Optometry and Vision Sciences - Theses
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ItemClinical applications of omega-3 fatty acids for corneal and peripheral nerve health( 2020)Peripheral neuropathy, a common complication of diabetes, can lead to debilitating functional impairment and adversely impact daily living. In diabetes, damage to small sensory nerves in the cornea, visible using in vivo confocal microscopy (IVCM), precedes large nerve fibre involvement. Quantitative corneal nerve parameters, derived from IVCM images, provide reliable markers for evaluating small fibre damage and repair in diabetic peripheral neuropathy. A current challenge in the management of diabetic peripheral neuropathy is a lack of effective treatments. Omega-3 polyunsaturated fatty acids (PUFAs) modulate systemic inflammation and impart neurotrophic effects and, thus, show promise as neuroprotective agents. Although omega-3 PUFAs have established utility in the management of a number of ocular conditions, including dry eye disease, their potential role for modulating corneal and peripheral nerve health in diabetes has not been thoroughly investigated. This thesis focuses on evaluating the role of omega-3 PUFAs in improving peripheral nerve health using corneal nerve parameters as surrogate markers. First, a clinician survey was developed and administered to explore current practices relating to omega-3 fatty acids in eye care settings. The survey outcomes provide an overview of Australian and New Zealand optometrists’ knowledge and practice patterns relating to omega-3 PUFAs and identify potential avenues for improving clinical implementation. To assist in providing a tailored clinical approach, a dietary questionnaire for quantifying an individual’s omega-3 PUFAs intake was designed and validated. In a cross-sectional study, the association between systemic omega-3 fatty acid levels and corneal nerve parameters was evaluated in healthy controls and individuals with diabetes. This study identified a relationship between corneal nerve structural parameters and the systemic Omega-3 Index, a metric combining erythrocyte docosahexaenoic acid (DHA) and eicosapentanoic acid (EPA) levels. Furthermore, an association between corneal nerve structure and DHA levels, but not EPA levels, was identified. Using a systematic review methodology, randomised controlled trials (RCTs) evaluating the effects of oral omega-3 PUFA supplementation on peripheral nerve structure and function were identified, appraised, and synthesised. This review found, with low certainty, that omega-3 PUFAs attenuate sensory function deficits in chemotherapy-induced peripheral neuropathy. It also identified a paucity of RCTs evaluating the role of omega-3 PUFAs in treating diabetic peripheral neuropathy. These finding supported the rationale for conducting an RCT evaluating the effects of six-months of omega-3 PUFA supplementation in individuals with type 1 diabetes. This study found that, relative to placebo, oral omega-3 PUFA supplementation for six months significantly improved corneal nerve parameters, consistent with a corneal neuroregenerative effect. However, no significant differences were found for small or large nerve fibre function relative to placebo. Overall, this body of work advances scientific understanding of the clinical practices relating to omega-3 PUFAs in eye care settings and provides a dietary assessment tool for improving clinical implementation. Using corneal nerve health as a marker, findings from the prospective clinical studies provide evidence for the role of omega-3 PUFAs in modulating peripheral nerve health.
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ItemOxygen levels at the human cornea during contact lens wear( 1981)Contact lenses can restrict the amount of oxygen available to the cornea, often resulting in corneal swelling and visual dysfunction. However, despite the obvious importance of corneal oxygen availability, few studies have attempted to measure the level of oxygen at the anterior corneal surface of the in vivo human eye during contact lens wear. The aim of this thesis is to acquire such information. The rate at which the unanaesthetised cornea consumes oxygen from the membrane of a polarographic oxygen sensor forms an arbitrary index of corneal oxygen demand. An equivalent oxygen technique was modified for use in the in vivo human eye; this technique involved equating the corneal oxygen demand following a test condition (such as contact lens wear), to that following exposure to known oxygen levels. Human eye models relating static equivalent oxygen percentages (EOP) to lens oxygen transmissibility (Dk/L) were derived using seven hard and soft gas-permeable contact lenses to allow predictions to be made of the likely physiological response to contact lens wear. The discrepancies between these and other models were to be expected in view of the differing criteria upon which they were based. The effects of blinking and lens fit on the EOP beneath hydrogel lenses were found to be minimal, confirming earlier predictions. The significant increases in rigid lens EOP results upon blinking could be explained in terms of the tear pumping mechanisms known to exist with such lenses. An EOP of 7.7% O2 is available to the cornea of the closed eye. In some instances, a significant amount of oxygen is derived not only from the paipebral conjunctiva, but also from the atmosphere as a result of an imperfect paipebral aperture seal. Attempts to derive and EOP v Dk/L model for contact lens wearing under closed-lid conditions were unsuccessful; individual variations in the supply of oxygen to the cornea under these conditions were thought to contribute to this failure. The validity of similar models was questioned in view of the simplified assumption which were made.