Charles Bonnet syndrome

How it works

Charles Bonnet syndrome is the name for visual hallucinations in people with poor sight and no mental illness¹. It usually appears in older adults with macular degeneration, glaucoma, cataracts, or diabetic eye disease. Anything that cuts the amount of visual signal reaching the brain can cause it¹.

The mechanism is called deafferentation. The visual cortex is built to process a constant stream of input. When that input drops, the cortex does not shut down. The cells become more excitable and start to fire on their own¹. The brain then presents that spontaneous activity as images. The same thing happens in people kept in the dark, and in the ear it produces musical ear syndrome.

The images range from simple to elaborate. Simple ones are lines, grids, and coloured patterns. Complex ones include faces, animals, buildings, and groups of small figures, often in old-fashioned or fancy dress¹. The images are silent. They can last seconds or hours and usually appear when the person is calm and the light is dim¹.

A key feature is insight. Most people with the syndrome know the images are not real, even when they look solid¹. Charles Bonnet, a Swiss naturalist, first described the condition in 1760 in his grandfather, who had cataracts and saw people, birds, and carriages that were not there².

Is it a problem?

The hallucinations themselves are harmless. They are a side effect of sight loss, not a mental illness, and not a sign of dementia¹. Many people with the syndrome hide it for fear of being thought mad. Studies find that a large share of them never mention it to a doctor¹.

Knowing the cause is usually enough to make it bearable. The images often become less frequent over one or two years. Some people can make an image go away by blinking, moving their eyes, or changing the lighting¹.

See a doctor if hallucinations come with confusion, memory loss, or hearing things, or if you are not sure the images are unreal. Those are not the pattern of this syndrome.