Shortsightedness in children is soaring due to the Covid-19 pandemic forcing them to spend longer on their screens, warns a new study.
The switch to remote learning - and spending less time outdoors - is to blame, say scientists.
Cases have almost doubled since the Covid-19 outbreak - and the study warns that the phenomenon is a "potential public health crisis."
It raises the risk of blindness - decades later. The finding is based on almost 1,800 six- to eight year-olds in Hong Kong. A similar trend has been identified in the UK and China.
Excessive computer use can cause the condition - known medically as myopia. Daylight helps reduce the risk.
Lead author Dr Jason Yam said: "Our initial results show an alarming myopia progression that warrants appropriate remedial action."
They serve to warn eye care professionals, policy makers, educators and parents that collective efforts are needed to prevent childhood myopia, a potential public health crisis as a result of Covid-19."
He added: "Short-sightedness in children matters.
"It puts them at risk of developing complications that increase the risk of irreversible impaired eyesight or blindness later in life."
Children have been particularly badly affected by lockdown - with restrictions or bans on leaving the home and socialising severely limited.
By last September more than 180 countries had closed schools and colleges to slow the spread of the coronavirus.
It affected one billion young people - eight-in-ten of the world's pupils and students.
Dr Yam, of The Chinese University of Hong Kong, said: "An increase in close working and screen time and a decrease in time spent outdoors have been implicated in short-sightedness.
"The shape of the eye changes, causing light rays to bend, or refract, incorrectly - focusing images in front of the retina instead of on the surface."
Analysis of data from the Hong Kong Children Eye Study enabled the team to compare those who joined before or after the pandemic.
The numbers newly diagnosed with short-sightedness were much higher among the latter group.
Estimated one year incidence was 28, 27 and 26 percent, respectively, for six, seven and eight year olds - dropping to 17, 16 and 15 in the former.
The changes coincided with an increase in screen time from around 2.5 to seven hours-a-day.
Time spent outdoors also fell from a daily average of an hour and 15 minutes to 24 minutes.
It suggests the enforced behavioural and lifestyle changes has significantly affected vision, said Dr Yam.
The researchers also took into account other factors including age, gender, length of monitoring period and parental short-sightedness.
Dr Yam said: "Another alarming finding from our report is the significant changes in children's lifestyle during the Covid-19 pandemic, with 68% decreased outdoor time and 2.8 fold increased screen time.
"Evidence suggests that when children are out of school, they are physically less active and have much longer screen time.
"Of all the environmental risk factors that have been studied, increased outdoor time has been consistently shown to have a protective role against the development of myopia."
Hong Kong is one of the most densely populated cities in the world with residents living in high-rises with little outdoor space.
Some 709 of the children were recruited at the start of the pandemic between December 2019 to January 2020 and monitored for 8 months.
The other 1,084 entered before it started and were tracked for about three years.
Visual acuity, or sharpness, was measured and they filled in regular lifestyle questionnaires - including time spent outdoors and on close work.
Added Dr Yam: "We showed a potential increase in myopia incidence, significant decrease in outdoor time and increase in screen time among schoolchildren in Hong Kong during the COVID-19 pandemic."
Earlier this year a survey found almost four-in-ten Britons believe their eyesight has worsened during the pandemic.
Eye health charity Fight for Sight advises people to learn the "20-20-20" rule - looking at something 20 feet away for 20 seconds every 20 minutes you look at a screen.
Chief executive Sherine Krause said: "More than half of all cases of sight loss are avoidable through early detection and prevention methods.
"Regular eye tests can often detect symptomless sight-threatening conditions."
In January a study of 120,000 Chinese schoolchildren found a threefold rise in shortsightedness among six- to eight-year-olds in 2020.
Scientists believe it was caused by being confined to their home with schoolwork delivered online between January and May.
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