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A visual hallucination is when an individual can see something, which other people in the room cannot see. Typically these are thought to be symptomatic of a mental illness such as schizophrenia or a form of dementia such as Parkinson’s disease or Alzheimer’s disease. However, this is not always the case, particularly for patients with poor vision due to glaucoma or age-related macular degeneration, instead they may be experiencing Charles Bonnet Syndrome.

Diagnosis of Charles Bonnet Syndrome

  • Formed visual hallucinations
  • Some degree of vision loss
  • Patient has insight into the unreal nature of the visual phenomena
  • No other psychological or neurological diagnosis to explain the symptoms

It was Charles Bonnet, a Swiss philosopher, who first documented the case of his nearly blind grandfather “seeing” men, birds and tapestries before his eyes. This condition of visual hallucinations in people with visual deficits, who are psychologically normal, is now called Charles Bonnet Syndrome. Most patients with Charles Bonnet Syndrome are not convinced that these images are real, so the images would be better termed an “illusion” rather than a “hallucination or delusion”. However, the clarity of the images can be startling to some patients.

It is thought that the visual processing circuits of the brain “fill in the gaps” like a daydream or mirage, just within the blind areas. When a patient continues to feel sensations and pain from a limb that has been amputated, it is called “phantom limb” syndrome. Therefore Charles Bonnet Syndrome is like “phantom vision” syndrome, resulting in vivid, complex and recurring visual illusions. These visions can be in the form of shapes and colours, or visual scenes with people, landscapes and animals. Sometimes when faces and people can be seen, they may seem to make eye contact or wear elaborate costumes. While some patients describe life-sized objects, others will see things appearing in miniature. For example, one man describes seeing small monkeys in red hats parading the garden.

It is thought that nearly one third of people with low vision (of any cause) will develop Charles Bonnet Syndrome. In studies of glaucoma, it was found that nearly 1 in 4 (23%) glaucoma patients experienced this phenomenon. This could occur even if central vision was quite normal, but there was severe visual field loss.

The images are more likely to occur while patients are alone, in dim light or during periods of physical inactivity. The syndrome was typically thought to dissipate within a year, however more recent research suggests that the illusions can last for 5 years or more.

Most people are not frightened by the hallucinations themselves, but they are fearful of being diagnosed with a mental illness. Therefore, after some reassurance that there is no mental illness or dementia, patients with Charles Bonnet Syndrome generally do not need any specific treatment. For those who do find the hallucinations are fear inducing, the first step is to turn on the lights, stand up or move slightly and to concentrate on doing something else. Emotional supports and sharing the symptoms are also crucial to reduce social isolation that some patients feel with these illusions.

Written by Dr Clare L Fraser: Consultant Neuro-Ophthalmologist - Sydney Eye Hospital. April 2015