User:Doctor Cornea/Eye aging

Eye aging is the natural changes that occur in the eyes as people get older. There are highly specialised tissues which are spread throughout the eyes. Refractive surfaces like the cornea and the lens help refract light rays to the retina,  which is responsible for detecting the light to give vision. The cornea, the ciliary body, the retina, and the connective tissues within the optic nerve undergo physiological changes as people age. These changes may lead to malfunction of tissues and affect vision. The prevalence and specific causes of vision loss differed across regions, but generally, the prevalence of vision impairment increases with age. There are a range of eye diseases related to eye aging. Eye aging can be delayed in numerous ways. The future research in eye aging can develop more strategies to promote healthy aging of the eye.

Function and anatomy of the eye
Highly specialised tissues are spread throughout the eyes. They are fundamental to maintaining the complex vision-generating process machinery. Light rays entering the front eyes are focused by two major refractive surfaces, which are the cornea and the lens. The cornea contributes about two thirds of the refractive power while the lens contributes the remaining1. Then, the light rays would pass through the vitreous humor reaching the retina, where densely populated first light-sensitive neurons (rod and cone photoreceptors) are located. (fig.1)

The retina located at the back of the eye is an organised structure and has an indispensable role in the transduction and transmission of signals. As the photoreceptors are stimulated by lights, neural signals would be generated and passed through the second neurons (bipolar cells) and tertiary neurons (retinal ganglion cells)2. Eventually, the signals would be coalesced into the optic nerve to be sent to the visual cortex of the brain for interpretation.

Physiological changes and causes
Physiological change in all ocular tissues would occur along with age. The progressive change of structure would devastatingly lower its efficiency, or even lead to the malfunction of tissues.

Cornea, which is a power refractive surface, would flatten and there would be an attrition of endothelial cells. The corneal endothelial cells are responsible for maintaining the cornea’s structural integrity3.undefined However, with aging, there is a decrease in corneal endothelial cell density. Approximately 2000 cells/mm2 corneal endothelial cell density would be decreased from birth to older age4. Not only the cornea, the shape of the trabecular meshwork also alters. Research tracking specimens from newborns to 81 years old has found that there would be a progressive decrease in trabecular endothelial cell density and absolute cell number caused by aging5. This phenomenon would then lead to the change in conformation of the trabecular meshwork. The configuration transforms from a wedge shape into a shorter and rhomboidal form6.

Moreover, the ciliary body would become collagenized as time passes. Ciliary body contains ciliary muscles that could change the thickness and curvature of the lens. As the cellularity of the ciliary body diminishes with age, they appear to be shorter and fail to complete its function7. In addition, the aging of the crystalline lens would also occur. Its shape would be changed from a reinform configuration to a more oval shape. Apart from the stiffening of lens with advancing age, the crystallins protein in the lens gradually lose its transparency. This could be due to the increasing oxidation stress along with age8.

Due to the function of the retina, the cells has been continuously exposed to light, leaving them vulnerable to damage caused by light. In addition to the reduction of nuclei in the outer nuclear layer of the retina may occur with age9. Various problems, like the decline in visual acuity and color perception threshold, are inevitably arising with age10. Research has also shown that rods would be degenerated before cones in the macula when time passes. Therefore, rod-mediated (scotopic) sensitivity has a larger decline then then cone-mediated (photopic) sensitivity. The dysfunction of rod would then slow down the dark adaptation11.

Furthermore, connective tissues within the optic nerve would become more abundant with age, affecting the exchange of materials between the capillaries and nerve fibres. This results in the accumulation of cellular and extracellular materials in the optic nerve fiber bundles12. Such accumulation would inhibit and the transmission of nerve signals to the brain, hindering the generation of vision.

Prevalence
The prevalence of vision impairment increases with age. Through tracking the change in distribution of vision impairment population from 1999 to 2019 in China, people with moderate vision impairment increased by 67%, while 14% increase in those with severe vision impairment and 35% increase in those with blindness13.

Related diseases
Age-related eye diseases affect the eyes as people get older. These conditions can lead to loss of vision and may require medical treatment. Common age-related eye diseases include age-related macular degeneration (AMD), cataracts, glaucoma and dry eye syndrome.

Age-related macular degeneration (AMD) is an eye disorder in which the macula, a part of the retina responsible for central vision, progressively deteriorates. This can result in a loss of central vision, making it difficult for people to read, see fine details, and recognize faces. There are two major types of AMD: dry AMD, which progresses relatively slower, and wet AMD, which can lead to rapid vision loss14.

Cataracts is another common age-related eye disease that occurs when the lens of the eye becomes opaque or cloudy. This can cause the vision to become hazy, blurry, or distorted. Cataracts can develop gradually over time and may require surgical treatment15.

Glaucoma is a common eye condition where the optic nerve is damaged16. The optic nerve is responsible for transmitting visual information from the eye to the brain. In the early stages, glaucoma may not show noticeable symptoms. However, as the condition progresses, it can cause loss of vision, usually starting with peripheral vision, and possibly leading to total blindness if the condition is left untreated17.

Dry eye syndrome is an eye condition in which the eyes cannot produce enough tears, or when the tears produced evaporate too quickly18. This can lead to discomfort in eyes, including itching, redness, and sensitivity to light. It can also result in blurred or distorted vision19. Common treatments for dry eye syndrome include eye drops, change in lifestyle and other therapies.

Early detection and treatment of these age-related eye diseases are important to preserve vision and eye health. It is recommended to have regular eye exams to monitor for these conditions and address any potential eye concerns.

Delay and prevention
Healthy lifestyle habits, which include having a balanced diet, having regular exercise and avoiding smoking can help delay eye aging. These habit also help prevent health conditions such as diabetes and high blood pressure, which may contribute to eye diseases.

Protecting the eyes from harmful ultraviolet (UV) radiation can help delay the aging of eyes. By wearing sunglasses that block out UV radiation, we can protect the eyes from the damage caused by the harmful rays from the sun. Wearing protective eyewear when taking part in activities which may expose the eyes to strong lights can also help preserve the eyes.

Regular eye exams are effective in detecting any change in the eyes early on. They are essential in maintaining eye health. The exams can identify the various age-related eye diseases before their progression and the resulting significant loss of vision20, 21.

Eye exercise and relaxation techniques can help delay eye aging. The exercise improves the strength and flexibility of eye muscle, reduces eye strain and fatigue, and improves visual acuity (the ability of the eye to distinguish the shapes and the details of objects).

Research and future development
Future research directions in the field of eye aging include the development of new diagnostic tools and technology for early detection of age-related eye diseases, the development of new drugs, and the research in novel therapies, such as gene therapy and stem cell therapy22.

Research can also focus on the impact of different lifestyle factors, such as diets and exercise, on the condition of eye aging, to develop more strategies for promoting healthy aging of the eye23.