Charles Bonnet syndrome

Someone whose sight has failed badly suddenly starts seeing things that are not there: a pattern on a blank wall, a branch stretching across the living room, an unfamiliar face, a small child standing in the corner. They know perfectly well that nothing is there, and that is exactly why most of them say nothing at all: they are afraid of being taken for senile and sent to the wrong kind of doctor. Yet what is happening has been described and explained for a long time. It is how the brain responds when the stream of pictures from the eye dries up. The condition is named after the Swiss naturalist who, in the eighteenth century, wrote down what his grandfather told him as he was going blind, and it has nothing to do with mental illness or with dementia.

What people actually see

The images vary a great deal, yet the range is surprisingly narrow:

• simple repeating patterns: grids, checks, brickwork, lines, honeycomb. These are the commonest and the most harmless;
• objects, animals, trees, buildings, whole landscapes that are not in the room;
• people, often in old-fashioned clothes and hats, sometimes tiny, sometimes with distorted features such as oversized eyes or teeth;
• the picture may be black and white or unnaturally vivid, still or drifting, and it can block part of the visual field or lie on top of the real surroundings.

An episode starts without warning and lasts anything from a few seconds to a few hours. It comes most often at rest, in dim light, in silence, when the person is alone, and when they are tired. Three features identify it almost unmistakably. The visions are purely visual: they do not speak, they have no smell, they cannot be touched, and no voices come with them. They demand nothing and are not addressed to the person seeing them. And insight is preserved: the person knows this is not reality, however convincing it looks.

Why the brain makes pictures up

The easiest comparison is phantom limb pain. After an amputation the missing arm is still felt: the part of the brain that used to serve it has lost its input and begins generating activity of its own. Vision works the same way. The visual cortex is used to processing a continuous stream of images; when the eye stops delivering that stream, the cortex fills the gap with ready-made pictures from its own stock. The brain is healthy and the eye is the diseased part. The image is born exactly where sight ends.

That explains who tends to get it. Almost always someone who has lost a noticeable amount of vision: age-related macular degeneration, cataract, glaucoma, diabetic damage to the retina, the after-effects of a stroke in the occipital lobe. The risk is higher when sight has dropped quickly and when both eyes are affected. It is not confined to older people either: it has been described in children and in young adults after an eye injury. Among people who see poorly it is far from rare. What is rare is talking about it, which is why everyone who has it assumes they are the only one.

What to do when an image appears

There is no drug designed for this, but an episode can often be interrupted. What works is anything that changes the working conditions of the visual cortex:

• move the eyes quickly from side to side without turning the head: fifteen to thirty times, pause, then repeat;
• change the lighting: switch a lamp on if the room was dark, or dim it if the light was harsh;
• stand up, walk about, move to another room, put the radio on or start a conversation, giving the brain a different stream of input;
• look at something else, blink hard, reach a hand out towards where the object appears to be;
• say out loud that this is the brain at work and not something happening in the room. That alone removes most of the fear.

Episodes come less often with enough sleep and without pushing on to exhaustion: tiredness, anxiety and isolation are their most reliable companions. At home it helps to brighten the lighting, use a magnifier and high-contrast markers, and read large print. The harder the remaining sight works, the fewer reasons the cortex has to invent. It is also worth checking carefully whether any vision can be restored: cataracts can be operated on, glasses can be updated, swelling of the retina can be treated, and when sight improves the visions usually fade with it. Medication has been tried, but it is only discussed when the visions are genuinely distressing: in older people the side effects of such drugs are often worse than the problem itself.

When it is not Charles Bonnet syndrome

This deserves proper thought, because several very different conditions can produce a similar complaint. The following should raise concern:

• the person does not only see things but also hears voices, smells odours, feels touch or senses someone present nearby;
• they believe the vision is real: they talk to it, argue with it, act on what it tells them;
• the visions appeared on their own, with no preceding loss of sight;
• alongside them, forgetfulness is increasing, alertness swings from day to day, and stiffness and tremor have appeared: visual hallucinations can be an early sign of dementia with Lewy bodies;
• behaviour has changed within a few days, the person is muddled about time and place and has become drowsy or, conversely, agitated: that is what confusion looks like when caused by an infection, dehydration or a new medicine;
• the episodes are short, identical to one another, a few seconds long, always the same flash or coloured circle in the same half of the visual field: this may be a seizure arising in the occipital lobe;
• a shimmering zigzag spreads across the field of vision over about twenty minutes and is followed by a headache: that is migraine aura;
• it all began soon after a new drug was started, or after sleeping tablets or alcohol were stopped abruptly.

Signs that cannot wait

The vision itself is not dangerous. What can be dangerous is the condition it appeared against. Seek help immediately, without waiting for morning, if:

• sight has dropped abruptly over hours or days, in one eye or both, or a curtain, a shadow at the side or a missing patch of the visual field has appeared;
• the person is over fifty and, along with the failing vision, has a new headache in the temples, a tender scalp when combing the hair, or aching jaw muscles while chewing. This may be giant cell arteritis: an inflammation of the arteries in which the clock runs in hours, and without treatment the second eye is lost as well;
• the eye is red and severely painful, there are coloured haloes around lights and there is nausea: an acute attack of glaucoma;
• an arm or leg has suddenly become weak, the face has drooped, or speaking or understanding speech has become difficult;
• the worst headache of a lifetime has come on within seconds;
• there is a high fever together with confusion.

With any of these, nobody waits until morning: call the emergency number for the country you are in (112 across Europe) or go straight to an emergency department.

What to expect over time

The outlook is reassuring. In most people the visions gradually weaken, come less often, and in many they disappear within a year or eighteen months after sight stops changing. In some they stay for much longer but lose their power to frighten: the person recognises them within a second and simply waits them out. What matters most here is not treatment but explanation. Those who are told in good time what is happening live with it calmly.

That is also why it is worth telling the family about the syndrome. Otherwise a casual “there is a man sitting over there” said out loud one day will frighten everyone, and the conversation will turn to psychiatry instead of ophthalmology. The main point is worth keeping in mind: seeing what is not there when sight is poor is not a sign of madness but the expected behaviour of a healthy brain left without a picture.

Online consultation

A remote appointment is well suited to the very question people sit on for years: whether what they are describing fits Charles Bonnet syndrome or points to something else. The doctor will ask what the visions look like, whether anything is heard at the same time, whether the sense that they are unreal is preserved, how and when the sight declined, and what medicines are being taken. That set of answers alone clears up a great deal. From there it is easy to ask who to see first, whether the eyes need another examination, and whether there is any prospect of getting some vision back. The warning signs listed above are not assessed on a screen: with those, in-person care is needed straight away.

This material is for information only and does not replace medical advice.