CORRELATION BETWEEN CENTRAL CORNEAL THICKNESS AND OCULAR PERFUSION PRESSURE

ABSTRACT Central corneal thickness (CCT) is the measurement of the thickness of the cornea at its center, typically quantified in micrometers (µm). Ocular perfusion pressure (OPP) on the other hand is a calculated parameter representing the pressure gradient driving blood flow to the eye, particularly the optic nerve head and retinal tissues. The present study was aimed at determining the correlation between central corneal thickness and ocular perfusion pressure. A total of 263 subjects (152 females and 111 males) between the ages of 18 and 60 years (mean age 23.65 ± 4.506 years) were included in this study. Individuals who had undergone eye surgery as well as those who were currently on medication that could alter blood pressure and/or intraocular pressure were excluded from the study. Informed consent was obtained from all the participants followed by penlight examination, non-contact tonometry, ultrasonic pachymetry (mean central corneal thickness 540 ± 30.964 µm) and sphygmomanometry. From the data, the ocular perfusion pressure for each subject was then computed (mean ocular perfusion pressure 43.53 ± 8.489 mmHg). All values were arranged in tables and figures and analyzed using the using the statistical package for social sciences (SPSS) version 25. The Correlation Test (Analysis) was specifically used to test for significant association/correlation between ocular perfusion pressure and central corneal thickness (p = 0.300 > 0.05) while the Analysis of Variance (ANOVA) Test was used to for the significant effect of age on the correlation between ocular perfusion pressure and central corneal thickness (p = 0.721 > 0.05) as well as gender on the correlation between ocular perfusion pressure and central corneal thickness (p = 0.489 > 0.05). All conclusions were drawn at 95% confidence level. In conclusion this study found no significant correlation between ocular perfusion pressure and central corneal thickness. Age and gender also had no effect on the correlation between ocular perfusion pressure and central corneal thickness. CHAPTER ONE INTRODUCTION The central cornea may be regarded as more than just an anatomical landmark. The measurement of its thickness (CCT) serves as an indicator of corneal biomechanics and is implicated in various ocular conditions (Suzuki et al., 2015). On the other hand, Ocular Perfusion Pressure (OPP), which is derived from an interplay between intraocular pressure and systemic blood pressure, is crucial for maintaining the optic nerve’s blood supply. According to Costa et al. (2014), adequate perfusion is vital for the optic nerve’s health and function. This is turn affects visual acuity and field. Recent studies suggest that variations in central corneal thickness can affect ocular perfusion pressure, thereby influencing the risk and progression of glaucomatous damage. For instance, thinner corneas may correspond with altered OPP, potentially exacerbating optic nerve susceptibility to pressure-induced damage (Tham et al., 2014). This relationship between central corneal thickness and ocular perfusion pressure, thus becomes very critical, as glaucoma remains a leading cause of irreversible blindness globally, with its pathogenesis already linked to both intraocular pressure and ocular perfusion abnormalities (Jonas et al., 2017). This thesis aims to deepen the understanding of the correlation between central corneal thickness and ocular perfusion pressure as this could provide valuable insights into early detection of glaucoma, leading to adoption of more tailored management choices for the condition. For instance, patients with thinner corneas might require different therapeutic approaches to mitigate their increased risk of optic nerve damage (Prum et al., 2018). Furthermore, the exploration of this correlation has significant clinical implications. It could help with refined risk assessment models. As the interplay between central corneal thickness and ocular perfusion pressure is better understood, it shall aid transformation in clinical practices, leading to improved outcomes for patients worldwide. The exploration of this correlation not only contributes to already existent knowledge of ocular physiology but also paves the way for innovative approaches in preventing and managing glaucoma. Through rigorous data collection and analysis, this experimental study seeks to foster advancements that benefit both optometrists and patients alike. 1.1 Background to the Study 1.1.1 The Cornea The cornea is the transparent, dome-shaped surface that covers the front of the eye. It functions as a critical component of the eye's optical system. According to Eghari et al. (2015), it has significant refractive and barrier functions. The cornea is composed of five distinct layers, each with unique functions. These are: i. Epithelium: This is the outermost layer of the cornea. It acts as a barrier against dust, debris and bacteria. It also absorbs oxygen and nutrients from the pre-corneal tear film. The epithelium possesses a special ability to regenerate. This regenerative capacity is vital for wound healing, as damaged cells are rapidly replaced through mitosis of basal cells, ensuring continuous protection and function (Kaufman & Alm, 2021). Moreso, the epithelial cells secrete antimicrobial peptides and proteins that contribute to the eye's immune defense. ii. Bowman's Layer: The Bowman’s layer is a tough layer that lies just below the epithelium. It is composed of randomly arranged collagen fibers that provide structural support and contribute to the cornea's overall biomechanical stability. Despite its thinness, this layer plays a significant role in maintaining the cornea's shape and protecting the underlying stroma from injury and infection (Meek & Knupp, 2015). Unlike the corneal epithelium, the Bowman's membrane does not regenerate once damaged, which can lead to scarring and potential vision impairment if significantly injured (Kaufman & Alm, 2021). iii. Stroma: Constituting about 90% of the cornea's thickness, the stroma is composed of collagen fibers and keratocytes. The orderly arrangement of these collagen fibers is responsible for the cornea's transparency and refractive properties, allowing light to pass through with minimal scattering. The stroma also contains proteoglycans that help maintain proper spacing between the collagen fibers, aiding its optical clarity. Due to the inability of the stroma to regenerate, injury to it from any sort of pathology (e.g., keratitis) leads to permanent loss of transparency and possibly, loss of functional vision. iv. Descemet's Membrane: This is a thin but resilient layer situated between the corneal stroma and the endothelium. This membrane serves as the basement membrane for the endothelial cells. Composed predominantly of collagen types IV and VIII, Descemet's membrane provides elasticity and mechanical strength to the cornea (Schlotzer-Schrehardt, 2015). One of the unique features of Descemet's membrane is its ability to regenerate and thicken throughout an individual's life, a property not observed in the Bowman's layer (Fuchsluger et al., 2021). This regenerative capability is particularly important in surgical procedures such as Descemet Membrane Endothelial Keratoplasty (DMEK), where damaged portions of the membrane and endothelium are replaced to restore corneal clarity and function (Price et al., 2009). In addition to its structural role, Descemet's membrane acts as a barrier against infection and inflammation, preventing pathogens and inflammatory cells from penetrating deeper into the cornea and anterior chamber (Mimura et al., 2022). v. Endothelium: The corneal endothelium is the innermost layer of the cornea. This monolayer of hexagonal cells is responsible for regulating fluid and solute transport between the aqueous humor in the anterior chamber and the corneal stroma, thereby preventing stromal swelling and maintaining the cornea's clarity (Joyce, 2018). The endothelium achieves this through a pump-leak mechanism, where endothelial cells actively pump out excess water from the stroma into the anterior chamber, balancing the passive leak of fluid into the stroma (Bonanno, 2012). According to Yoshida et al. (2020), recent advances in cell therapy and tissue engineering are showing promise in treating endothelial dysfunction. Techniques involving the transplantation of cultured HCECs and the development of bioengineered corneal endothelium aim to restore endothelial function without the need for donor tissue. Also, Okumura et al. (2012) has reported research into pharmacological agents that can stimulate endothelial cell proliferation and migration. The functions of the cornea include: i. It is responsible for approximately two-thirds of the eye's total refractive power. It refracts light rays to help focus them on the retina for the formation of a “circle of least confusion.” ii. It serves as a physical barrier against dust, debris, pathogens, and other environmental hazards, protecting the inner components of the eye. iii. The cornea's unique structure, including the orderly arrangement of collagen fibers and absence of blood vessels, maintains its transparency, allowing light to pass through without obstruction. iv. The corneal epithelium absorbs oxygen and nutrients from the tear film and the ambient environment, vital for the health and function of corneal cells. v. The cornea filters out some of the harmful ultraviolet (UV) radiation from sunlight, protecting the inner eye structures from UV damage. vi. It has a high density of nerve endings, providing sensory input that triggers protective reflexes such as blinking and tearing, which help to protect the eye from injury and maintain a moist environment. vii. The cornea participates in the eye's immune defense by producing antimicrobial peptides and proteins, helping to ward off infections. These functions collectively ensure that the cornea not only contributes to sharp and clear vision but also protects the eye from various external threats and maintains overall ocular health (Bhatia et al., 2022). In all, the cornea is a vital component of the visual system, as its overall structure and function impacts the overall health and optical performance of the eye. Basically, the cornea is indispensable in vision, meaning that vision is impossible without this structure. For individuals with compromised corneas and who are not candidates for traditional corneal transplants, artificial corneas (keratoprostheses) provide an alternative. Recent innovations in biocompatible materials and design have improved the integration and performance of these devices (Yoon et al., 2023). Plate 1.1: Structure of the Cornea 1.1.2 Ocular Perfusion Ocular perfusion refers to the blood flow to the eye, particularly the delivery of oxygen and nutrients to the ocular tissues and the removal of metabolic waste products. This process is vital for maintaining the health and function of the eye, especially the retina and optic nerve. Adequate ocular perfusion is necessary for proper visual function and the prevention of various ocular diseases. The blood supply to the eye is primarily provided by the ophthalmic artery (a branch of the internal carotid artery). The ophthalmic artery further branches into the central retinal artery and the posterior ciliary arteries, which supply the retina and the optic nerve head, respectively (Brazitikos, 2019). According to Kur et al. (2012), the retinal vasculature is organized in a way that supports the high metabolic demands of the retinal ganglion cells and photoreceptors, crucial for visual processing. This circulation is autoregulated, meaning it can adjust blood flow based on the metabolic needs of the retina, ensuring consistent perfusion even with fluctuations in systemic blood pressure (Tolentino, 2009). Ocular perfusion is important in the following ways: i. Nutrient and Oxygen Supply: Proper blood flow ensures that the retina, optic nerve, and other ocular tissues receive sufficient oxygen and nutrients to function effectively. ii. Waste Removal: Effective perfusion helps remove metabolic waste products from ocular tissues, preventing accumulation that could lead to cellular damage. iii. Maintenance of Retinal Function: The retina has one of the highest metabolic rates of any tissue in the body and is highly dependent on continuous and adequate blood supply to maintain its function and support visual processing. iv. Prevention of Ocular Diseases: Inadequate ocular perfusion is associated with various ocular diseases, including glaucoma and diabetic retinopathy. Low ocular perfusion pressure has been identified as a risk factor for glaucomatous damage due to reduced blood flow to the optic nerve (Leske et al., 2002). Ocular perfusion can be tracked and monitored via an index known as “Ocular Perfusion Pressure (OPP),” a concept that will be explored later in this study. OPP is critical in the clinical setting, especially for patients with glaucoma or other ocular vascular diseases. Therapeutic strategies may involve the use of medications to lower IOP or manage systemic blood pressure to ensure adequate perfusion to the eye (Costa et al., 2014). 1.2 Statement of Problem The relationship between central corneal thickness