Charles Bonnet Syndrome

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Charles Bonnet syndrome is a common but often surprising condition experienced by people who are losing, or have lost, their sight. It involves seeing things which are not really there (having visual hallucinations). The hallucinations are most marked in low light or when relaxing and are often complicated scenes involving faces, children and wild animals.

Charles Bonnet was an eighteenth century swiss lawyer and philosopher who studied natural science as a hobby. He first described visual hallucinations in a patient who did not have dementia or a mental illness. That patient was his grandfather, who had lost vision due to cataracts and who saw people, birds, carriages, buildings and tapestries.

CBS involves visual hallucinations. Having a visual hallucination means experiencing something which is not really there. This can be frightening or distressing, particularly when it first occurs and you don't know the cause. People experiencing the hallucinations of CBS sometimes fear they are going mad, or believe there could be a supernatural cause.

The visual hallucinations can be black and white, or in colour. They can just involve patterns, walls or grids. More often, however, they are complicated moving images. People can experience a wide variety of visual images. However, most commonly they will see complicated coloured patterns, children, wild animals and faces. The faces may be of people they recognise - and this may include people who have died. Plants, trees and beautiful countryside scenes are also common.

The hallucinations of CBS often fit into the person's surroundings, such as seeing:

  • Wild animals on the staircase.
  • Extra people, trees or animals at the bus stop.
  • Children surrounding the person's bed.

At first the affected person may need to reach out to touch them to prove to yourself that they are not there.

The hallucinations are purely visual - they don't have a sound or a smell. Sometimes text is involved - but typically it isn't possible to read it. They can last anything from seconds to hours. However, people say that they have no personal meaning - for example, they don't deliver messages or answer an unresolved question. It is not clear whether they are visual scenes from the past.

People who experience CBS tend to have their visions when in a state of quiet restfulness, or in conditions of low light. The visions are not dreams and people don't tend to experience them whilst falling asleep.

People with CBS visions usually come to realise that the images are not real - that they can't touch them and they are 'mirages'. In medical terms they are illusions (false images which the person knows to be false) rather than delusions (false images which the person firmly believes are real). At first, however, those experiencing the hallucinations may think that they are real. They may prove to themselves that they are not by turning on lights or attempting to touch the images.

CBS affects people who have experienced a sharp decline in their vision. It can occur in anyone who has experienced moderate to severe visual loss. This includes people experiencing conditions such as macular degeneration, cataracts, and diabetic retinopathy.

CBS is much more common in older patients because visual loss is more common in older people. However, anyone of any age may develop CBS, as any condition which causes sight loss can trigger it. It is therefore also seen in younger adults and children with visual loss.

CBS is fairly common, although patients often don't tell their doctor - or indeed anyone - about what they are experiencing. This may be due to the fear of being thought foolish or 'mad.' Some patients fear that the symptoms are an early sign of dementia. CBS has nothing whatsoever to do with dementia and is entirely due to loss of sight.

Despite this uncertainty, the condition is believed to occur in 10-15% of all people with moderate visual loss and up to 50% of people with severe visual loss.

The main cause of the symptoms of CBS is loss of vision. The loss may be caused by retinitis pigmentosa, Stargardt's disease or any other accident or disease which can affect the eyesight. Researchers believe that when you have normal vision the information your eyes send to your brain stops the brain from 'making up' its own pictures. When you lose your sight and the brain stops receiving this information it 'fills the gap' by reproducing its own images, which are created from stored visual memories. You experience these images as (silent) visual hallucinations.

There are no specific tests for CBS. If you experience visual hallucinations your doctor will consider CBS as a likely cause. In order to make the diagnosis an eye examination and some neurological and memory tests may be done. This is to rule out some other conditions (see below) which can also cause visual hallucinations. Usually these other conditions have other symptoms in addition to visual hallucinations.

Conditions which can also produce simple visual hallucinations (for example, flashing lights, zigzags) include:

Conditions capable of producing complicated visual hallucinations, often with colour and dramatic appearance:

  • Dementia, particularly Lewy body dementia. Lewy body dementia is a common form of dementia which typically causes hallucinations. However, other symptoms include:
    • Changes in memory, judgement, concentration and recognition (more than would be expected for your age).
    • Symptoms of stiffness, slowed movement and tremor.
    • Drowsiness, sleep disturbance and falls. which are also common.
  • Parkinson's Disease. This typically causes low mood, tremor, stiffness and slowness. Visual hallucinations are not typical.
  • Epilepsy (complex focal seizures).
  • Schizophrenia and other severe mental illness (psychosis). In these conditions people show other signs of disturbed thought processes. They usually firmly believe the visual hallucinations are real (and often experience associated sound and smell).
  • Drug misuse (particularly opiates such as heroin, and hallucinogens such as 'magic mushrooms' and LSD). These may cause complex hallucinations which can be very unpleasant.
  • Stroke can cause CBS if it causes a sudden loss of vision, or if it affects an area of the brain called the midbrain. The visions are not usually accompanied by sound or smell.
  • Alice in Wonderland Syndrome (a rare condition mainly seen in children with brain inflammation, some drugs or migraine, in which objects appear larger or smaller than they really are).

These conditions have other distinguishing features, whereas CBS consists only of visual hallucinations in people with reduced vision.

There is no cure for CBS. Doing the following can all help to reduce the frequency of hallucinations:

  • Increasing lighting levels in the evening.
  • Being active both physically and mentally.
  • Spending more time in the company of others.

Anxiety treatments such as antidepressants are sometimes offered to those who find their symptoms upsetting.

For most patients, understanding the cause of the symptoms - and realising that they are not becoming mentally ill - is all that is needed.

CBS tends not to last for ever. Typically the visions last for about 18 months then begin to lessen.  It is believed that the brain becomes used to the low vision. Unfortunately for some people, the visions may never fade.

Original Author:
Dr Mary Lowth
Current Version:
Peer Reviewer:
Dr Hayley Willacy
Document ID:
29181 (v1)
Last Checked:
Next Review:
The Information Standard - certified member
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