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Charles Bonnet syndrome is a common 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.

People often feel distressed, sometimes because they are worried that they may be considered as having a psychiatric condition if they tell anyone what they're experiencing. However, Charles Bonnet syndrome is caused by problems with the eyes rather than the mind.

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 whose hallucinations were of people, birds, carriages, buildings and tapestries.

Charles Bonnet syndrome (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 of CBS can be black and white, or in colour. They can involve patterns, walls or grids, but more often they are complicated moving images. A huge variety of visual images have been described, as the pictures are created by the subconscious mind and the memory, and the human imagination is so extraordinary.

Most commonly people see complicated coloured patterns, children, wild animals and faces. The faces may be of people they recognise. This may (and often does) include seeing people from the past, and 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 night.

The hallucinations are purely visual - they don't have a sound or a smell. Sometimes written text is seen - but typically it isn't possible to read it. The images 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. About half of patients see the same objects each time. About half see them in colour and about half in black and white.

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). In a Canadian study, 8 out of 10 of participants knew that the visual hallucinations were not real after the first episode, and 9 out of 10 by the second, although a few did not realise until they had experienced 10 or more episodes.

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 by reaching out and attempting to touch the images. One patient described trying to fight off the images of wild animals by using an umbrella.

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 degenerationcataracts 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.

The condition is believed to occur in 10-15% of all people with moderate visual loss and up to 60% of people with severe visual loss. There are thought to be around 100,000 people in the UK with CBS today.

The main cause of the symptoms of CBS is loss of vision.

The loss of vision 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, and imagined images. 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, as a result of some drugs or in some forms of migraine. The syndrome causes objects to 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.

A form of dementia called Lewy body dementia can mimic CBS, particularly early on in its course. This means that sometimes cases which are diagnosed as CBS in fact turn out to be Lewy body dementia. The important difference in this condition is that visual hallucinations are not the only symptoms.

CBS, when correctly diagnosed, is not itself a symptom of dementia, and does not mean you are more likely to develop dementia. CBS can occur at any age, and is more common in elderly people only because loss of vision is more common in this group.

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 and then begin to lessen. It is believed that this happens because the brain becomes used to the low vision. A recent study has found that, although most people find that their symptoms become less frequent, they still have occasional hallucinations five years after they first started.

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