IN VIVO analysis of ocular morphological changes during phakic accommodation

The principal theme of this thesis is the in vivo examination of ocular morphological changes during phakic accommodation, with particular attention paid to the ciliary muscle and crystalline lens. The investigations detailed involved the application of high-resolution imaging techniques to facilitate the acquisition of new data to assist in the clarification of aspects of the accommodative system that were poorly understood. A clinical evaluation of the newly available Grand Seiko Auto Ref/ Keratometer WAM-5500 optometer was undertaken to assess its value in the field of accommodation research. The device was found to be accurate and repeatable compared to subjective refraction, and has the added advantage of allowing dynamic data collection at a frequency of around 5 Hz. All of the subsequent investigations applied the WAM-5500 for determination of refractive error and objective accommodative responses. Anterior segment optical coherence tomography (AS-OCT) based studies examined the morphology and contractile response of youthful and ageing ciliary muscle. Nasal versus temporal asymmetry was identified, with the temporal aspect being both thicker and demonstrating a greater contractile response. The ciliary muscle was longer in terms of both its anterior (r = 0.49, P <0.001) and overall length (r = 0.45, P = 0.02) characteristics, in myopes. The myopic ciliary muscle does not appear to be merely stretched during axial elongation, as no significant relationship between thickness and refractive error was identified. The main contractile responses observed were a thickening of the anterior region and a shortening of the muscle, particularly anteriorly. Similar patterns of response were observed in subjects aged up to 70 years, supporting a lensocentric theory of presbyopia development. Following the discovery of nasal/ temporal asymmetry in ciliary muscle morphology and response, an investigation was conducted to explore whether the regional variations in muscle contractility impacted on lens stability during accommodation. A bespoke programme was developed to analyse AS-OCT images and determine whether lens tilt and decentration varied between the relaxed and accommodated states. No significant accommodative difference in these parameters was identified, implying that any changes in lens stability with accommodation are very slight, as a possible consequence of vitreous support. Novel three-dimensional magnetic resonance imaging (MRI) and analysis techniques were used to investigate changes in lens morphology and ocular conformation during accommodation. An accommodative reduction in lens equatorial diameter provides further evidence to support the Helmholtzian mechanism of accommodation, whilst the observed increase in lens volume challenges the widespread assertion that this structure is incompressible due to its high water content. Wholeeye MRI indicated that the volume of the vitreous chamber remains constant during accommodation. No significant changes in ocular conformation were detected using MRI. The investigations detailed provide further insight into the mechanisms of accommodation and presbyopia, and represent a platform for future work in this field.

Ocular biometric correlates of early-and late-onset myopia

Myopia is a refractive condition and develops because either the optical power of the eye is abnormally great or the eye is abnormally long, the optical consequences being that the focal length of the eye is too short for the physical length of the eye. The increase in axial length has been shown to match closely the dioptric error of the eye, in that a lmm increase in axial length usually generates 2 to 3D of myopia. The most common form of myopia is early-onset myopia (EO M) which occurs between 6 to 14 years of age. The second most common form of myopia is late-onset myopia (LOM) which emerges in late teens or early twenties, at a time when the eye should have ceased growing. The prevalence of LOM is increasing and research has indicated a link with excessive and sustained nearwork. The aim of this thesis was to examine the ocular biometric correlates associated with LOM and EOM development and progression. Biometric data was recorded on SO subjects, aged 16 to 26 years. The group was divided into 26 emmetropic subjects and 24 myopic subjects. Keratometry, corneal topography, ultrasonography, lens shape, central and peripheral refractive error, ocular blood flow and assessment of accommodation were measured on three occasions during an ISmonth to 2-year longitudinal study. Retinal contours were derived using a specially derived computer program. The thesis shows that myopia progression is related to an increase in vitreous chamber depth, a finding which supports previous work. The myopes exhibited hyperopic relative peripheral refractive error (PRE) and the emmetropes exhibited myopic relative PRE. Myopes demonstrated a prolate retinal shape and the retina became more prolate with myopia progression. The results show that a longitudinal, rather than equatorial, increase in the posterior segment is the principal structural correlate of myopia. Retinal shape, relative PRE and the ratio of axial length to corneal curvature have been indicated, in this thesis, as predictive factors for myopia onset and development. Data from this thesis demonstrates that myopia progression in the LOM group is the result of an increase in anterior segment power, owing to an increase in lens thickness, in conjunction with posterior segment elongation. Myopia progression in the EOM group is the product of a long posterior segment, which over-compensates for a weak anterior segment power. The weak anterior segment power in the EOM group is related to a combination of crystalline lens thinning and surface flattening. The results presented in this thesis confirm that posterior segment elongation is the main structural correlate in both EOM and LOM progression. The techniques and computer programs employed in the thesis are reproducible and robust providing a valuable framework for further myopia research and assessment of predictive factors.

Ocular response to silicone-hydrogel contact lenses

The thesis investigates the ocular response to silicone-hydrogel (SiH) contact lens wear, a relatively new contact lens material that has a higher modulus of rigidity and different surface coating than used in conventional hydrogel materials. The properties of SiH materials differ significantly from conventional hydrogels and, using subjective and objective means of assessment, the thesis examines how these properties affect reflection and biometry, ocular physiology, tear film characteristics, symptomatology, adverse events and complications. A range of standard and newly designed investigative techniques were employed, and latter involving novel imaging techniques, for the objective assessment of physiological changes which occur with contact lens wear. The study is the first to combine these techniques with biochemical analyses of the tear film composition. Forty-seven subjects were fitted with SiH lenses and randomly allocated to one of the two materials currently on the market (Lotrafilcon A or Balafilcon A) on an either daily or continuous wear basis. An additional control group of 14 age-matched non-contact lens wearers were monitored over the same period. Measurements were taken before and 1, 3, 6, 12 and 18 months after initial fitting. The findings reported in this thesis will enable contact lens practitioners and manufacturers to understand further the optical, physiological and biochemical nature of the ocular response to SiH contact lenses and hence facilitate the development of this important generation of contact lens material.

Behavioural correlates of ocular accomodation and the autonomic nervous system

The binding issue of th is thesis was the examination of workload, induced by relinotopic and spatiotopic stimuli, on both the ocu lomotor and cardiovascular systems together with investigating the covariation between the two systems - the 'eye-heart' link. Further, the influence of refractive error on ocular accommodation and cardiovascular function was assessed. A clinical evaluation was undertaken to assess the newly available open-view infrared Shin-Nippon NVision-K 5001 optometer, its benefit being the capability to measure through pupils = 2.3 mm. Measurements of refractive error taken with the NVision-K were found to be both accurate (Difference in Mean Spherical Equivalent: 0.14 ± 0.35 D; p = 0.67) and repeatable when compared to non-cycloplegic subjective refraction. Due to technical difficulties, however, the NVision-K could not be used for the purpose of the thesis, as such, measures of accommodation were taken using the continuously recording Shin-Nippon SRW-5000 openview infrared optometer, coupled with a piezo-electric finger pulse transducer to measure pulse. Heart rate variability (HRV) was spectrally analysed to determine the systemic sympathetic and parasympathetic components of the autonomic nervous system (ANS). A large sample (n = 60), cross-sectional study showed late-onset myopes (LOMs) display less accurate responses when compared to other refractive groups at high accommodative demand levels (3 .0 0 and 4.0D). Tonic accommodation (TA) was highest in the hypermetropes, fo llowed by emmetropes and early-onset myopes while the LOM subjects demonstrated statistically significant lower levels of TA. The root-meansquare (RMS) value of the accommodative response was shown to amplify with increased levels of accommodative demand. Changes in refractive error only became significant between groups at higher demand levels (3.0 D and 4.0 D) with the LOMs showing the largest magnification in oscilIations. Examination of the stimulus-response cross-over point with the unit ratio line and TA showed a correlation between the two (r = 0.45, p = 0.001), where TA is approximately twice the dioptric value of the stimulus-response cross-over point. Investigation of the relationship between ocular accommodation and systemic ANS function demonstrated covariation between the systems. Subjects with a faster heart rate (lower heart period) tended to have a higher TA value (r = -0.27, p < 0.05). Further, an increase in accommodative demand accompanies a faster heart rate. The influence of refractive error on the cardiovascular response to changes in accommodative demand, however, was equivocal. Examination of the microfluctuations ofacconunodation demonstrated a correlation between the temporal frequency location of the accommodative high Frequency component (HFC) and the arterial pulse frequency. The correlation was present at a range of accommodative demands from 0.0 D to 4.0 D and in all four refractive groups, suggesting that the HFC was augmented by physiological factors. Examination of the effect of visual cognition on ocular accommodation and the ANS confirmed that increasing levels of cognition affect the accommodative mechanism. The accommodative response shifted away from the subject at both near and far. This shift in accommodative response accompanied a decay in the systemic parasympathetic innervation to the heart. Differences between refractive groups also existed with LOMs showing less accurate responses compared to emmetropes. This disparity, however, appeared to be augmented by the systemic sympathetic nervous system. The investigations discussed explored Ihe role of oculomotor and cardiovascular fu nction in workload enviromnents, providing evidence for a behavioural link between the cardiovascular and oculomotor systems.

Ocular biometric investigation of anisometropia

The thesis aims to define further the biometric correlates in anisometropic eyes in order to provide a structural foundation for propositions concerning the development of ametropia.Biometric data are presented for 40 anisometropes and 40 isometropic controls drawn from Caucasian and Chinese populations.The principal finding was that the main structural correlate of myopia is an increase in axial rather than equatorial dimensions of the posterior globe. This finding has not been previously reported for in vivo work on humans. The computational method described in the thesis is a more accessible method for determination of eye shape than current imaging techniques such as magnetic resonance imaging or laser Doppler interferometry (LDI). Retinal contours derived from LDI and computation were shown to be closely matched. Corneal topography revealed no differences in corneal characteristics in anisometropic eyes, which supports the finding that anisometropia arises from differences in vitreous chamber depth.The corollary to axial expansion in myopia, that is retinal stretch in central regions of the posterior pole, was investigated by measurement of disc-to-fovea distances (DFD) using a scanning laser ophthalmoscope. DFD was found to increase with increased myopia, which demonstrates the primary contribution made by posterior central regions of the globe to axial expansion.The ocular pulse volume and choroidal blood flow, measured with the Ocular Blood Flow Tonograph, were found to be reduced in myopia; the reductions were found to be significantly correlated with vitreous chamber depth. The thesis includes preliminary data on whether the relationship arises from the influx of a blood bolus into eyes of different posterior volumes or represents actual differences in choroidal blood flow.The results presented in this thesis show the utility of computed retinal contour and demonstrate that the structural correlate of myopia is axial rather than equatorial expansion of the vitreous chamber. The technique is suitable for large population studies and its relative simplicity makes it feasible for longitudinal studies on the development of ametropia in, for example, children.

Physiological and pathological human ocular perfusion characteristics

There were three principle aims to this thesis. Firstly, the acquisition protocols of clinical blood flow apparatus were investigated in order to optimise them for both cross-sectional and longitudinal application. Secondly, the effects of physiological factors including age and systematic circulation on ocular blood flow were investigated. Finally, the ocular perfusion characteristics of patients diagnosed with ocular diseases considered to be of a vascular origin were investigated. The principle findings of this work are:- 1) Optimisation of clinical investigationsPhotodiode sensitivity of the scanning laser Doppler flowmeter should be kept within a range of 70-150 DC when acquiring images of the retina and optic nerve head in order to optimise the reproducibility of capillary blood flow measures. Account of the physiological spatial variation in retinal blood flow measures can be made using standard analysis protocols of the scanning laser Doppler flowmeter combined with a local search strategy. Measurements of pulsatile ocular blood flow using the ocular blood flow analyser are reproducible, however this reproducibility can be improved when consecutive intraocular pressure pulses are used to calculate pulsatile ocular blood flow. Spectral analysis of the intraocular pressure pulse-wave is viable and identifies the first four harmonic components of the waveform. 2) Physiological variation in ocular perfusionAge results in a significant reduction in perfusion of the retinal microcirculation, which is not evident in larger vessel beds such as the choroid. Despite known asymmetry in the systemic vasculature, no evidence of interocular asymmetry in ocular perfusion is apparent. 3) Pathological variation in ocular perfusionIn primary open angle glaucoma, perfusion is reduced in the retinal microcirculation of patients classified as having early to moderate visual field defects. However, ocular pulsatility defects are masked when patients and subjects are matched for systemic variables (pulse rate and mean arterial pressure); differentiation is facilitated by the application of waveform analysis to the continuos intraocular pressure curve even in the early stages of disease. Diabetic patients with adequate glycaemic control, exhibit maintenance of macular blood flow, macular topography and visual function following phacoemulsification.

Dynamic ocular thermography

The ability to measure ocular surface temperature (OST) with thermal imaging offers potential insight into ocular physiology that has been acknowledged in the literature. The TH7102MX thermo-camera (NEC San-ei, Japan) continuously records dynamic information about OST without sacrificing spatial resolution. Using purpose-designed image analysis software, it was possible to select and quantify the principal components of absolute temperature values and the magnitude plus rate of temperature change that followed blinking. The techniques was examined for repeatability, reproducibility and the effects of extrinsic factors: a suitable experimental protocol was thus developed. The precise source of the measured thermal radiation has previously been subject toe dispute: in this thesis, the results of a study examining the relationships between physical parameters of the anterior eye and OST, confirmed a principal role for the tear film in OST. The dynamic changes in OST were studied in a large group of young subjects: quantifying the post-blink changes in temperature with time also established a role for tear flow dynamics in OST. Using dynamic thermography, the effects of hydrogel contact lens wear on OST were investigated: a model eye for in vivo work, and both neophyte and adapted contact lens wearers for in vivo studies. Significantly greater OST was observed in contact lens wearers, particularly with silicone hydrogel lenses compared to etafilcon A, and tended to be greatest when lenses had been worn continuously. This finding is important to understanding the ocular response to contact lens wear. In a group of normal subjects, dynamic thermography appeared to measure the ocular response to the application of artificial tear drops: this may prove to be a significant research and clinical tool.

Objective methods for anterior ocular grading

This thesis set out to develop an objective analysis programme that correlates with subjective grades but has improved sensitivity and reliability in its measures so that the possibility of early detection and reliable monitoring of changes in anterior ocular surfaces (bulbar hyperaemia, palpebral redness, palpebral roughness and corneal straining) could be increased. The sensitivity of the program was 20x greater than subjective grading by optometrists. The reliability was found to be optimal (r=1.0) with subjective grading up to 144x more variable (r=0.08). Objective measures were used to create formulae for an overall ‘objective-grade’ (per surface) equivalent to those displayed by the CCLRU or Efron scales. The correlation between the formulated objective verses subjective grades was high, with adjusted r2 up to 0.96. Determination of baseline levels of objective grade were investigated over four age groups (5-85years n= 120) so that in practice a comparison against the ‘normal limits’ could be made. Differences for bulbar hyperaemia were found between the age groups (p<0.001), and also for palpebral redness and roughness (p<0.001). The objective formulae were then applied to the investigation of diurnal variation in order to account for any change that may affect the baseline. Increases in bulbar hyperaemia and palpebral redness were found between examinations in the morning and evening. Correlation factors were recommended. The program was then applied to clinical situations in the form of a contact lens trial and an investigation into iritis and keratoconus where it successfully recognised various surface changes. This programme could become a valuable tool, greatly improving the chances of early detection of anterior ocular abnormalities, and facilitating reliable monitoring of disease progression in clinical as well as research environments.

An optical study of human ocular dimensions

Many workers have studied the ocular components which occur in eyes exhibiting differing amounts of central refractive error but few have ever considered the additional information that could be derived from a study of peripheral refraction. Before now, peripheral refraction has either been measured in real eyes or has otherwise been modelled in schematic eyes of varying levels of sophistication. Several differences occur between measured and modelled results which, if accounted for, could give rise to more information regarding the nature of the optical and retinal surfaces and their asymmetries. Measurements of ocular components and peripheral refraction, however, have never been made in the same sample of eyes. In this study, ocular component and peripheral refractive measurements were made in a sample of young near-emmetropic, myopic and hyperopic eyes. The data for each refractive group was averaged. A computer program was written to construct spherical surfaced schematic eyes from this data. More sophisticated eye models were developed making use of linear algebraic ray tracing program. This method allowed rays to be traced through toroidal aspheric surfaces which were translated or rotated with respect to each other. For simplicity, the gradient index optical nature of the crystalline lens was neglected. Various alterations were made in these eye models to reproduce the measured peripheral refractive patterns. Excellent agreement was found between the modelled and measured peripheral refractive values over the central 70o of the visual field. This implied that the additional biometric features incorporated in each eye model were representative of those which were present in the measured eyes. As some of these features are not otherwise obtainable using in vivo techniques, it is proposed that the variation of refraction in the periphery offers a very useful optical method for studying human ocular component dimensions.

Measurement of ocular component contributions to residual astigmatism in adult human eyes

The aim of this study was to determine whether an ophthalmophakometric technique could offer a feasible means of investigating ocular component contributions to residual astigmatism in human eyes. Current opinion was gathered on the prevalence, magnitude and source of residual astigmatism. It emerged that a comprehensive evaluation of the astigmatic contributions of the eye's internal ocular surfaces and their respective axial separations (effectivity) had not been carried out to date. An ophthalmophakometric technique was developed to measure astigmatism arising from the internal ocular components. Procedures included the measurement of refractive error (infra-red autorefractometry), anterior corneal surface power (computerised video keratography), axial distances (A-scan ultrasonography) and the powers of the posterior corneal surface in addition to both surfaces of the crystalline lens (multi-meridional still flash ophthalmophakometry). Computing schemes were developed to yield the required biometric data. These included (1) calculation of crystalline lens surface powers in the absence of Purkinje images arising from its anterior surface, (2) application of meridional analysis to derive spherocylindrical surface powers from notional powers calculated along four pre-selected meridians, (3) application of astigmatic decomposition and vergence analysis to calculate contributions to residual astigmatism of ocular components with obliquely related cylinder axes, (4) calculation of the effect of random experimental errors on the calculated ocular component data. A complete set of biometric measurements were taken from both eyes of 66 undergraduate students. Effectivity due to corneal thickness made the smallest cylinder power contribution (up to 0.25DC) to residual astigmatism followed by contributions of the anterior chamber depth (up to 0.50DC) and crystalline lens thickness (up to 1.00DC). In each case astigmatic contributions were predominantly direct. More astigmatism arose from the posterior corneal surface (up to 1.00DC) and both crystalline lens surfaces (up to 2.50DC). The astigmatic contributions of the posterior corneal and lens surfaces were found to be predominantly inverse whilst direct astigmatism arose from the anterior lens surface. Very similar results were found for right versus left eyes and males versus females. Repeatability was assessed on 20 individuals. The ophthalmophakometric method was found to be prone to considerable accumulated experimental errors. However, these errors are random in nature so that group averaged data were found to be reasonably repeatable. A further confirmatory study was carried out on 10 individuals which demonstrated that biometric measurements made with and without cycloplegia did not differ significantly.

Modelling ocular monochromatic aberrations using schematic eyes with homogeneous optical media

Previous research has indicated that schematic eyes incorporating aspheric surfaces but lacking gradient index are unable to model ocular spherical aberration and peripheral astigmatism simultaneously. This limits their use as wide-angle schematic eyes. This thesis challenges this assumption by investigating the flexibility of schematic eyes comprising aspheric optical surfaces and homogeneous optical media. The full variation of ocular component dimensions found in human eyes was established from the literature. Schematic eye parameter variants were limited to these dimensions. The levels of spherical aberration and peripheral astigmatism modelled by these schematic eyes were compared to the range of measured levels. These were also established from the literature. To simplify comparison of modelled and measured data, single value parameters were introduced; the spherical aberration function (SAF), and peripheral astigmatism function (PAF). Some ocular components variations produced a wide range of aberrations without exceeding the limits of human ocular components. The effect of ocular component variations on coma was also investigated, but no comparison could be made as no empirical data exists. It was demonstrated that by combined manipulation of a number of parameters in the schematic eyes it was possible to model all levels of ocular spherical aberration and peripheral astigmatism. However, the unique parameters of a human eye could not be obtained in this way, as a number of models could be used to produce the same spherical aberration and peripheral astigmatism, while giving very different coma levels. It was concluded that these schematic eyes are flexible enough to model the monochromatic aberrations tested, the absence of gradient index being compensated for by altering the asphericity of one or more surfaces.

The detection of automated perimetric stimuli in ocular disease

Automated perimetry has made viable a rapid threshold examination of the visual field and has reinforced the role of perimetry in the diagnostic procedure. The aim of this study was twofold: to isolate the influence of certain extraneous factors on the sensitivity gradient, since these might limit the early detection and accurate monitoring of visual field loss and to investigate the efficacy of certain novel combinations of stimulus parameters in the detection of early visual field loss. The work was carried out with particular reference to glaucoma and to ocular hypertension. The effects of media opacities on the visual field were assessed by forward intraocular light scatter (n= 15) and were found to mask diffuse glaucomatous visual field loss and underestimate focal loss. Correction of the visual field indices for the effects of forward intraocular light scatter (n= 26) showed the focal losses to be, in reality, unaffected. Measurements of back scatter underestimated forward intraocular light scatter (n= 60) and the resultant depression of the visual field. Perimetric sensitivity improved with patient learning (n= 25) and exhibited eccentricity- and depth-dependency effects whereby improvements in sensitivity were greatest for peripheral areas of the field and for those areas which initially demonstrated the lowest sensitivity. The effects of practice were retained over several months (n= 16). Perimetric sensitivity decreased during prolonged examination due to fatigue effects (n&61 19); these demonstrated a similar eccentricity-dependency, being greatest for eccentricities beyond 30o. Mean sensitivities over the range of adaptation levels employed obeyed the Weber-Fechner law (n= 10) and, as would be expected, were independent of pupil size. No relationship was found between short-term fluctuation and adaptation level. Detection of diffuse glaucomatous visual field loss was facilitated using a size III stimulus of duration 200msec at an adaptation level of 31.5asb, compared with a size III stimulus of duration 100msec at an adaptation level of 4asb (n= 20). In a pilot study (n= 10), temporal summation was found to be higher in glaucomatous patients compared with age-matched controls, although the difference was not statistically significant.

Biometric and physiological factors in human ocular perfusion

By addressing the vascular features that characterise myopia, this thesis aims to provide an understanding of the early structural changes associated with human myopia and the progression to co-morbidity with age. This thesis addresses three main areas of study: 1. Ocular perfusion features and autoregulatory mechanisms in human myopia; 2. Choroidal thickness at the macular area of myopic eyes; 3. Effect of chronic smoking on the ocular haemodynamics and autoregulation. This thesis demonstrated a reduced resting ocular pulse amplitude and retrobulbar blood flow in human myopia, associated with an apparent oversensitivity to the vasodilatory effects of hypercapnia, which may be due to anatomical differences in the volume of the vessel beds. In young smokers, normal resting state vascular characteristics were present; however there also appeared to be increased reactivity to hypercapnia, possibly due to relative chronic hypoxia. The systemic circulation in myopes and smokers over-reacted similarly to hypercapnia suggesting that physiologic differences are not confined to the eye. Age also showed a negative effect on autoregulatory capacity in otherwise normal eyes. Collectively, these findings suggest that myopes and smokers require greater autoregulatory capacity to maintain appropriate oxygenation of retinal tissue, and since the capacity for such regulation reduces with age, these groups are at greater risk of insufficient autoregulation and relative hypoxia with age.