The Eyes: A Window to our Health?

It is famously said that ‘eyes are windows to the soul’, however a recent study conducted by Mass Eye and Ear, a Harvard Ophthalmology research centre, extends the proverb to the physical body as well. Researchers have recently discovered biomarkers in layers of the retina linking to ocular, but more importantly, systemic health, and this article will explore the importance of this to the emerging world of personalised medicine.

The retina is the bedrock of our vision, and its complexity lies in the several layers of cells (Figure 1). Cone and rod cells are photoreceptor cells; they absorb the photons that form light waves, and this triggers a cascade of reactions that activate the cone or rod cell, and produce an electrical signal. Rod cells are more sensitive to light (detect low amounts of photons), which is why we use them for vision in dark environments, and cone cells need many photons to be activated, useful for bright light, but are specialised for colour vision as well. The electric impulse is transmitted to the bipolar cell, which is an intermediary neuron to the ganglion cell, the top layer of the retina. Ganglion cells are the output neurons: transmitting the impulse to the brain to process, allowing you to read this article. Researchers explored retinal thinning, which damages the layers of the retina and compresses nerve connections between layers, leading to impaired vision, as well as retinal thickening, due to fluid accumulation, which separates layers of the retina and distorts their shape, also damaging visual acuity. This study explored the biomarker of retinal thickness at every layer of the retina, and applied it to predicting disease risk within the eye and body, a first of its kind.

The main finding of the study, utilising data from 44,828 patients, was that a thinner retina is strongly correlated with an increased risk of cardiovascular, pulmonary and ocular disease. In the methodology, optical coherence tomography (OCT) imaging was used (Figure 2), which is a more realistic form of Figure 1. Imaging analysis was combined with a decade of health data on the patients. Several conclusions were made based on the relative thickness of each layer of the retina, the main findings are listed below:

Thinner photoreceptor layers (rod and cone cells) strongly correlated with a 25% increased risk of heart failure, as well as 31% increased risk of obstructive airway diseases such as asthma. • Thinner ganglion cell layer strongly correlated with liver damage (alcohol related) and multiple sclerosis This study is potentially instrumental in strengthening medicine’s approach of prevention over cure, as OCT imagery could be used in routine health checks. Furthermore, it reinforces the interconnected nature of our body: there are thousands of unknown biomarkers revealing key information about various parts of the human body, and further research into their accuracy could develop the fields of pathology, cardiovascular health, and several other areas of interest.

Despite such a strong correlation, further research is needed to validate the causality between retinal thickness and systemic diseases, as that could open the pathway of disease prevention, instead of being limited to an indicator of health. Some of the correlations can be explained, such as Type 1 and 2 diabetes leading to high blood pressure which can cause fluid accumulation and retinal thickening of the blood vessels in the retina. However, some findings, such as the relationship between retinal thinning and multiple sclerosis, need further attention, as this will reaffirm the reliability of using retinal thickness in health checks. Also, the big data at hand needs to be questioned, as despite the large sample size, the conclusions drawn are specific to the European-dominated patients, with a mean age of the patients being 56.8 years. Especially in this case, at a mean age of 56.8 years, many of the correlations could be due to a third factor which links retinal thickness and the disease, as the elderly are more prone to a wider range of health complications. Variables like ethnicity, age and gender need to be explored on a global scale, in order to offer personalised treatments to any patient when using eye-health as a biomarker, as different ethnicities, or ages could have different accuracies in predicting systemic and ocular diseases.

Key words

Systemic health: The health of the body, not a singular organ. A systemic disease or disorder affects the whole body, such as high blood pressure, or the flu.

Photon: A bundle of electromagnetic radiation that is massless and forms ‘light waves’, or put more simply ‘particles of energy’.

Biomarker: ‘‘Characteristic that is objectively measured and evaluated as an indicator of normal biological processes.’’

Visual acuity: ‘‘Measure of the ability of the eye to distinguish shapes and the details of objects at a given distance.’

Multiple Sclerosis: chronic disease that damages the brain and spinal cord.

Synoptic Links GCSE - 4.5.2.3 The Eye The importance of the retina in vision, and how images are created through light absorption and translation into electrical signals through the optical nerve.

• 4.5.2.1 Structure and function of the human nervous system The parts of a neuron which increase the speed of transmission of an electric impulse (such as myelin sheath), which are present in the bipolar and ganglion cell. A-Level - 3.6.1.2 Receptors (A-level only) The layers of the retina, and looking and cone and rod cells in more depth, with respect to the different pigments of the photoreceptor cells that allow us to see colour and contrast.

Reference List

1. ScienceDaily. (n.d.). Retinal imaging and genetics data used to predict future disease risk. [online] Available at: https://www.sciencedaily.com/release s/2024/01/240124164518.htm
2. Zekavat, S.M., Saman Doroodgar Jorshery, Rauscher, F.G., Horn, K., Sayuri Sekimitsu, Koyama, S., Thi, T., Costanzo, M.C., Jang, D., Burtt, N.P., Kühnapfel, A., Yusrah Shweikh, Ye, Y., Raghu, V.K., Zhao, H., Ghassemi, M., Elze, T., Segrè, A.V., Wiggs, J.L. and Lucian Del Priore (2024). Phenomeand genome-wide analyses of retinal optical coherence tomography images identify links between ocular and systemic health. Science Translational Medicine, 16(731). doi:https://doi.org/10.1126/scitranslm ed.adg4517.
3. News-Medical (2010). What is a Biomarker? [online] News-Medical.net. Available at: https://www.newsmedical.net/health/What-is-aBiomarker.aspx. 4. Dotinga, R. (2024). Can the Eye Reveal Cardiac and Lung Disease -- Plus Much More? [online] Medpagetoday.com. Available at: https://www.medpagetoday.com/opht halmology/generalophthalmology/108 437