Aspects of Dynamic Anterior Surface Aberrations
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Introduction: The measurement of tear film stability/regularity is very critical in the diagnosis of dry eye. The tear breakup time, which is used as a diagnostic tool in diagnosing dry eye, is very subjective in nature and variations among individual clinicians exists. The exact mechanism of the tear breakup is also unclear due to the involvement of so many other factors other than the tear film itself. As the prevalence of dry eye is increasing, the need for an objective technique which can be used universally to differentiate between dry eye and normal values increases. Studies have shown that aberrations can be used as an objective technique in diagnosing dry eye, as there is a direct involvement of the tear film in the optics of the eye. However, very few studies have studied the dynamic nature of the anterior surface using aberrations and suggested using dynamic surface aberrations as an objective measure of surface quality. Hence, a series of studies were conducted to understand the aberrations produced by the anterior surface of the eye (tear film and corneal surface) and to measure objectively the anterior surface quality using surface aberrometry. The objectives of each study chapter are as follows: Chapter 3 i): To obtain the noise associated with the instrument using a non-dynamic measuring surface, and ii) to design appropriate acquisition settings for the measurements with ocular surface. Chapter 4: To determine і) the spectral characteristics of the Placido disc light sources of two corneal analysers, іі) the thermal characteristic for a variety of inanimate objects, human ocular surface and the adnexa in the presence of Placido disc light source at normal working distance, and ііі) to compare the ocular surface aberrations obtained using both the corneal analysers Chapter 5: To determine i) the optimal method for acquisition with respect to normal physiological processes, by examining the blink regimen and head position that elicits the most consistent response over the largest region on repeated measurement; and iі) the largest region selected for analysis by investigating the effect on the individual and summary aberration metrics of the inclusion of non-measurement areas (i.e. where the Placido disc cannot be projected onto the cornea or contact lens). The proportion of non-measurement area that elicits a significantly different result will be determined. Chapter 6: To evaluate і) a new method of analyzing dynamic ocular surface aberrations using segmented liner regression, and іі) the inter-ocular characteristics of the dynamic ocular surface aberrations using the segmented linear regression. Methods: Chapter 3: The characteristics of the surface aberrometer and the noise associated with the measurements of surface aberrations were evaluated using a non-dynamic surface (model eye). Measurements were obtained in different frame rates and focus positions to evaluate the optimal acquisition technique. At each focus position, a set of three repeated measurements were obtained to analyse the repeatability of the measurements obtained using a surface aberrometer. Chapter 4: The spectral characteristics of the Placido disc light source were obtained by using a PR650 SpectraScan photometer and the thermal characteristics of the objects were obtained using THI-500 non-contact infrared thermometer. The surface aberration measurements were compared between the corneal analysers. The spectral measures were obtained from the light sources, whereas the thermal measures were obtained from three different surfaces and surface of the eye and adnexa of ten participants. The dynamic anterior surface aberrations were obtained after obtaining the thermal measurements from the surface of the eye. Chapter 5: Twelve participants were enrolled by screening twenty participants. Participants were screened with their habitual lenses for contact lens wettability and non-invasive tear breakup time (NITBUT) without contact lenses. The participants were enrolled according to the inclusion and exclusion criteria and categorized into normal and dry eye group for study visits. The measurements of NITBUT and surface aberrations were obtained with and without contact lenses, and study lens wettability were also obtained in two visits on consecutive days. The surface aberration measurements were obtained in natural and forced blinking condition and in two different head positions. All the measurements were randomized between eye and between instruments. Chapter 6: Seventeen non- symptomatic and non- contact lens participants were recruited in this study. NITBUT and dynamic anterior surface aberration measurements were obtained. The order of the measurements was randomized between the eyes. Two open intervals of at least 10 sec and a maximum of 15 sec were used in the analysis of segmented fit. The dynamic vertical prism coefficients and higher order aberrations were used for the analysis. Results: Chapter 3: i. Data acquisition at an inter-frame interval of 0.25s gave the least number of dropped frames across focus positions, therefore this is the preferred frame rate for data acquisition. ii. Data obtained in the initial ~15s reflects the focusing procedure and needs to be manually removed prior to analysis of tear dynamics. iii. Even in the optimal focus position there were significant (small) differences in the distributions between repeated measures. For this reason repeated samples have to be obtained where possible. iv. The green and red focus positions showed the most consistency within repeated measurements. The variability of the measurements was also more similar between the red and green focus positions than the blue focus positions, both at the extreme positions of defocus and with incremental defocus away from the optimal focus position. When obtaining the dynamic sampling of human ocular surface measurements, the optimal position of focus should be obtained at the blink such that as the tear film dissipates between blinks the measurements are obtained in the (relatively) red focus position. Chapter 4: i. CA200 is the preferred device because of the consistent luminance. ii. Although aberrations were not significantly different between devices, the HOA RMS were higher with the CA200 and, combined with different luminance and possible tear response, indicates the devices are not interchangeable. In both instruments, there was no indication that there was a thermal response induced by the power of the light source. Therefore, this aspect of the source does not likely contribute to any difference in the aberrations measured by the two devices Chapter 5: i. Obtain data in the straight-ahead position, as there is no significant increase in target size with head turn. ii. With the CA100F, the forced blink paradigm is preferred as this enables blink dynamics to be examined. With the CA200F, either forced or natural blink paradigms are interpretable for tear dynamics. iii. Differentiation between dry eye and normal groups was best determined with the slope of the RMS aberrations within a blink. iv. Differentiation between performance with and without a contact lens in the dry eye and normal groups was best determined by analysing the width of the confidence interval of the moving average. Chapter 6: i. The location breakpoints one and two are significantly different between eye, open eye interval and order of the measurements for both vertical prism and HOA RMS values. ii. The highest positive slope for the HOA RMS was, on average, higher in the second eye measured (p= 0.0407) and tended to occur later after the blink (p= 0.0676). iii. The location of breakpoint 2 is not significantly different from the NITBUT values (p>0.05), even though the correlation was found to be low and not significant. iv. The average HOA RMS for segmented fit parameter intervals of vertical prism was found to be higher in the second open eye interval compared to first open eye interval. Conclusion: From the results of each chapter, it was observed that choosing the blink paradigm is very important to obtain and analyse the dynamic anterior surface aberrations. Choosing a forced blink paradigm (chapter 5) was showed to be useful when the information regarding blink location were not available. The repeatability of the measurements using a non-dynamic surface (chapter 3) shows that the measurements of surface aberrations are repeatable and it is important to choose a criterion closer to the natural tear film dynamics to obtain more repeatable measurements of anterior surface aberrations (chapter 4, 5 and 6). It also shows that the three phased segmented linear regression techniques can be used to analyse the anterior surface aberrations. The segmented linear regression technique was able to differentiate different stages of the tear film and the location of the second breakpoint calculated using segmented regression was closer to the clinical values of tear breakup time, indicating a possible use of segmented linear regression as an objective measure of surface quality.