Retinal Age: A Novel Biomarker of Ageing

Globally, the number of adults aged 60 years and over is projected to more than double by 2050, increasing from 962 million in 2017 to 2.1 billion.¹ The shift in distribution of the population towards older ages inevitably leads to a substantial increase in morbidity and mortality.² The Australian Institute of Health and Welfare reported that 3.7 million Australians were aged 65 and over in 2016, but accounted for one-third of the health burden across the Australian population.³ These numbers are expected to grow even further, imposing an increasing burden on an already strained healthcare system. Healthy ageing is one of the priorities of the World Health Organisation’s work between 2015 and 2030.⁴ 

The retina shares similar embryological, anatomic and physiological characteristics with vital organs such as the brain and the heart

CHRONOLOGICAL AGE VS. BIOLOGICAL AGE

Chronological age refers to the number of years a person has been alive. Ageing is only loosely associated with chronological age, and the rate at which this occurs can range greatly between different people.⁵ Some 70-year-olds live healthy and energetic lives that are similar to those lived by 30-year-olds. Other 70-year-olds may require extensive support for daily basic activities like walking and eating. This suggests that a person’s chronological age may not fully account for the ageing process.⁵ 

Biological age is a measurement reflecting functional decline, which results from the accumulation of a wide range of molecular and cellular damage over time.⁶ Functional decline in physical and mental capacity leads to an increased risk of morbidity and mortality.^7,8 The concept of biological age is of significance, as it is a better indicator in quantifying the ageing process and assessing disease risk, and is more reliable and accurate than chronological age.

The relationship between chronological age and biological age characterises different ageing rates. When chronological age is equal to biological age, it represents a healthy ageing rate. Delayed ageing is characterised by a decelerated biological age when compared to chronological age, while faster ageing is characterised by an accelerated biological age when compared to chronological age. Figure 1 illustrates these three ageing trajectories.

In the context of the unprecedented growth rate of the world’s ageing population, there is a clear need for a simple and reliable quantification of the biological age to track the state of biophysiological ageing and implement tailored prevention strategies and interventions.

HOW BIOLOGICAL AGE IS DETERMINED

Extensive efforts have been made to develop methods to determine biological age during the past decades, such as molecular biomarkers, imaging biomarkers, and composite biomarkers. The epigenetic clock, based on DNA methylation status at CpG sites across the genome, is one of the most robust molecular ageing biomarkers with a high correlation with chronological age and a small deviation from chronological age.^10,11 Studies have shown that the epigenetic clock is associated with a wide spectrum of ageing phenotypes (e.g., cognition and physical function),¹² non-communicable age-related diseases (e.g., cancer),13 and mortality.¹⁰ Although quantification of the ageing process at the molecular level is promising and clinically relevant, it requires invasive procedures to collect biosamples and special facilities with appropriate laboratory equipment, which limits its clinical utility.

Recently, brain age based on neuroimaging has been developed and further validated as a reliable and valid aging biomarker with the prominent advantage of being non-invasive.¹⁴ The introduction of deep learning (DL), the state-of-the-art approach in image processing, has further improved its accuracy.^15,16 The clinical values of brain age have been validated in neurodegenerative diseases, such as dementia¹⁷ and Parkinson’s disease (PD).¹⁸ However, the cost and accessibility are two major barriers in the wide adoption of brain age.¹⁹ 

Compared to single biomarkers, composite biomarkers that combine multiple clinical measures are more informative by reflecting diverse aspects of ageing.²⁰ Frailty index and Klemera-Doubal method-biological age are two commonly used composite biomarkers, with great potentials in predicting agerelated outcomes.^21,22 Nevertheless, assessing a panel of biomarkers across different systems is complicated and time-consuming, and therefore may not be practical for largescale population screening.

THE RETINA: A WINDOW TO THE BODY

The retina shares similar embryological, anatomical and physiological characteristics with vital organs such as the brain and the heart.^23,24 Originating from the diencephalon, the retina is an extension of the central nervous system (CNS) via the optic nerve, which is composed of axons of retinal ganglion cells (RGC).²⁴ RGCs are characterised by the typical features of CNS neurons, with a cell body, dendrites and an axon. In terms of the microvasculature, the retinal microvessels share common features with the vasculature of the brain and the heart. RGCs can be measured using optical coherence tomography (OCT) and retinal microvessels can be visualised and quantified using colour fundus photography. The easily accessible neurons and vessels of the eye are, therefore, a window to study subclinical pathology in the brain and the heart.

There is growing evidence of the association between retinal changes and neurodegenerative diseases.²⁵ Neurodegeneration in the retina, characterised by the thinning of retinal nerve fibre layer, has been observed in patients with neurodegenerative diseases.²⁶ The retinal vasculature alterations, including venular dilation, arteriolar narrowing, vascular fractal dimension reduction, the loss of vessel density and perfusion density, may be potential biomarkers for early detection of neurodegenerative diseases.^27-32 Similarly, recent studies have sought to identify, quantify and link retinal microvascular changes with cardiovascular and cerebrovascular diseases. A growing body of evidence has indicated that retinal vessel calibre (e.g., artery narrowing and venules dilation) and qualitative retinal signs (e.g., microaneurysms and retinal haemorrhage) are associated with cardiovascular and cerebrovascular diseases.^33-37 

RETINAL AGE – A NOVEL BIOMARKER FOR AGEING

A growing body of evidence has shown that the retina is a window to the brain and the heart.^38-40 Throughout the aging process, the retina is expected to undergo changes in molecular metabolism, cytology and morphology that are similar to changes to the brain and heart.

The neurological and vascular aspects of ageing are particularly important to general health. A reliable and valid retinabased biomarker of ageing may have the potential to identify individuals deviating from a healthy neural and vascular ageing trajectory. Furthermore, the retina is amenable to rapid, non-invasive and costeffective assessments. These features of the retina sparked the interest of our team in exploring the potential of using retinal imaging as an accurate and clinically significant ageing biomarker.

To realise the full potential of retinal imaging in the study of ageing, our interdisciplinary team from Centre for Eye Research Australia, University of Melbourne and University of Monash proposed the concept of retinal age (i.e., biological age based on retinal images). Currently, there is no gold standard to determine biological age. Consistent with previous studies,^23,24,41 chronological age was equal to the biological age in relatively healthy individuals. Therefore, the DL model to predict retinal age was trained and validated using over 19,000 retinal images from individuals who were deemed disease free. Results showed that the predicted retinal age, based on the retinal images, correlated closely with the chronological age, with a correlation index of 0.81 (the closer the index is to one, the stronger the positive correlation). Additionally, the predicted retinal age achieved a high accuracy, with a mean average deviation of 3.55 years from the chronological age (also known as mean absolute error [MAE], where a lower value is better). The retinal age achieved excellent accuracy, which outperformed most well-established ageing biomarkers. When comparing the accuracy of other methods that measure biological age using MAE, the epigenetic clock has achieved accuracy of 3.3-5.2 years,^42,43 blood measures of 5.5-5.9 years,^44,45 transcriptome ageing clock of 6.2-7.8 years,^46,47 brain age of 4.3-7.3 years,^48,49 and three-dimensional facial age of 2.8-6.4 years.^50,51 In addition to better accuracy in the prediction of age, the fast, simple, cost-effective, and user-friendly features makes retinal ageing more accessible, especially in remote and rural areas.

CLINICAL VALUE OF RETINAL AGE

Beyond good performance in age prediction, a clinically relevant ageing biomarker needs to be closely related to the risk of age-related morbidity and mortality. To further explore the clinical value of the retinal age, we estimated the retinal age gap – the difference between the predicted retinal age using retinal images and the chronological age – in a largescale cohort of more than 35,000 middleaged and older adults. A positive retinal age gap indicated an ‘older’ appearing retina compared to the chronological age, while a negative one indicated a ‘younger’ appearing retina compared to the chronological age. Following 11 years of follow-up, participants were linked to the death registry to capture vital status and, if available, date of death. We evaluated the risk of mortality for those who had ‘older’ or ‘younger’ appearing retinas. Our findings showed that for each one-year increase in retinal age gap there was an associated 2% increase in mortality risk. In addition, we found a J-shaped relationship between retinal age gap and the risk of mortality, where the ‘older’ appearing retina was associated with substantially increased risk of mortality (Figure 2).

VISION FOR THE FUTURE

More Diverse Data 

The current DL model was based on a UK population, which is mainly Caucasian. To improve the generalisability, retinal images from different ethnic populations, from different facilities, and with different field of views, are needed to refine and optimise this model.

Black Box in AI 

Even though we performed attention maps to track the contributions of every pixel from fundus images and found that the areas around retinal vessels were highlighted in the prediction of age, the black box in the DL model may raise concerns in real-world applications.⁵³ Recent efforts have been made to uncover the invisible decision-making process to make AI techniques more transparent and interpretable,^54,55 thus providing the opportunity for a future study to specify the mechanism underlying ageing.

Clinical Validation 

Clinical validation of retinal age gap in the prediction of mortality has been confirmed by our multidisciplinary research group. Further clinical validation in specific agerelated diseases, such as cardiovascular diseases and neurodegenerative diseases, will add further value in the application of the retinal age gap in the future.

Influencing Factors 

Investigations on protective and risk factors influencing retinal age, especially modifiable factors, raise the possibility of delaying ageing at an individual level. For example, environmental factors, lifestyle changes, and pharmacological interventions can change the trajectory of ageing.

Clinical Application 

The simple, fast, non-invasive and costeffective features of retinal imaging enable retinal age to be an accessible screening tool for the identification of people at high risk of accelerated ageing. The recent development of smartphone-based retinal cameras has further corroborated potentials for large-scale population-based screening. For those fast agers with ‘older’ appearing retinas, individualised and tailored interventions could be proposed, such as lifestyle changes (e.g., smoking cessation, physical activity, healthy sleep, and dietary changes), and intensive management of chronic diseases (e.g., blood glucose control, and weight control).

CONCLUSION 

We are the first group to propose the concept of retinal age as a novel biomarker of ageing. The accuracy and clinical utility to predict the risk of mortality for retinal age has been verified. There is more to explore in this field, such as refinement and validation of the current DL model, identification of factors influencing retinal age, uncovering the black box of the DL model, and further investigation into the clinical validation of specific age-related diseases.

In the context of population ageing globally, the concept of retinal age may help to achieve accurate risk stratification and assist personalised prevention and interventions, ultimately leading to tremendous public health benefits.

Dr Lisa Zhuoting Zhu is a research fellow in the Ophthalmic Epidemiology Department at the Centre for Eye Research Australia. Her work has focused on how images of the eye can be used to view a person’s overall health. She has presented at national and international conferences, published over 45 manuscripts and attracted more than 220 citations. Her research has been supported by a number of grants, including a NHMRC Investigator Grant, and been awarded multiple patents. 

Dr Alice Ruiye Chen is a PhD student in the Ophthalmic Epidemiology Department at the Centre for Eye Research Australia. Her PhD project focuses on the concept of retinal age. She is a certified medical practitioner in China with a background in the field of medical imaging. She is a recipient of the Melbourne Research Scholarship from the University of Melbourne. 

Professor Mingguang He is the Professor of Ophthalmic Epidemiology at the University of Melbourne. He undertook his medical training in China and holds a Master of Public Health degree from Johns Hopkins University in Baltimore, and a PhD in ophthalmology at Moorfields Eye Hospital in London. 

Professor He is a global expert in vision-related clinical and epidemiologic research. He has led some important epidemiological studies and clinical trials, including the first population-based study on myopia in China, the first population-based study on glaucoma in China, a clinical trial to prove the efficacy of increased outdoor time on myopia prevention published in JAMA (2015) and a prophylactic clinical trial on angle closure glaucoma published in Lancet (2019). His publications have attracted more than 13,000 citations. 

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