The eye resembles a camera that detects, captures and transmits images to our brain.
The cornea, a transparent layer covering the front of the eyeball, allows light through.
When it reaches the lens, which has the camera-like ability to 'autofocus', the light is bent to create sharp images on the retina, a light-sensitive layer at the back of the eye.
These images are then transmitted through the optic nerve to the brain, enabling us to see.
Hence, when one or more of these components malfunction, our vision becomes poorer. In some cases, we may even go blind.
Mind Your Body looks at four serious eye diseases that have the potential to cause blindness.
1: CATARACT
Simply put, cataract is clouding of the lens, said Dr Cheryl Lee, a consultant ophthalmic surgeon at Pacific Healthcare Specialist Centre.
Vision becomes blurred, akin to 'looking through a fog', said Dr Lee.
'Cataracts are commonly due to chemical changes in the lens that are related to ageing,' said Dr Leonard Yip, a consultant ophthalmologist at Tan Tock Seng Hospital.
Other than blurred vision, sufferers may experience glare and loss of colour intensity, he added.
Although cataracts are the second most common cause of blindness, this loss of sight can be restored by removing the clouded lens and implanting artificial ones.
At the Singapore National Eye Centre (SNEC), cataract surgery is the most common operation, with over 10,000 done per year, said Professor Donald Tan, the centre's medical director. Success rate is very high - close to a 100 per cent.
2: GLAUCOMA
Here, the optic nerve is damaged due to high eye pressure, which may also injure blood vessels.
Glaucoma is the top cause of blindness in Singapore, making up 34 per cent of all blindness cases, said Dr Yip. There are two kinds - the slow, progressive type and the acute form.
The former, known as open-angle glaucoma, is where peripheral vision gradually gets 'eaten away'. However, the person may notice changes in vision only at the very late stages. By then, 80 per cent of the optic nerve might have been damaged, said Prof Tan.
'It's like having high blood pressure - you don't know you have it until you get a stroke,' he said.
If detected early, further vision loss can be prevented by the use of eye drops, laser treatment and surgery if necessary, said Dr Yip.
Most glaucoma sufferers here are believed to have the open-angle variant. In Singapore, about 3 per cent of those over the age of 50 have glaucoma.
The acute form, or closed-angle glaucoma, is more dramatic. However, only about 300 to 400 Singaporeans are affected by this acute form annually, said Dr Yip.
In this case, the drainage system in the eye suddenly closes or becomes blocked, causing eye pressure to shoot up, said Prof Tan. Sufferers tend to experience sudden blindness and acute eye pain.
Prof Tan said the acute form is especially prevalent in East Asians, particularly among the Chinese. It also tends to affect the elderly more.
The reason may be due to East Asians' narrower eye angle, which increases the likelihood of blockage in the eye's drainage system.
3: AGE-RELATED MACULAR DEGENERATION (AMD)
There are two types of AMD - dry and wet. Dry AMD progresses slowly, over months or even years, and is more common, accounting for about 90 per cent of all cases. In wet AMD, the deterioration is swift and one can go blind overnight, said Dr Lee.
Because the macula is responsible for clear central vision, sufferers may see distortion or dark splotches in the middle but details in the periphery appear sharp, she said.
The third most common cause of blindness here, it results from the breakdown of light receptors in the macular, or from the formation of abnormal blood vessels that damage these receptors, said Dr Yip.
Treatment options include laser treatment and injections into the eye.
4: DIABETIC RETINOPATHY
In diabetic retinopathy, the retina is damaged, usually through bleeding, which may then cause blindness, said Prof Tan.
Because the disease progresses slowly, regular screening is crucial.
If detected early, laser treatment can be administered to induce abnormal blood vessels to regress and prevent the condition from worsening.
Late detection, however, may require complex surgery to remove blood and affected membranes.
However, good control of blood sugar levels is perhaps the most important preventive measure as the risk of getting this disease hinges on how well one's diabetes is controlled, said Prof Tan.
