Charles Bonnet syndrome (CBS) involves visual hallucinations due to eye disease, usually associated with a sharp decline in vision. Interesting features of the condition are the complexity of the hallucinations and the fact that there is some consistency between people in the types of images seen, most notably images of faces, children and wild animals.
Charles Bonnet (1720-1792) was a Swiss naturalist and philosopher whose elderly grandfather, Charles Lulin, experienced visual hallucinations. Lulin's vision had been failing for some years but his health was otherwise good and he had no mental health problems. Charles Bonnet recognised that visual hallucinations secondary to eye disease are distinct from those caused by mental illness. The condition was named after him in the 1930s.
The phenomenon is seen in patients with moderate or severe visual impairment. It can occur spontaneously as the vision declines or it may be precipitated, in predisposed individuals, by concurrent illness such as infections elsewhere in the body.
CBS predominantly affects people with visual loss due to old age, diabetes or other damage to optic pathways, particularly if problems are bilateral. In particular, central vision loss due to macular degeneration combined with peripheral vision loss due to glaucoma may predispose to CBS, although most people with such deficits do not develop the syndrome. It can also develop after toxic damage to the optic nerves by methyl alcohol.
It is not clear why CBS develops or why some individuals appear to be predisposed to it. It is particularly noted in patients with advanced macular degeneration. It has been suggested that reduced or absent stimulation of the visual system leads to increased excitability of the visual cortex (deafferentation hypothesis). This release phenomenon is compared to phantom limb symptoms after amputation.
Interestingly, there are reports of inducing CBS under experimental conditions (blindfolding healthy individuals over a period of time) and even following therapeutic eye patching in an otherwise healthy and previously asymptomatic man.
Some researchers have questioned whether CBS may, in some patients, be an early stage of dementia with Lewy bodies. This condition is certainly in the list of differential diagnoses.
- CBS can affect anyone who has experienced a sharp decline in vision, including people experiencing conditions such as macular degeneration, cataracts and diabetic retinopathy.
- CBS is much more common in older patients because conditions causing marked visual loss are more common in older people. However, it can occur at any age and has been described in children.
- The prevalence is hard to assess due to considerable under-reporting, perhaps because patients frequently fear that it is a sign of mental illness or dementia.However, it is thought to occur in:
Visual hallucination is defined as a perception of an external object when no such object is present. The nature of the hallucination depends on the part of the brain that is activated. The hallucinations may be black and white or in colour. They may involve grids/brickwork/lattice patterns but are typically much more complex:
- Figures (often children), faces, wild animals and moving vehicles have all been described. These may be recognisable individuals, including people long since deceased but known to the patient.
- Images of complex coloured patterns and images of people are most common, followed by animals, plants or trees and inanimate objects. The hallucinations also often fit into the person's surroundings.
- Patients may report seeing texts or letters but, typically, they can never quite read them when they try to.
- The hallucinations are always outside the body.
- The hallucinations are purely visual - other senses are not involved.
- The hallucinations have no personal meaning to the patient.
- Hallucinations may last seconds, minutes or hours.
- CBS tends to occur in a 'state of quiet restfulness'. This may be after a meal or when listening to the radio (but not when dozing off).
- Symptoms also have a tendency to occur in dim lighting conditions.
- Patients may report high levels of distress, with some patients reporting anger, anxiety and even fear associated with the hallucinations.
There are no universally accepted diagnostic criteria. Consider it in an older patient who has:
- Vivid, elaborate and often stereotyped visual hallucinations (eg, groups of people or children, faces, wild animals and panoramic countryside scenes).
- Images which are frequently 'lilliputian' (smaller than usual).
- Partial sight (eg, due to macular degeneration, cataract or glaucoma).
- Partial or total insight into the fact that what they are seeing is not real (they may have to keep reassuring themselves).
- An ability to modify the image or make it disappear by closing their eyes.
- A normal cognitive status.
- An understanding that the images are not real.
- Absence of:
- Organic disease - eg, urinary tract infection.
- Impaired conscious level.
- Dementia (particularly Lewy body dementia).
- Metabolic abnormalities.
- Focal neurological illness.
- Some have therefore argued that it can occur when there are lesions that are not associated with the visual system. This is rare.
- Some have suggested that auditory hallucinations may be part of this syndrome but this is contested and most would concur that the hallucinations are only visual.
Simple hallucinations (flashes of light, zig-zag patterns, circles):
- Ocular disease (eg, retinal detachment).
- Neuro-ophthalmic disease (eg, optic neuritis).
- Neurological disease (eg, migraine, occipital lobe seizures or tumours).
Complex hallucinations (well formed and relatively stereotyped and often involving animals and figures in bright colours and dramatic settings):
- Dementia, particularly Lewy body dementia.
- Parkinson's disease.
- Complex partial seizures.
- Schizophrenia and other psychotic mental illness.
- Drug misuse.
- Peduncular, hypnagogic and hypnopompic hallucinations.
- Alice in Wonderland syndrome, a disorienting neurological condition that affects size perception. It is a temporary condition, more common in children, often associated with migraines, organic brain disease, psychoactive drugs and the Epstein Barr virus.
Symptom overlap with Lewy body dementia
There is a strong association of hallucinations with dementia with Lewy bodies. This is associated with significant prognostic and therapeutic implications for the patient. Both patients with CBS and patients with dementia with Lewy bodies can present with formed visual hallucinations. Ophthalmologists and retina specialists need to be particularly familiar with the features of dementia with Lewy bodies. Early diagnosis of this condition can help prevent drug-related side-effects. If there is any suspicion of early dementia in such patients, they may benefit from neuropsychiatric evaluation.
- If CBS is suspected but the patient has not been diagnosed with an eye problem, assess vision using a Snellen chart. If vision is normal, consider alternatives in the differential diagnosis.
- If the vision is abnormal, refer to an optometrist or the local eye unit for a complete ocular examination including dilated fundus check.
- It is important to consider other conditions in the differential diagnosis even if eye disease is present, since this is commonly found with advancing age.
You may be fairly sure of the diagnosis on the initial history. If CBS is a likely diagnosis:
- Investigate appropriately.
- Avoid an early but potentially erroneous diagnosis of a psychiatric condition unless other features are present to support this.
- Reassure the patient by explaining what the problem is. Many will fear that they are 'going mad'. Understanding that they are not will help them to accept their symptoms.
There is no treatment with proven benefit. For most patients, understanding that they do not have a mental illness seems to be the best treatment, as it improves their ability to cope with the hallucinations, although these can still interfere with daily life. Interrupting vision for a short time by closing the eyes or blinking is sometimes helpful.
There are some practical suggestions that can be implemented immediately:
- In some patients, eye movement can help dispel the hallucination.
- As the hallucinations tend to occur in similar situations (quiet activity), it may help to get up and do a distracting activity.
- Increasing the retinal impulses can counterbalance the effect (eg, increasing the ambient light).
- Reducing social isolation and therefore increasing both sensory and cognitive stimulation can be helpful.
Pharmaceutical treatment should only be considered in patients experiencing distressing hallucinations and who cannot tolerate them despite reassurance and non-pharmaceutical approaches. There have been reports of successful cases treated with a variety of drugs, including risperidone, valproate, carbamazepine, clonazepam, selective serotonin reuptake inhibitors, gabapentin and olanzapine. However, their effectiveness in treating CBS remains disputed and the side-effects can cause a number of problems - including hallucinations.
- Social isolation.
- Injury - patients have been known to battle imaginary wild animals in the semi-darkness.
The course is variable. People experience hallucinations for anywhere from a few days up to many years and these hallucinations can last only a few seconds or continue for most of the day. For many, the symptoms subside but some only experience improvement if the sight improves.Additionally, some only experience improvement if the sight is totally lost.
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- Visual Hallucinations (Charles Bonnet syndrome hallucinations); Macular Society
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