Description¹

In about 1760, Charles Bonnet (1720-1792), a Swiss natural philosopher, noted that his elderly grandfather, Charles Lulin, began to experience visual hallucinations. Lulin's vision had been failing over a number of years but his health was otherwise good with, notably, no history of any mental health problems. It wasn't until the 1930s that the phenomenon was named after Charles Bonnet in recognition of his realisation that visual hallucinations secondary to eye disease are quite distinct from those caused by mental illness.

Today, this interesting phenomenon is well recognised 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. In over half the cases, the hallucinations lessen and eventually disappear about 18 months after onset. However, they can reappear should there be a further sharp decline in vision. There are some techniques and drugs that can help the hallucinations to a certain extent but, for most, education and reassurance remain the mainstay of management.

Pathogenesis

It is not clear why Charles Bonnet syndrome (CBS) develops, why some individuals get it and others do not and why certain conditions predispose individuals more than others (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.² Interestingly, there are reports of inducing CBS under experimental conditions (blindfolding healthy individuals over a period of time)² and even following therapeutic eye patching in an otherwise healthy and previously asymptomatic man.³ However, this does not explain the small number of reported cases where the visual system is intact.

Epidemiology

• Charles Bonnet syndrome (CBS) tends to occur in elderly individuals (probably because of the prevalence of visual impairment in this group).⁴
• The prevalence of this condition is hard to assess due to considerable under-reporting⁴ but it is thought to occur in about 10%-15% of patients with moderate visual loss⁵ and possibly up to 50% of people with severe visual loss.^1,6

Presentation

• Figures, faces, moving vehicles have all been described.
• Patients may report seeing texts or letters but, typically, they can never quite read them when they try to.¹
• The hallucinations are always outside the body.
• The hallucinations have no personal meaning to the patient.
• Hallucinations may last seconds, minutes or hours.

Charles Bonnet syndrome (CBS) tends to occur in a 'state of quiet restfulness'.¹ 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.⁴ Patients may report high levels of distress, with some patients reporting anger, anxiety and even terror associated with the hallucinations.⁶

Diagnosis

There are no universally accepted diagnostic criteria. Consider it in an older patient who has:⁴

• Vivid, elaborate and often stereotyped visual hallucinations (e.g. groups of people or children, animals and panoramic countryside scenes).
• Partial sight (e.g. 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).²
• An ability to modify the image or make it disappear by closing their eyes.
• A normal cognitive status.
• Absence of:
  • Psychosis.
  • Impaired sensorium.
  • Dementia (particularly Lewy body dementia).⁵
  • Intoxication.
  • Metabolic abnormalities.
  • Focal neurological illness.

Charles Bonnet syndrome (CBS) has been reported in people with normal vision^2,5 and some have 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.²

Differential diagnosis⁴

Simple hallucinations (flashes of light, zig-zag patterns, circles):

• Ocular disease (e.g. retinal detachment).
• Neuro-ophthalmic disease (e.g. optic neuritis).
• Neurological disease (e.g. 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):

• Delirium.
• Dementia.
• Parkinson's disease.
• Complex partial seizures.
• Schizophrenia and other psychotic mental illness.
• Drug misuse.
• Peduncular, hypnogogic and hypnopompic hallucinations.
• Migraine coma.
• Alice in Wonderland syndrome.

Investigations

If the patient has not been diagnosed with an eye problem but you suspect this condition, do a simple Snellen chart visual acuity assessment to give you an idea of their level of vision (if vision has deteriorated slowly, they may still be remarkably functional with very little residual sight).

If the vision is normal, investigate to rule out conditions outlined 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 still worth ruling out the above conditions, even if ocular disease is found.

Management

Think about this syndrome for two reasons:

1. You need to investigate appropriately and avoid an erroneous diagnosis of a psychiatric condition, starting the patient on inappropriate drugs or even admitting them to a mental health unit.
2. You can reassure the patient greatly by explaining what the problem is and that they are not 'going mad'. Many patients find this reassurance a major source of relief⁴ and are then able to accept their symptoms and manage these much better.

There are some practical suggestions that can be implemented immediately:

• In some patients, eye movement can help dispel the hallucination.¹
• 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 (e.g. increasing the ambient light).
• Reducing social isolation can be helpful.⁴

Pharmaceutical treatment should only be considered in patients experiencing distressing hallucinations and who cannot tolerate these despite reassurance and non-pharmaceutical approaches. There have been reported successful cases treated with a variety of drugs, including risperidone, valproate, carbamazepine, clonazepam, selective serotonin reuptake inhibitors, gabapentin and olanzapine.² However, their effectiveness in treating Charles Bonnet syndrome (CBS) remains disputed and the side-effects can cause a number of problems - including hallucinations.

Complications

• Anxiety ('I'm going mad!')
• Depression
• Social isolation

Prognosis

The course is variable. For many, the symptoms subside after about 18 months but some only experience improvement if the sight improves⁷ or is totally lost.⁴

Document references

1. Ffytche D; Charles Bonnet Syndrome; go to right-hand link to access full article
2. Jackson ML, Ferencz J; Cases: Charles Bonnet syndrome: visual loss and hallucinations. CMAJ. 2009 Aug 4;181(3-4):175-6.
3. Khadavi NM, Lew H, Goldberg RA, et al; A case of acute reversible Charles Bonnet syndrome following postsurgical Ophthal Plast Reconstr Surg. 2010 Jul-Aug;26(4):302-4. [abstract]
4. Jacob A, Prasad S, Boggild M, et al; Charles Bonnet syndrome--elderly people and visual hallucinations. BMJ. 2004 Jun 26;328(7455):1552-4.
5. Terao T, Collinson S; Charles Bonnet syndrome and dementia. Lancet. 2000 Jun 17;355(9221):2168.
6. Russell G; Age related macular degeneration. Is associated with Charles Bonnet syndrome. BMJ. 2010 Mar 24;340:c1611. doi: 10.1136/bmj.c1611.
7. Singh A, Sorensen TL; Charles Bonnet syndrome improves when treatment is effective in age-related Br J Ophthalmol. 2011 Feb;95(2):291-2. Epub 2010 Aug 23.

Acknowledgements