study design was utilized with each child acting as his/her own control. Fourteen children and teenagers aged 8-18 with DS were recruited and underwent a basic optometric exam including measurement of their accommodative ability and a cycloplegic refraction. Seventy nine percent required a change in their spectacle prescription and were prescribed single vision (SV) lenses. One hundred percent had reduced accommodation both before and after new SV glasses were prescribed. Distance visual acuity did not significantly improve with SV lenses (p>0.05) but near visual acuity showed a significant improvement (p-=0.015) from 0.64±0.25 logMAR to 0.54±0.20 logMAR. A high prevalence of high refractive errors, including both hyperopia and myopia, was observed t andnear visual acuity even with a habitual correctionwas reduced compared to distance VA. A full battery of reading and visual-perceptual tests was administered with SV lenses. Thereafter the participants were followed for 6 months and monthly subtests (probes) of literacy skills and printing tasks were administered. These “probes” acted as immediate indicators of the child’s performance with his/her correction and change in performance over this time period was monitored. Over the 6 months the participants showed no noteworthy progression in their literacy skills. The group of participants performed at an age-equivalent between 3-10 years. The quality of printing formation in this population has been studied for the first time and showed no significant change over time. It was observed that some aspects of visual-perceptual and early literacy skills could be measured in all the participants. Chronological age and receptive vocabulary were significantly correlated with visual motor integration and Word Identification. Eighty five percent of the participants were prescribed bifocals with additions ranging from +1.00D to +3.50D at the 6th month after the provision of SV lenses. Post-bifocal measures of visual acuity, accommodation, visual-perceptual and early literacy skills were taken 1-2 weeks, and finally 5 months, after bifocal correction. Throughout the pre- and post-bifocal period, verbal compliance with spectacle wear was assessed through school and parental reports. The mean near logMAR VA improved with bifocals (p=0.007) compared to SV lenses. Accommodative accuracy improved with bifocals (less accommodative lag) compared to SV lenses (p=0.002) but there was no change in the accommodation exerted through the distance portion of the lens compared to SV lenses (p=0.423). There was a main effect of bifocals on sight words (p=0.013), Word Identification (p=0.047), and 2 out of 3 tests of visual perception (p<0.05). It was observed that bifocals have a positive impact on the children’s visual and school performance and this was supported by reports of improved performance in school for nine out of eleven individuals who were prescribed bifocals. The children adapted to bifocals more readily than the SV glasses, wearing them for the majority of their waking time. All the sessions of early literacy and visual-perceptual skills administered throughout the duration of the study were videotaped and were then analyzed by a naïve examiner. The time taken to perform each task was calculated and compared between the main single vision and bifocal visits. There was a significant decrease in the completion times on the test battery with bifocals for Word Identification (p=0.0015) and the Dolch sight words (p=0.048). All participants who completed the monthly probes took less time to complete the Dolch sight words (p= 0.025) and the number writing task (p=0.001) with bifocals. Similar results were not observed for the visual-perceptual tests. Performance in the monthly probes was compared before and after bifocal prescription in terms of the average raw scores and time taken. The rate of improvement in performance with bifocals was calculated by plotting the test scores against time and determing the regression lines. There was an overall significant improvement in the monthly probe scores of Word Identification (p=0.050), Dolch sight words (p=0.025) and the number test (p=0.023) with bifocals. The rate of progression in scores increased with bifocals for the Word Identification (p=0.008). Evidence of improved and faster performance with bifocals on some literacy skills was seen. It was concluded that bifocals, which result in improved near focusing, help individuals with DS to maximize their educational potential. It is suggested that more children and teenagers with DS will benefit from bifocal prescription, as they were observed to improve near visual acuity and enable better focusing for near. This thesis has provided a comprehensive analysis of the some tests of literacy, visual- perceptual and early printing skills before and after a bifocal prescription in a group of children and teenagers with Down syndrome. This is the first study to longitudinally monitor the educational impact of bifocals in a population with Down syndrome. Furthermore, the quality of printing formation in this population is a previously unstudied area and was studied longitudinally prior to and after bifocal intervention. The impact of bifocals on printing skills is also discussed. Another novel approach was that all the literacy, writing and visual-perceptual tasks sessions were videotaped to calculate the time taken to complete each task pre- and post-bifocals. This thesis is an addition to the existing literature on bifocal prescription in Down syndrome populations. From the findings in this thesis, the following recommendations are made in order to improve the standard of clinical eye care in this population. Measurement of accommodation should be considered a routine test in the clinical ocular examination for young individuals with DS, now that it is known that many of them present with accommodative deficits. When accommodation is found to be reduced, prescription of bifocals is indicated and should also become the standard of care in this population.
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    Binocular adaptation to near addition lenses in emmetropic adults
    (Elsevier, 2008-05) Sreenivasan, Vidhyapriya; Irving, Elizabeth; Bobier, William
    Near addition lenses are prescribed to pre-presbyopic individuals for treatment of binocular motor problems such as convergence excess and to control the progression of myopia. To date, no investigation has looked at the complete sequence of binocular motor responses during a period of near work with +2 D lenses. This investigation evaluated changes to accommodation and vergence responses when young adults sustained fixation at 33 cm with +2 D addition lenses. In addition, the effect of the accommodative vergence cross-link (AV/A) on the magnitude and the completeness of binocular adaptation to these lenses were evaluated. The results showed that +2 D lenses initiate an increase in exophoria and convergence driven accommodation. The degree of the initial induced phoria was dependant upon the magnitude of the AV/A ratio. Vergence adaptation occurred after 3 min of near fixation and reduced the exophoria and convergence driven accommodation. The magnitude of vergence adaptation was dependant upon the size of the induced phoria and hence the AV/A ratio. The completeness of adaptation was seen to vary inversely with induced exophoria and thus the AV/A ratio.
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    Calibration and validation the Dynamic Photorefraction System (DPRS)
    (University of Waterloo, 2014-09-25) Labhishetty, Vivek
    Introduction: Our group has been measuring accommodation in myopic children. The instrument of choice has been the Power Refractor (Multichannel Systems). However, this instrumentation is no longer supported and runs on an outdated platform (Windows 98). There is a need to develop a new photorefraction system that has the ability of the Power Refractor to accurately measure accommodation and its dynamic characteristics. The resolution of measures can be increased with modern video cameras especially in regards to the measurement of the first and second order dynamics of the accommodative response. Photorefraction is a rapid video based means to measure the refractive status of the eye. Light from infra-red LEDs set eccentric to the aperture of the camera is reflected back from the eye forming intensity gradients across the pupil that varies with the degree of ocular defocus relative to camera. However, this gradient can be influenced by various factors such as pupil size, fundus brightness etc. Thus the relationship between defocus and the intensity gradient must be calibrated empirically in order that valid measures of accommodation can be made from the photorefractor. Calibration trials determine a conversion factor which will be used to convert intensity gradients into diopters where other optical parameters such as pupil brightness and pupil size are controlled. Furthermore, once designed, this new instrument should be validated to see if it can accurately measure the ocular accommodative response and its dynamic characteristics.  Methods: Two calibration procedures were employed for conversion of the luminance slope in to units of diopters. First being the relative calibration where relative changes in the accommodation were induced and was related to the absolute changes showed in the luminance slope output from the photorefractor. Second, pupil size calibration was done to account for the changes in the pupil size where the conversion factor obtained from each subject was related to their respective luminance measure across the pupil (fundal brightness). For relative calibration, varying degree of ocular focus ranging from +5D to -5D (1D steps) was induced using trial lenses in-front of the right eye in which visible light was filtered out using an infra-red filter. Subjects (20-40 yrs.) fixated at a distance target with the other eye. Intensity gradients obtained in each condition were then plotted against the induced lens to get the linear regression slope value (conversion factor). This conversion factor was used later to convert the luminance output given by the photorefractor in to diopters. Repeatability measures were taken on 4 subjects (20-40 yrs.) on a different day. Bland-Altman plots were used to test the repeatability of conversion factor between the two visits. To address the inter-individual variability of the conversion factor, pupil size calibration was employed wherein fundus brightness was plotted against conversion factor and the linear regression equation of this plot was used to calculate the conversion factor. This procedure was used to account for changes in the pupil size during an accommodative response and thus to enhance the accuracy of the estimation. For validation, accommodation was stimulated using high contrast vertical line targets placed in a Badal optical system. A 2D step was provided with a 4sec presentation time and the onset of the stimulus was randomized. The video output obtained was loaded into DPRS for analysis. The raw position data obtained was further loaded in to MATLAB for analyzing the dynamics characteristics of the response using the velocity threshold criterion. Results: Calibration slopes of 12 subjects (29.16 ± 3.13 years) ranged from 1.77 to 3.41. Mean (±SD) slope obtained was 2.86±0.46. Bland-Altman plots showed a good repeatability of the conversion factor calculation between the visits for the four subjects (30.75 ± 4.11 years) with a coefficient of repeatability (Mean+1.96SD) of the slope values was 0.20 (≈0.05D). Fundus brightness showed a negative correlation with the calculated conversion factor across the subjects and then linear regression (y = -0.008x + 2.66; R2 = 0.73) was used to calculate the conversion factor. Validation was performed on 9 subjects (7 adults (29.57±2.69 years) and 2 children (11±1.4 years)). Conversion factors were obtained from all the subjects and it ranged from 1.48 to 2.08. Four out of nine subjects showed sluggish accommodative responses with an accommodative gain value less than 0.5. Bland-Altman plots showed good agreement between the DPRS (both individual and pupil size calibration) and the dynamic retinoscopy (mean deviation of 0.16D and 0.24D respectively). Individual calibration was slightly more accurate than the pupil size calibration and a mean deviation of 0.06D was noted between the two methods for estimating the defocus using the calculated conversion factors. Discussion: A new dynamic photorefraction along with an offline dynamic photorefraction system (DPRS) was calibrated and validated. Modifications done during this process made the system more robust with a linear operating range for a dioptric range of +5D to -5D. Photorefraction measures showed significant inter-individual variability in conversion factors as suggested previously (Schaeffel et al., 1993) with a good repeatability. In order to address the pupil size variability during a dynamic accommodative response and inter-individual variability of the conversion factors, the fundus brightness of the subjects were plotted against their respective conversion factors. The linear regression (y = -0.008x + 2.66; R2 = 0.73) was used to calculate the conversion factors directly from the fundal brightness. This procedure not only accounted for changes in the pupil size but also for calculation of conversion factors directly. Further, the system was validated on nine subjects and the responses obtained were in agreement with the previous literature on accommodation dynamics (Campbell et al., 1960; Tucker et al., 1979; Suryakumar et al., 2005). Bland-Altman plots showed agreement between the DPRS and the gold standard dynamic retinoscopy with a clinically insignificant mean deviation in the accommodative gain estimation between the methods.
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    Dose-effect autonomic responses to ocular surface stimulation
    (University of Waterloo, 2018-04-26) Alabi, Emmanuel
    Introduction: The ocular surface is one of the most densely innervated superficial tissues of the human body supplied extensively by autonomic and sensory nerve fibres. Studies have shown that ocular surface sensory neurons respond to thermal, chemical and mechanical stimuli, but investigation into the functional response of the autonomic nervous system (ANS) to ocular surface stimulation is lacking. The ANS is cardinal to human functioning as it acts below the level of consciousness to regulate the internal organs of the body thus controlling secretory cells, smooth muscle and cardiac muscle. Within the eye the ANS is responsible for the control of pupillary reflexes, accommodation and regulation of blood flow, thus monitoring these mechanisms can provide information about ANS functionality. The primary objective of this research was to determine the response of the ANS to ocular surface stimuli by measuring pupil size, conjunctival blood flow and accommodation changes after the delivery of noxious and innocuous corneal stimuli. Methods: A calibrated and computerised Belmonte pneumatic esthesiometer was used to determine detection thresholds (using ascending method of limits), and to randomly deliver mechanical and chemical stimuli from levels of detection threshold (100% threshold) to twice the threshold (200% threshold) in 50% steps, to the central cornea of 43 healthy subjects, aged 19 - 35 years. Statistical analyses were performed using SPSS (Chicago, SPSS Inc.) and p ≤ 0.05 was considered statistically significant. Chapter 3: 15 participants enrolled in this study. For each suprathreshold stimulus a spectrophotometer (Spectrascan650; Photoresearch Inc, Chatsworth, VA) was used to measure ipsi- and contralateral redness before and after delivery of the corneal stimulus, the change in redness represented the ocular vascular response to noxious stimuli. Conjunctival redness between the stimulated and unstimulated eye was analyzed using dependent t-tests. The effects of stimulus intensity and modality on conjunctival redness were analyzed using repeated measures ANOVA. Tukey HSD tests were used for all post hoc analyses. Chapter 4: 15 participants were enrolled in this study. For each suprathreshold stimulus, imaging of the stimulated and unstimulated eye was performed using two modified and calibrated Logitech c920 digital cameras (Logitech c920; Logitech International S.A., Newark, CA), for 4 seconds before (pre-stimulus capture) and 4 seconds after the delivery of the stimulus (post-stimulus capture). The data were processed with a custom segmentation algorithm to help identify the pupils and pupil diameter (average of horizontal and vertical measures) was measured using ImageJ software (NIH, Bethesda, MD). Pupil dilation response differences between the ipsi- and contralateral eye was analyzed using dependent t-tests. The effect of stimulus intensity, modality and sex of subjects were analyzed using repeated measures ANOVA. Chapter 5 13 participants were enrolled in Part A. For each suprathreshold stimulus the accommodative response at a sampling rate of 25Hz, over a 5 second period (while the subjects fixated on a high contrast (85%) color cartoon frame at 66cm) prior to (baseline) and after stimulus delivery was acquired with an eccentric infra-red (IR) photorefractor (Power Refractor, Multi-channel Systems, Reutlingen, Germany). The accommodative response for the left and right eye were averaged. Quantitative differences in accommodative response, stimulus intensity and modality were analyzed using repeated measures ANOVA. The data used in Part B were acquired from the same subjects in Part A. The pupil response (while the eyes were accommodating to a 66cm target) to ocular surface stimulation was acquired using the same device and methods as in Part A. Quantitative differences in pupil response, stimulus intensity and modality were analyzed using repeated measures ANOVA. Results: Chapter 3 In mechanical and chemical stimulation experiments, the stimulated eye became redder than the unstimulated eye (all dependent t-test p > 0.05). On average, redness increased from baseline as the corneal stimulus intensity increased. This happened regardless of whether mechanical or chemical stimulation occurred (ANOVA p < 0.05). At 200% threshold, conjunctival redness was greater than all stimulus intensities (Tukey HSD, all p < 0.05). There was a difference between chemical and mechanical stimulation based on stimulus intensity (ANOVA p < 0.05), chemical stimulation produced greater conjunctival redness than mechanical stimulation at all stimulation levels (all Tukey HSD p < 0.05). Chapter 4: In mechanical and chemical stimulation experiments, there was no difference in pupil responses between the stimulated eye and the unstimulated eye, (all dependent T-test p > 0.05). On average, pupil diameter increased from baseline as the corneal stimulus intensity increased. This happened regardless of whether mechanical or chemical stimulation occurred (ANOVA p < 0.05). At 200% threshold, pupil diameter was greater than at all stimulus intensities (Tukey HSD, all p < 0.05). There was a difference in pupil diameter between male and female subjects based on stimulus intensity (ANOVA p < 0.05); females had greater pupil diameters than males at levels of 150% threshold and 200% threshold (all Tukey HSD p < 0.05). Chapter 5 Part A: On average, accommodation increased from baseline as the corneal stimulus intensity increased. This happened regardless of whether mechanical or chemical stimulation occurred (ANOVA p < 0.05). At 200% threshold, accommodation was greater than all stimulus intensities (Tukey HSD, all p < 0.05). There was no difference in accommodation between chemical and mechanical stimulation based on stimulus intensity. Part B: On average, pupil constriction response (during accommodation) was different between baseline and 200% threshold but there was no dose dependent pupil response to ocular surface stimulation. This happened regardless of whether mechanical or chemical stimulation occurred (ANOVA p < 0.05). There was no difference in pupil response between chemical and mechanical stimulation based on stimulus intensity (ANOVA p > 0.05). Conclusion: Suprathreshold stimulation of the cornea appears to evoke dose dependent autonomic responses in the pupils, conjunctival vasculature and the accommodative mechanism. These autonomic measures are accessible, relative easy and cost effective to acquire. The components that respond to noxious corneal stimulation are linked in a homeostatic loop of complex sympathetic, parasympathetic and sensory neural control and therefore, understanding the characteristics of the local stimulus-response neural circuitry relating nociceptive stimuli to autonomic nervous functionality is important. It also promises the development of clinical procedures and instruments to better understand how these neural responses are impacted by pain.
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    Near addition lenses as a tool to investigate vergence adaptation in myopic children
    (University of Waterloo, 2011-08-12T19:42:50Z) Sreenivasan, Vidhyapriya
    Accommodation and vergence are two interacting ocular motor systems that function to maintain clear and single vision across a wide range of distances. Sustained fixation results in the adaptation of these ocular motor systems and has been widely investigated in adults but not in children. Moreover, limited reports have measured adaptation to disparities induced by ophthalmic lenses. This thesis used near addition lenses as a means to investigate binocular adaptation in children. The specific aims of this thesis were three-fold. First, the thesis aimed to gain insight into the mechanism of changes to accommodation and vergence during binocular adaptation in children. The second objective was to determine the role of vergence-bias category (eso/exo/normals) on adaptation. Lastly, this thesis evaluated the influence of myopia on binocular adaptation. Thirty- eight myopic and 38 emmetropic children between 7-14 years of age were examined for the purpose of this thesis. A series of studies were performed to evaluate adaptation using varying demands for accommodation and vergence, stimulated by binocular fixation at near (33 cm), through the addition of +2D and -2D over corrective lenses (closed loop accommodation) and using 10 base-out prisms (open-loop accommodation at 4M). In each closed-loop condition, measures of binocular and monocular accommodation (PowerRefractor, Multichannel systems) and near phoria (modified Thorington technique) were recorded at frequent intervals when children binocularly fixated a high contrast near target (33 cm) for 20 min. For the open-loop condition (obtained using 0.5 mm pinhole pupils), binocular accommodation and tonic vergence (distance heterophoria through pinhole pupils) were determined at frequent intervals when binocular fixation was sustained at 4M for 20 min. For all conditions, tonic accommodation was measured before and after the near task to measure accommodative adaptation. The results of this thesis make three major contributions to the literature. First, it outlines that the addition of +2D and -2D lenses alters both accommodation and near phoria during sustained binocular fixation, which can be explained based on the models of accommodation and vergence. Second, it shows that the direction of phoria influences the pattern of binocular vs. monocular accommodation in closed-loop conditions and alters the degree of vergence adaptation in both closed and open-loop accommodation. These changes have been primarily attributed to the varying demands on fusional vergence. Lastly, this thesis demonstrates that myopic children show reduced vergence adaptation when fusional convergence was initiated through plus adds or base-out prisms but not when fusional divergence was initiated through minus addition lenses. Further, myopic children also showed variations in other ocular motor parameters such as higher accommodative lags, greater variability of accommodative response, larger accommodative after-effects, and higher AV/A ratios compared to emmetropes. Consistent with the models of accommodation and vergence, the thesis highlights that it is necessary to measure changes to both accommodation and vergence when evaluating the response of the ocular motor system. The direction of phoria and type of refractive error play a significant role in determining binocular adaptation in children. Future studies should differentiate these parameters when evaluating adaptation of the ocular motor system.
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    Understanding the sensory and motor behavior of accommodation in progressive myopic children
    (University of Waterloo, 2017-09-27) Labhishetty, Vivek
    Introduction: Accommodation and vergence, in unison, help in maintaining a clear and single binocular vision, a linchpin for normal vision development. Under natural viewing conditions, accommodation and vergence systems mutually interact with each other through a unique cross-link mechanism. Based on the empirical data from visually normal adults, several control theory models were proposed to predict the behavior of accommodation and vergence. Progressive myopic children, however, exhibit a transient but consistent abnormal accommodative behavior which is not predicted by the currently accepted models. Progressive myopes exhibit high accommodative adaptation, elevated AC output coupled with high accommodative lags. Several predictions, both sensory and motor origin, have been proposed to explain this behavior however they failed to predict this behavior completely. Furthermore, empirical conclusions on the impact of myopia over accommodation and blur sensitivity were mostly based on data obtained from adult population. Accordingly, this dissertation examined the sensory and motor mechanism of accommodation in young children with and without progressive myopia. Data from children were also compared to adults. Methods: 12 children (8-13 years), 6 myopic and 6 non-myopic, and 6 naïve adults (25-33 years) were recruited for the purpose of this dissertation. Varying demands of accommodation were stimulated using a Badal optical system and recorded using custom-built dynamic photorefraction system (DPRS). Theoretical and empirical designs were developed to identify the ideal sampling rate necessary to measure a dynamic accommodative response. First and second order main sequence relationship and strength of the cross-link (CA) response was tested in the first experiment. In the second experiment, blur discrimination thresholds were tested psychophysically using simulated blur targets at two different stimulus demands. Furthermore, objective DOF and sensitivity of accommodative system to blur were examined using known demands of lens defocus and compared across different subject groups. Results: The new custom built high speed photorefractor (DPRS) was found to be superior in measuring dynamic accommodation compared to the commercially available Power Refractor (p <0.001). The current dissertation makes four major contributions to the field of accommodation and myopia: first, a motor deficit such as a sluggish or rigid plant cannot explain the abnormal behavior of accommodation in myopes. This was established based on the findings of a similar first and second order main sequence relationship between myopes and non-myopes (p > 0.05). Also, CA/C measures were not found to be attenuated as would be expected with a rigid plant. Novel simulations with reduced blur sensitivity coupled with a motor recalibration of AC gain did predict the myopic behavior. Second, atypical response patterns of accommodation such as the dynamic overshoots and double step responses were reported to exist with blur-driven accommodation. These patterns were suggested to occur due to an incorrect initiation of the response based on the similar main sequence findings between typical and atypical responses. Refractive error had no influence on the frequency of the atypical response in children. However, the influence of age reflects a developmental pattern in these dynamic atypical responses. Third, young progressive myopes showed large DOF and reduced accommodative sensitivity to lens defocus compared to non-myopes and adults. However, the ability to detect blur was similar between myopes and non myopes. Finally, depth of focus was found to increase with an increase in the stimulus demand. This change in the depth of focus was strongly correlated to a change in the microfluctuations but was independent of changes in the pupil size. Discussion: These findings confirm that school aged myopic children exhibit an altered pattern of accommodation. The pattern appears to be associated to progressive myopia but cannot be readily explained by current motor models of accommodation and vergence. Empirical data and model simulations suggest that a sluggish or rigid motor plant (lens and ciliary muscle) cannot predict this altered behavior completely. The current dissertation speculates a reduced sensory gain (or reduced blur sensitivity) as the prime factor coupled with a compensatory motor recalibration in the accommodative convergence cross link. The large lags of accommodation reflect a large objective depth of focus in myopes. However, the capacity to detect blur was found to be independent of the retinal defocus present in the myopic eyes. The elongating eye of a progressive myopes appears to compensate the persistent retinal defocus on the retina by some form of a perceptual adaptation.