Biomarkers of canine glaucoma

Abstract

Glaucoma is a leading cause of blindness worldwide1 and successful management of the disease is complicated by a limited understanding of pathophysiological mechanisms, a slow and insidious onset and progression, and difficulties directly observing endpoints of disease (and treatment). Progress in our understanding of the disease is therefore limited by the need for prolonged and costly clinical trials. These difficulties mean monitoring the effect of, and response to novel therapeutic interventions in clinical trials is limited. Therefore, the use of animal models that show the characteristic progressive loss of retinal ganglion cell (RGC) function becomes necessary. Primary glaucoma occurs in dogs at a similar rate to that in humans, and the dog has been suggested as a suitable model of the disease due to the occurrence of natural disease in heterogenous populations, the size of the eye, and the visual system which functions at day and at night. Although presented as a model of glaucoma, the canine eye has never been validated as such. To be an effective model of disease, similarities and differences between these species must be known, quantifiable methods of identifying and monitoring disease progression established, and objective methods of determining response to treatment are required. With establishment of these entities, the validity of a potential biomarker, or more likely a group of biomarkers, may be investigated for relevant outcomes of interest in both human and veterinary patients. Chapter 1 outlines the current knowledge regarding canine glaucoma. Currently used diagnostic and therapeutic strategies are summarised and gaps in current understanding of the disease as it relates to the dog, are identified. In Chapter 2 and 3, preliminary studies describing tests to obtain quantifiable measures of functional vision in dogs for use in a clinical setting are presented. Initial development and validity of the canine Vision Impairment Score (VIS) is described (Chapter 2). Using a proxy (owner)-based questionnaire about daily performance and abilities, the VIS provides a measure that differed between blind and sighted eyes, between dogs based on the number of sighted eyes, and between sighted dogs with some vision impairment compared to those without impairment. In Chapter 3, a forced-choice preferential looking task that measures estimated visual acuity in dogs is described. This paradigm, modelled on testing used in human infants, demonstrated measures of visual acuity in emmetropic dogs that showed no evidence of vision impairment and had no prior training. In Chapter 4, a modified pupillometry assessment using equipment that is both available and cost effective for interested veterinarians to have, enabling evaluation of function that is not dependent on vision. Having investigated methods of assessing visual function, Chapters 5-7 describe cross-sectional studies using imaging techniques to investigate potential structural biomarkers of glaucoma. In these studies, structural measures in eyes with and without glaucoma, and those predisposed to the development of glaucoma but before the onset of clinically recognisable disease, were compared. In Chapter 5 structures affecting aqueous humour outflow in the anterior segment are evaluated with the use of ultrasound biomicroscopy. We identified measures of the ciliary cleft and iridocorneal angle that differed between groups. In Chapter 6 optical coherence tomography (OCT) is used to compare the peripapillary retina, retinal nerve fibre layer, and the optic nerve head between normal eyes and those predisposed to glaucoma. Structural differences are identified that discriminate predisposed eyes from healthy eyes. The potential that these changes are suggestive of early disease, and that serial monitoring may facilitate monitoring of disease progression at earlier stages than are currently possible, is proposed In Chapter 7, diffusion tensor imaging (DTI), a magnetic resonance technique that measures the properties of water diffusion, is used to evaluate the integrity and architecture of white matter tracts of the visual pathway beyond the retina. Using this non-invasive technique where indices correlate with tissue microstructure and pathology, we demonstrate measurable differences in diffusivity within the visual pathway of dogs with and without PACG. In Chapter 8 mass spectrometry is used to investigate the potential use of the canine tear film as a source of biomarkers of disease in glaucoma. Tear samples were collected using Schirmer tear test strips and protein expression as well as pathway analyses were performed. Tear proteins unique to the tears from eyes with both primary open and closed angle glaucoma are identified, and proteins uniquely expressed in the tears of eyes predisposed to PACG are also identified. Chapter 9 outlines the technique and outcomes in a series of dogs with naturally occurring glaucoma that is refractory to medical treatment. Placement of a Baerveldt-350mm2 implant with modifications commonly used in human glaucoma patients is described. The outcomes of this surgical technique, which is not previously described in dogs, are described. In Chapter 10, a review of clinical outcomes seen in dogs undergoing surgical treatment of glaucoma at a veterinary referral hospital is performed. Comparisons between dogs treated with the technique described in Chapter 7 and dogs treated with trans-scleral ciliary body destruction using a diode laser are made. The merits of techniques to shunt aqueous humour from the eye compared to cyclodestructive techniques are discussed with the suggestion these techniques treat the disease in a more physiologically appropriate manner. Chapter 11 forms the conclusion of this thesis. A discussion of how the objectives of this research to establish a paradigm for the assessment of glaucoma in dogs is presented. The significance of the findings to clinical veterinary practice as well as vision science researchers is outlined. An outline of initial proposals for subsequent work to validate and build on this research is provided with a description of how this body of knowledge may contribute to our understanding and management of glaucoma.