People who see frightening hallucinations often think they are losing their minds, but they may actually be losing their vision, British doctors are warning.
The Royal College of Ophthalmologists in London has launched a media campaign warning people that phantom images are a normal consequence of going blind.
But a lack of awareness among doctors means many patients are misdiagnosed with dementia or psychosis.
The college said the condition is caused by Charles Bonnet Syndrome, which occurs when visual images are no longer channeled into the brain. Because of this, the brain goes on to replace real images with its own stored images.
"Patients often see grotesque, snarling, distorted faces looming up in front of them," Dr. Dominic Ffytche, a leading expert on the condition, told Sky News. "Figures wearing costumes often with elaborate hats, extended landscape scenes and sometimes even vehicles."
Rita Blakey, 76, suffered with terrifying night visions before learning of the syndrome.
"It was the middle of the night," she said. "Three men in black, dressed like undertakers, suddenly appeared in my bedroom. They were beckoning to me. I thought my days were up".
She suffered a nervous breakdown before eventually being told the cause of her hallucinations.
The Macular Disease Society receives 10 calls a week to its helpline from desperate patients.
"We know of several people who have gone through a period with a psychiatrist, trying to find out what it is in their mind causing these things to happen," the society's CEO Tom Bremridge said.