In vivo measurements of prelamina and lamina cribrosa biomechanical properties in humans

Purpose: To develop and use a custom virtual fields method (VFM) to assess the biomechanical properties of human prelamina and lamina cribrosa (LC) in vivo. Methods: Clinical data of 20 healthy, 20 ocular hypertensive (OHT), 20 primary open-angle glaucoma, and 16 primary angle-closure glaucoma eyes were analyzed. For each eye, the intraocular pressure (IOP) and optical coherence tomography (OCT) images of the optic nerve head (ONH) were acquired at the normal state and after acute IOP elevation. The IOP-induced deformation of the ONH was obtained from the OCT volumes using a three-dimensional tracking algorithm and fed into the VFM to extract the biomechanical properties of the prelamina and the LC in vivo. Statistical measurements and P values from the Mann-Whitney-Wilcoxon tests were reported. Results: The average shear moduli of the prelamina and the LC were 64.2 ± 36.1 kPa and 73.1 ± 46.9 kPa, respectively. The shear moduli of the prelamina of healthy subjects were significantly lower than those of the OHT subjects. Comparisons between healthy and glaucoma subjects could not be made robustly due to a small sample size. Conclusions: We have developed a methodology to assess the biomechanical properties of human ONH tissues in vivo and provide preliminary comparisons in healthy and OHT subjects. Our proposed methodology may be of interest for glaucoma management

Ocular rigidity : a previously unexplored risk factor in the pathophysiology of open-angle glaucoma : assessment using a novel OCT-based measurement method

Le glaucome est la première cause de cécité irréversible dans le monde. Bien que sa pathogenèse demeure encore nébuleuse, les propriétés biomécaniques de l’oeil sembleraient jouer un rôle important dans le développement et la progression de cette maladie. Il est stipulé que la rigidité oculaire (RO) est altérée au travers les divers stades de la maladie et qu’elle serait le facteur le plus influent sur la réponse du nerf optique aux variations de la pression intraoculaire (PIO) au sein du glaucome. Pour permettre l’investigation du rôle de la RO dans le glaucome primaire à angle ouvert (GPAO), la capacité de quantifier la RO in vivo par l’entremise d’une méthode fiable et non-invasive est essentielle. Une telle méthode n’est disponible que depuis 2015. Basée sur l'équation de Friedenwald, cette approche combine l'imagerie par tomographie par cohérence optique (TCO) et la segmentation choroïdienne automatisée afin de mesurer le changement de volume choroïdien pulsatile (ΔV), ainsi que la tonométrie dynamique de contour Pascal pour mesurer le changement de pression pulsatile correspondant. L’objectif de cette thèse est d’évaluer la validité de cette méthode, et d’en faire usage afin d’investiguer le rôle de la RO dans les maladies oculaires, particulièrement le GPAO. Plus spécifiquement, cette thèse vise à : 1) améliorer la méthode proposée et évaluer sa validité ainsi que sa répétabilité, 2) investiguer l’association entre la RO et le dommage neuro-rétinien chez les patients glaucomateux, et ceux atteints d’un syndrome de vasospasticité, 3) évaluer l’association entre la RO et les paramètres biomécaniques de la cornée, 4) évaluer l’association entre la RO et les pics de PIO survenant suite aux thérapies par injections intravitréennes (IIV), afin de les prédire et de les prévenir chez les patients à haut risque, et 5) confirmer que la RO est réduite dans les yeux myopes. D’abord, nous avons amélioré le modèle mathématique de l’oeil utilisé pour dériver ΔV en le rendant plus précis anatomiquement et en tenant compte de la choroïde périphérique. Nous avons démontré la validité et la bonne répétabilité de cette méthodologie. Puis, nous avons effectué la mesure des coefficients de RO sur un large éventail de sujets sains et glaucomateux en utilisant notre méthode non-invasive, et avons démontré, pour la première fois, qu'une RO basse est corrélée aux dommages glaucomateux. Les corrélations observées étaient comparables à celles obtenues avec des facteurs de risque reconnus tels que la PIO maximale. Une forte corrélation entre la RO et les dommages neuro-rétiniens a été observée chez les patients vasospastiques, mais pas chez ceux atteints d'une maladie vasculaire ischémique. Cela pourrait potentiellement indiquer une plus grande susceptibilité au glaucome due à la biomécanique oculaire chez les patients vasospastiques. Bien que les paramètres biomécaniques cornéens aient été largement adoptés dans la pratique clinique en tant que substitut pour la RO, propriété biomécanique globale de l'oeil, nous avons démontré une association limitée entre la RO et ces paramètres, offrant une nouvelle perspective sur la relation entre les propriétés biomécaniques cornéennes et globales de l’oeil. Seule une faible corrélation entre le facteur de résistance cornéenne et la RO demeure après ajustement pour les facteurs de confusion dans le groupe des patients glaucomateux. Ensuite, nous avons présenté un modèle pour prédire l'amplitude des pics de PIO après IIV à partir de la mesure non-invasive de la RO. Ceci est particulièrement utile pour les patients à haut risque atteints de maladies rétiniennes exsudatives et de glaucome qui nécessiteraient des IIV thérapeutiques, et pourrait permettre aux cliniciens d'ajuster ou de personnaliser le traitement pour éviter toute perte de vision additionnelle. Enfin, nous avons étudié les différences de RO entre les yeux myopes et les non-myopes en utilisant cette technique, et avons démontré une RO inférieure dans la myopie axiale, facteur de risque du GPAO. Dans l'ensemble, ces résultats contribuent à l’avancement des connaissances sur la physiopathologie du GPAO. Le développement de notre méthode permettra non seulement de mieux explorer le rôle de la RO dans les maladies oculaires, mais contribuera également à élucider les mécanismes et développer de nouveaux traitements ciblant la RO pour contrer la déficience visuelle liée à ces maladies.Glaucoma is the leading cause of irreversible blindness worldwide. While its pathogenesis is yet to be fully understood, the biomechanical properties of the eye are thought to be involved in the development and progression of this disease. Ocular rigidity (OR) is thought to be altered through disease processes and has been suggested to be the most influential factor on the optic nerve head’s response to variations in intraocular pressure (IOP) in glaucoma. To further investigate the role of OR in open-angle glaucoma (OAG) and other ocular diseases such as myopia, the ability to quantify OR in living human eyes using a reliable and non-invasive method is essential. Such a method has only become available in 2015. Based on the Friedenwald equation, the method uses time-lapse optical coherence tomography (OCT) imaging and automated choroidal segmentation to measure the pulsatile choroidal volume change (ΔV), and Pascal dynamic contour tonometry to measure the corresponding pulsatile pressure change. The purpose of this thesis work was to assess the validity of the methodology, then use it to investigate the role of OR in ocular diseases, particularly in OAG. More specifically, the objectives were: 1) To improve the extrapolation of ΔV and evaluate the method’s validity and repeatability, 2) To investigate the association between OR and neuro-retinal damage in glaucomatous patients, as well as those with concomitant vasospasticity, 3) To evaluate the association between OR and corneal biomechanical parameters, 4) To assess the association between OR and IOP spikes following therapeutic intravitreal injections (IVIs), to predict and prevent them in high-risk patients, and 5) To confirm that OR is lower in myopia. First, we improved the mathematical model of the eye used to derive ΔV by rendering it more anatomically accurate and accounting for the peripheral choroid. We also confirmed the validity and good repeatability of the method. We carried out the measurement of OR coefficients on a wide range of healthy and glaucomatous subjects using this non-invasive method, and were able to show, for the first time, that lower OR is correlated with more glaucomatous damage. The correlations observed were comparable to those obtained with recognized risk factors such as maximum IOP. A strong correlation between OR and neuro-retinal damage was found in patients with concurrent vasospastic syndrome, but not in those with ischemic vascular disease. This could perhaps indicate a greater susceptibility to glaucoma due to ocular biomechanics in vasospastic patients. While corneal biomechanical parameters have been widely adopted in clinical practice as surrogate measurements for the eye’s overall biomechanical properties represented by OR, we have shown a limited association between these parameters, bringing new insight unto the relationship between corneal and global biomechanical properties. Only a weak correlation between the corneal resistance factor and OR remained in glaucomatous eyes after adjusting for confounding factors. In addition, we presented a model to predict the magnitude of IOP spikes following IVIs from the non-invasive measurement of OR. This is particularly useful for high-risk patients with exudative retinal diseases and glaucoma that require therapeutic IVIs, and could provide the clinician an opportunity to adjust or customize treatment to prevent further vision loss. Finally, we investigated OR differences between non-myopic and myopic eyes using this technique, and demonstrated lower OR in axial myopia, a risk factor for OAG. Overall, these findings provide new insights unto the pathophysiology of glaucomatous optic neuropathy. The development of our method will permit further investigation of the role of OR in ocular diseases, contributing to elucidate mechanisms and provide novel management options to counter vision impairment caused by these diseases

Scleral structure and biomechanics

As the eye's main load-bearing connective tissue, the sclera is centrally important to vision. In addition to cooperatively maintaining refractive status with the cornea, the sclera must also provide stable mechanical support to vulnerable internal ocular structures such as the retina and optic nerve head. Moreover, it must achieve this under complex, dynamic loading conditions imposed by eye movements and fluid pressures. Recent years have seen significant advances in our knowledge of scleral biomechanics, its modulation with ageing and disease, and their relationship to the hierarchical structure of the collagen-rich scleral extracellular matrix (ECM) and its resident cells. This review focuses on notable recent structural and biomechanical studies, setting their findings in the context of the wider scleral literature. It reviews recent progress in the development of scattering and bioimaging methods to resolve scleral ECM structure at multiple scales. In vivo and ex vivo experimental methods to characterise scleral biomechanics are explored, along with computational techniques that combine structural and biomechanical data to simulate ocular behaviour and extract tissue material properties. Studies into alterations of scleral structure and biomechanics in myopia and glaucoma are presented, and their results reconciled with associated findings on changes in the ageing eye. Finally, new developments in scleral surgery and emerging minimally invasive therapies are highlighted that could offer new hope in the fight against escalating scleral-related vision disorder worldwide

Development of optical Doppler interferometry for the visualization of ocular elasticity and ciliary activity

Functional optical imaging techniques have become increasingly important due to their high resolution and non-invasive nature, and have been used to address many unmet needs in the biomedical imaging field. In the area of ophthalmology, mechanical properties have been shown to be an early indicator of retinal disease, but current imaging modalities are unable to provide high resolution in-vivo imaging to capture the minute changes in the elasticity of thin tissue layers at the back of the eye. For respiratory diseases, the ciliary cell function inside the airway have been discovered to play an important role in respiratory health and the onset of disease. Similarly, current techniques are not equipped to image and characterize the cellular level changes in in-vivo tissues. Phase-resolved Doppler (PRD) imaging is a technology developed by our F-OCT lab, primarily for visualizing blood flow and angiography. Recently, it has been determined that the PRD technique is able to provide high phase sensitivity, which can be used to obtain the tissue displacement as well as particle motions. Using this principle, we developed two types of imaging systems: confocal acoustic radiation force optical coherence elastography (ARF-OCE) and spectrally-encoded interferometric microscopy (SEIM). Using the confocal ARF-OCE system, we present the first spatially mapped elasticity imaging in a live animal retina, and obtained a better understanding of the elasticity of different retinal layers. With the SEIM system, we introduced a novel method of spatially tracking ciliary activity in real-time of in vitro tracheal and oviduct tissues. We demonstrate that the SEIM system can image and quantify ciliary beating frequency and ciliary beating pattern with high speed and large field of view. While both these technologies use the PRD technique, the optical system has been optimized for the respective applications. The results in this dissertation serve as a stepping stone to the optimization and ultimately, the clinical translation of the PRD technique to diagnostic imaging. The developed technology has great potential for clinical diagnosis and management of a number of ocular disease, such as age related macular degeneration, glaucoma, presbyopia and myopia, as well as airway diseases such as asthma

Three-dimensional optical coherence tomography imaging of the optic nerve head

Background: the primary site of injury in glaucoma is likely to be at the lamina cribrosa (LC), deep within the optic nerve head (ONH). Optical coherence tomography (OCT) in glaucoma has, to date, focused on the detection of nerve fibre loss. Spectral domain OCT (SDOCT) has improved speed and axial resolution, allowing acquisition of three-dimensional ONH volumes and may capture targets deep within the ONH. This thesis explores the capabilities and potential of deep SDOCT imaging in the monkey ONH. Plan of research: an investigation was conducted into the detection of key landmarks that would be necessary for future quantification strategies. In particular, detection of the neural canal opening (NCO) was assessed and how the NCO relates to what is clinically identified as the disc margin. The next phase involved clarifying the anatomical and histological basis of ONH structures observed within SDCOT volumes, by comparison with histological sections and disc photographs. Finally, quantification strategies for novel parameters based on deep targets were developed and used to detect chronic longitudinal changes in experimental glaucoma and acute changes following IOP manipulation. Results: SDOCT reliably detects the NCO, which can be used as an anchoring structure for reference planes. Usually the NCO equates to the disc margin but disc margin architecture can be complex and highly variable. SDOCT captures the prelaminar tissue and anterior LC surface. Prelaminar thinning and posterior LC displacement were both detected longitudinally in experimental glaucoma. Prelaminar thinning was observed with acute IOP elevation; posterior LC movement was rare. Significance: deep ONH structures, including the LC, are realistic targets for clinical imaging. These imaging targets may be useful in the detection of glaucoma progression and in the verification of ex-vivo models of ONH biomechanical behaviour

In Vivo Assessment of Lamina Cribrosa Microstructure in Glaucoma

Glaucoma is an optic neuropathy that is the second leading cause of blindness worldwide. The disease is characterized by damage to the retinal ganglion cells, resulting in irreversible vision loss. While the exact pathogenesis remains unclear, damage due to glaucoma is believed to first occur at the lamina cribrosa (LC), a collagenous meshwork in the optic nerve head through which all retinal ganglion cell axons pass on their way to the brain. The mechanical theory of glaucoma postulates that elevated intraocular pressure deforms the LC, leading to a biological cascade resulting in retinal ganglion cell death. However, the interaction between intraocular pressure and glaucoma is complex; a substantial heterogeneity exists in the intraocular pressure at which a given patient experiences glaucoma. Recent studies have identified that perhaps intracranial pressure, which acts posterior to the LC, may play an important role in the disease process. Given the complex 3D microstructure of the LC, in vivo studies thus far have been limited to assessment of changes in its surface. However, because the axons are traversing through the entire volume of the LC, the axonal damage can occur at any level of the LC, rather than only at its surface. Therefore, full understanding of the damage caused by glaucoma requires systematic characterization of the 3D LC microstructure. In order to better characterize the 3D LC microstructure, we demonstrate here a novel automated 3D LC segmentation method that is reproducible and capable of accurately detecting the LC microstructural component. Using our segmentation analysis, we find in a primate model that the LC microstructure deforms according to both intraocular pressure as well as intracranial pressure, with significant interaction between the two. We then move to the translational aspect of our study to characterize the healthy LC in human eyes and identify a number of structural and biomechanical differences in the LC microstructure compared to glaucoma eyes. Our findings demonstrate that a novel automated 3D assessment of the LC microstructure is capable of 1) identifying in vivo difference in the LC microstructure and LC biomechanics in glaucoma eyes and 2) improving our understanding of glaucoma pathogenesis

Reduction of the Lamina Cribrosa Curvature After Trabeculectomy in Glaucoma

PURPOSE. To investigate whether the lamina cribrosa (LC) curvature is decreased after trabeculectomy. METHODS. Thirty-nine eyes of 39 patients with primary open-angle glaucoma who underwent trabeculectomy were included. Optic nerves were scanned by using enhanced-depth-imaging spectral-domain optical coherence tomography before and after trabeculectomy. The LC curvature was assessed by measuring the LC curvature index (LCCI) in seven horizontal Bscan images in each eye. RESULTS. The LCCI was significantly smaller at postoperative 6 months than at the preoperative level in all seven planes (all P < 0.001). Preoperative LCCI was associated with younger age at superior midperiphery, midhorizontal plane, inferior midperiphery (all P 0.005) and higher preoperative intraocular pressure (IOP) at superior and inferior midperiphery (both P ¼ 0.039). Younger age and larger preoperative LCCI were associated with a larger reduction of the LCCI at all three locations (P ¼ 0.003 and 0.031 at superior midperiphery, P ¼ 0.011 and 0.001 at midhorizontal plane, and P ¼ 0.014 and 0.005 at inferior midperiphery, respectively), whereas the percentage IOP lowering was associated at superior and inferior midperiphery (P ¼ 0.017 and 0.047, respectively). CONCLUSIONS. Lamina cribrosa curvature was reduced after trabeculectomy. This finding suggests that LC curvature may have value as a parameter relevant to optic nerve head biomechanics

Recent Clinical Research on Glaucoma

In the past few years, knowledge about glaucoma diagnosis and follow up has evolved dramatically through advances in intraocular pressure (IOP) measurement, corneal biomechanics, structural and functional assessment of the ocular surface, anterior chamber, retina, optic nerve and intracranial visual pathways, as well as the advent of artificial intelligence. In addition, the development of new modalities of IOP-lowering and non-IOP-lowering drugs, alternative deliveries, refined laser technologies, and minimally invasive glaucoma surgery (MIGS) techniques with different implants have widened the therapeutic possibilities for treating this disease. Finally, current insights into risk factors and quality of life in relation to glaucomatous impairment are emerging. The purpose of this Special Issue is to present the latest exciting clinical developments that are taking place in the field of glaucoma

Evaluation and Correlation of Morphological, Blood Flow and Physiological Retinal Changes in a Rat Model of Glaucoma with a Combined Optical Coherence Tomography and Electroretinography System

Glaucoma is a chronic disease associated with progressive dysfunction of the retinal ganglion cells (RGC), reduction of the retinal blood flow, thinning of the retinal nerve fiber layer (RNFL) and deformation of the optical nerve head (ONH). It is the second leading cause of blindness worldwide, with an estimate of 64.3 million people between the ages of 40 to 80 years affected in 2013, 76.7 million by 2020, and 111.8 million by 2040. Currently, there is no cure for glaucoma and any clinically available pharmaceutical or surgical approaches to treating the disease can only slow its progression. Therefore, early detection and treatment are essential for managing the glaucoma progression. Elevated intraocular pressure (IOP) is one of the most well studied and documented pathogenic risk factors for open-angle glaucoma (OAG), and as such, numerous animal models have been developed to study the acute and chronic IOP elevation effect on the ONH structure, retinal blood perfusion and RGC function. However, most of these studies utilized static chronic IOP elevation, while the relation between the IOP dynamics and the progression of glaucoma is still poorly understood. Joos et al proposed a rat model of glaucoma that utilized a dynamic approach to IOP elevation by use of a vascular loop that consists of short duration (~1h), intermittent IOP elevation. This model resembles closely the daily IOP spiking observed in glaucomatous patients, especially during the early stages of the disease. Better understanding of how the retina (human and animal) responds to such intermittent spikes of the IOP can provide ophthalmologists with valuable information on the origins and early stages of glaucoma development when treatment would be most efficient, as well as insights into developing new therapeutic approaches for glaucoma. Over the past few decades, a number of ex-vivo and in-vivo optical imaging modalities ranging from histopathology to confocal microscopy and optical coherence tomography (OCT) have been used to image changes in the morphology of the retina and the optic nerve head (ONH) in human subjects and animal models of OAG. Laser Doppler Flowmetry, Doppler OCT (DOCT) and Optical Coherence Angiography (OCTA) have been utilized to image and quantify changes in the total retinal blood flow and the blood perfusion in retinal capillaries during IOP elevation. Furthermore, electroretinography (ERG) has been used to assess changes in the retinal function (response to visual stimulation) during elevated IOP. However, all previous studies collected information about the morphological, functional and blood flow / perfusion changes in the retina during elevated IOP separately, at different time points, which prevented the researchers from correlating those changes and uncovering the relationship between them, typically referred to as neurovascular coupling. Since OCT provides both intensity and phase information in a single acquisition, this imaging technology is able to assess changes in the retinal morphology, function and blood flow/perfusion in-vivo and simultaneously. Therefore, the main goals of this PhD project were to: • Develop a combined OCT+ERG imaging system that can image in-vivo and record simultaneously, changes in the retinal morphology, retinal response to visual stimulation and retinal blood flow / perfusion at normal and elevated IOP. • Test the performance of the OCT+ERG system in a rat model of glaucoma. • Utilize the OCT+ERG technology and the dynamic IOP rat model of glaucoma based on the vascular loop, to investigate the effects of acute and chronic IOP elevation to ischemic and non-ischemic IOP levels on the rat retina. • Utilize the OCT+ERG technology to investigate neurovascular coupling in the rat retina at normal and abnormal IOP levels. Results from this PhD research have been published or summarized in manuscripts that are currently under review. Therefore, this PhD thesis was prepared in such a way that individual manuscripts represent separate thesis chapters