Charles Bonnet syndrome

Also called CBS, Visual hallucinations with sight loss

Vivid hallucinations in people losing their sight, with a completely healthy mind. Almost nobody is warned, so most think they are developing dementia.

What it is

Complex visual hallucinations experienced by people with significant sight loss, in the absence of any mental illness.

The mechanism is straightforward once explained: when the eyes stop sending signals, the visual part of the brain does not go quiet, and it generates images of its own. It is the visual equivalent of phantom limb pain.

It is common, affecting perhaps one in five people with substantial sight loss, and it is dramatically under-diagnosed. The reason is that people do not report it. They assume hallucinations mean they are going mad or developing dementia, and they say nothing, sometimes for years.

Being told what it is, in advance, is close to the entire treatment. People who understand it typically find it manageable or even interesting; people who do not can become severely distressed, and it is a recognised cause of depression and, occasionally, of being wrongly prescribed antipsychotics.

The hallucinations are purely visual. Hearing voices, smelling things or feeling touched is not this, and points elsewhere.

It commonly follows macular degeneration, glaucoma, cataracts, diabetic retinopathy or stroke affecting vision.

Signs you might notice

Seeing things that are not there, in someone with poor sight: patterns and lattices, faces, figures often in period costume, small people, animals, landscapes, or text that cannot be read.

The images are silent, and the person knows they are not real, or works it out quickly.

They may last seconds or hours, and are often more frequent in poor light or when doing nothing.

No voices, smells or physical sensations.

No confusion, no memory problems, and no change in the person otherwise.

Different, and needing assessment: hallucinations with confusion, drowsiness or fluctuating alertness (delirium or Lewy body dementia); hallucinations in other senses (psychosis); or a sudden change in vision itself.

How it can affect day-to-day life

The silence is the problem. People conceal it from family and clinicians because of what they fear it means, and then live with something frightening entirely alone.

Once explained, most people cope well. Knowing that other people have it, that it does not mean dementia, and that it usually settles over months to years is what changes the experience.

Poor light, tiredness, isolation and inactivity make episodes more likely, which gives some practical levers.

Anybody supporting a person with new sight loss should raise this proactively, before it happens. That is unusual advice in this library and it is the right advice here.

Supporting someone well

Tell people about it in advance, at the point sight loss is diagnosed. This is the intervention.

If it has started, say plainly: this is a known consequence of sight loss, the mind is fine, and it is not dementia.

Ask about it directly in anybody with sight loss. Most will not volunteer it.

Practical tricks that help during an episode: put a light on, move the eyes side to side, blink, change activity, get up and move to another room.

Keep rooms well lit and keep the person occupied and in company; boredom and dim light make it worse.

Get the eyes reviewed; any treatable cause of sight loss is worth treating for its own sake.

Treat resulting low mood, and resist antipsychotic medication, which does not help and carries real risks in older people.

Where to get help

An optometrist or GP practice, and the eye clinic. Ask about an eye clinic liaison officer, who is often the best-informed person about this.

RNIB helpline: 0303 123 9999, and Macular Society on 0300 3030 111, both of whom know this condition well.

Fight for Sight and the sight-loss charities campaign specifically on awareness of it.