Eyes are often said to be "the windows to the soul". But little do people know that they are also the windows to your health.
Eyes are the only part of the body in which you can see blood vessels, and a wide range of diseases can be diagnosed through an eye examination.
"You can't see blood vessels through the skin, but you can see blood vessels through the eyes," says Natalie Young Shui-ming, managing optometrist at LensCrafters opticians.
"Every systemic disease related to blood can be seen through the eyes."
The most common of these are hypertension and diabetes, both of which are on the rise in Hong Kong.
"There are a whole load of systemic diseases that have eye manifestations," says Dr Marten Brelén, an assistant professor of ophthalmology and visual sciences at Chinese University.
"The best example is probably diabetes. We get all our diabetic patients to have photographs taken of the back of their eyes every year."
Eight to 10 per cent of thyroid eye disease cases are diagnosed through the eyes.
Important clues to the diagnosis of systemic diseases can be found on nearly every part of the eye, including the outer surface (eyelids, conjunctiva and cornea), middle and retina.
Young recommends eye checks every one to two years for early detection and treatment of such diseases.
"Most people don't have this concept of primary eye care in Hong Kong," she says. "They will not have an eye check unless they have symptoms."
The eyes are composed of many different types of tissue, making them susceptible to a variety of diseases.
The leading cause of blindness (23 per cent of all cases) in Hong Kong is glaucoma, when blood pressure builds up in the eye. Because there are no symptoms in the early stages of the disease, Young says many sufferers only go for an eye check after they start losing their sight.
Johnny Liyu Chun-kit, a senior optometrist at LensCrafters, says many patients wrongly believe that if they have good vision and do not feel pain, their eyes are fine. "There are no pain receptors in the eye," he says. "The eyes only receive light."
Brelén says people with myopia, or short-sightedness, and a family history of glaucoma are at higher risk of developing the disease, and recommends they visit an ophthalmologist once a year.
Myopia usually develops in early childhood, and has taken on epidemic proportions in the city. A 2012 study found that 70 per cent of Hong Kong Chinese children aged 12 or above were short-sighted.
"Schoolchildren are now having more homework, and need to read or do assignments with short reading distance requirements," says Dr Carly Lam Siu-yin, professor of optometry at Polytechnic University.
If they don't use good lighting to read, "students will be more likely to have a higher degree of myopia", she says. Mobile phones and hand-held game consoles further affect the quality of vision.
A survey completed by children and parents showed that 30 per cent of children had not had an eye check in the past two years.
Almost 40 per cent thought that eye checks were only needed when there were vision problems, more than 80 per cent were not aware of the importance of regular eye checks, and only 20 per cent knew about eye diseases related to high myopia - a prescription of more than -6.
People with short-sightedness are also more at risk of retinal detachment, and those with high myopia are 15 times more at risk than normal.
A 20-year-old woman with normal vision and no ocular health problems visited LensCrafters after she noticed floaters in her right eye. A fundus photo of her eye - a photograph of the interior surface - showed a retinal hole and liquefied vitreous leakage.
She was diagnosed with retinal detachment and immediately referred to accident and emergency. She received laser photocoagulation and her vision was saved.
Liyu says floaters and flashes in the eye are signs of retinal detachment, and anyone with these symptoms should see an optometrist or ophthalmologist immediately.
"If you are not treated within 48 hours, you could go blind," he says.
All eye diseases, by and large, become more prevalent the older you get, says Brelén, who specialises in the treatment of age-related macular degeneration, "an accumulation of wear and tear damage at the back of the eye".
He says a particular subtype of the disease is especially prevalent in Hong Kong and "usually requires very early and very aggressive treatment". He says everyone will develop cataracts.
Young recommends that people over the age of 40 go for checks once a year, but laments the lack of accessible eye care in Hong Kong. There are 2,000 optometrists in the city, but only 800 can offer primary eye care.
"Care of the eyes is more important than any other part of the body," Young says. Teeth can be replaced, but the eyes cannot, she says.
Here are some tips for taking care of your eyes:
Eat lots of lutein - found in green, leafy vegetables such as spinach and kale - and zeaxanthin - found in yellow vegetables such as yellow peppers and courgettes.
Blink while using computers and follow the 20-20-20 rule: every 20 minutes, look at something 20 feet (six metres) away for 20 seconds.
Wear dark glasses when in strong sunlight for prolonged periods of time.
Quit smoking. Smoke affects many parts of the eyes and plays a big role in eye allergies, dry eyes and problems with ocular surface.
Do not sleep, shower or swim wearing contact lenses.