The condition is called “transient smartphone blindness” and has only been observed in women so far.
A team of eye experts from Moorfields Eye Hospital and several London universities have published an urgent warning about the illness in The New England Journal of Medicine.
They described the cases of two women who experienced temporary blindness in one eye.
A 22-year-old woman in England thought she was going blind in one eye. She could always see fine out of her left eye. But on some nights, the right eye failed her. All she could see out of it were vague shapes in the room.
At first, it happened about two or three times a week. Then it started happening every night.
When she went to the doctor, her vision appeared normal. So did brain scans. But it was a disturbing trend.
Around the same time, another woman noticed the same thing. On some mornings, she'd lose vision in just one eye for up to 20 minutes. It was bothersome enough to land her in the emergency room.
Vision loss in one eye can be a sign that a person is having a small stroke, which is why one of the women was put on blood thinners and the other got a brain scan. It can also signal a compressed optic nerve.
"They were looking at their smartphones and they just happened to have one eye covered because they were lying in bed," says Omar Mahroo, an ophthalmologist at Moorfields Eye Hospital in London and an author on the letter.
The first patient had a habit of gazing at her smartphone before falling asleep. She'd lie on her left side and look at the screen primarily with her right eye. Her left eye was often covered by the pillow.
The other patient, who was in her 40s, had similar problems when she woke up in the morning before sunrise and checked the news on her smartphone before sitting up. It had been going on for about a year, ever since she'd injured her cornea. But around the same time, she'd bought an iPhone.
In both cases, nothing bad was going on. Mahroo says. It's just that one retina was adapted to light, and the other to dark.
Anyone who lies on their side in a dark room whilst peeking at their gizmo could be clobbered with this very modern affliction.
"The retina is pretty amazing because it can adapt to lots of different light levels, probably better than any camera," he says. "It can reduce its sensitivity, so that when you're on the beach or in the bright snow you can still see relatively well," he says.
In the dark on a clear sky, it can adjust to detect extremely dim, far away points of light like stars in the sky.
Retinas constantly adjust when someone leaves a room and enters a slightly dimmer room, or goes inside after being outdoors. But these patients experienced a rare scenario in which that change would actually be noticeable.
Here's how it happens:
Light makes the pigment in rod-shaped cells change shape, which changes the electrical current running to another set of cells, which determines how much of a chemical those cells release to nerve cells, which eventually pass the message on to the brain.
After exposure to a bright light, it can take 40 minutes for that process to reset, after which a person can again see in the dark.
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