If you've ever wondered why you shouldn't direct a laser pointer at your eyes, let a 9-year-old boy in Greece be your cautionary tale: by repeatedly staring into a green laser pointer he was playing with, he managed to burn a hole in his retina.

And the damage may be permanent.

The boy's parents took him to the ophthalmology clinic when he started complaining of difficulty seeing out of the left eye. His doctors assessed his visual acuity - he still had 20/20 vision in his right eye, but his left eye had dropped to 20/100.

When the doctors examined the boy, they found a large hole in the macula - the central area of the retina, where light is focussed from the front of the eye. It's responsible for colour vision and visual acuity.

Macular holes are not uncommon in the elderly, so the effects are well known - they affect the centre of your vision, or what is right in front of your eyes. Not far from the hole, the doctors found two additional spots of further injury.

Retinal injuries from laser pointers are actually shockingly common - so much so that there are diagnostic criteria for helping to determine when eye injuries have been caused by the devices.

And it doesn't take long to cause damage, either: just one minute of exposure to a 5mW laser pointer caused significant damage to a 13-year-old boy's eyes, recorded in a 2012 case report.

His eyes healed, but another 13-year-old boy, according to a 2015 case report, was not so lucky. His brother had ordered a 50mW laser pointer from the internet, and shone it into the patient's eyes for one second - causing permanent eye damage.

In the US, there's a 5mW power limit on laser pointers, regulated by the FDA. In Britain and Australia, regulations are even more stringent, limiting the devices to just 1mW. To use anything stronger, you need a permit.

But that doesn't stop consumers from purchasing higher powered laser pointers from the internet.

The doctors didn't specify the strength of the laser pointer that damaged the boy's eye, but it's possible they didn't know it; his father had purchased it from a street vendor, Fox News reports, and it could have been mislabelled.

The hole in the boy's macula, the doctors said, was too large to be treated with surgery, which is how it is often treated in elderly patients, and they recommended conservative management instead. But it seems like the damage has been done.

"The patient's vision has remained unchanged during 18 months of follow-up," the doctors wrote.

The case report has been published in The New England Journal of Medicine.