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In 1760, the Swiss naturalist and philosopher Charles Bonnet wrote about his visually impaired, psychologically sound, grandfather experiencing detailed visual hallucinations: 'I know a respectable man full of health, of ingenuousness, judgement and memory, who, completely alert and independently from all outside influences, sees from time to time, in front of him, figures of men, of women, of birds, of carriages, of buildings etc.'1

So advanced were Bonnet's thoughts and observations on the syndrome that the understanding of the syndrome has progressed little since then. Today, Charles Bonnet syndrome (CBS) is considered to involve complex visual hallucinations, in cognitively normal patients often with visual impairment.1,2

Typical symptoms

Reported hallucinations are often complex (that is, formed images of people or objects), detailed, brightly coloured and mobile. The hallucinations rarely have any personal meaning to the subject (unlike clinical depression or schizophrenia). They are purely visual, that is they make no sound or smell and often disappear on approach. The subject is almost always aware that the images are not real. Examples of images seen include people (fairies, flower girls, ogres), animals from ants to elephants, cobwebs, vines, trees, buildings and toys. Hallucinations are often reported to grow, shrink, distort or multiply.2,3,4

Although the examples seem very diverse and random, there are common themes noticed from patient to patient, such as Roman chariots, misshapen faces, trees and puddles of water.

Such are these similarities that Ffytche et al5 classified the hallucinations into eight specific groups, including prosopometamorphopsia (facial distortion) and dendropsia (branching forms) and were able to link these groups to localised areas in the visual association cortex, where specificity is well known but not yet fully understood.

It has been noted that hallucinations are precipitated by solitude, ambient lighting and drowsiness. The images are often reported to disappear when the subjects close their eyes, walk away, undertake distracting tasks or blink rapidly.6 Images tend to reduce in frequency or halt altogether with time.7

Typical subjects

Subjects experiencing CBS are commonly elderly and have acquired visual impairment, due to ocular pathology or visual pathway disturbances.

It is not the severity of visual loss that is most significant regarding the onset of CBS but the rate at which the impairment develops. De Morsier8 said that the Charles Bonnet phenomenon can occur in the absence of visual loss.

Age-related macular degeneration is commonly the cause of visual loss in those suffering from CBS but often there is more than one cause. Sufferers do not tend to have any psychological disorders and are aware that the images are not real, often referred to as the patient having insight. Although patients very commonly have insight into the unreal nature of their hallucinations it may be delayed and initially the patient may be temporarily deceived. Insight has also been known to come and go or to be incomplete.9,10

Other risk factors include being of female gender, creativity, social isolation, loss of independence, stress, fatigue and depression. Many sufferers are within the early stages of visual loss and therefore this may contribute to some of the associations such as depression, isolation and greater dependence.7 Although true CBS is defined as complex visual hallucinations in the absence of any psychiatric/psychological condition, it is possible for conditions to co-exist. Psychiatric problems can be present alongside CBS.11 Depression is frequently present in CBS sufferers.12

Pathophysiology

The pathophysiology is not fully understood. There are many theories on the topic and it is still a much debated subject. Complex visual hallucinations such as those associated with CBS are thought to originate from the visual association area of the brain (area 19).13,14 This area is associated with visual memory and imagination.15 It has discrete highly specialised regions. The location of activity within this area correlates to the form of the hallucinations seen.5,16

Currently, a leading theory is that deprivation of input to nerve fibres from lack of visual stimulation (due to retinal or visual pathway lesions) causes loss of impulses into the brain (deafferentation of the neurone). This in turn can cause spontaneous firing of subsequent neurones and an increase in hyper-excitability of the deafferentated neurones causing the expression of hallucinations.17 It is also thought that according to this theory lowered sensory stimulation allows earlier visual memories to be perceived again consciously as Charles Bonnet hallucinations.18 This is comparable with phantom limb experiences after amputation.7 This theory is supported by the disappearance of hallucinations where visual stimulation is possible.19 This theory was also demonstrated by blindfolding visually normal patients, where 10 out of 13 reported visual hallucinations after 24 hours.19,20

Other schools of thought include the perceptual release theory. This is that the hallucinatory images are constantly formed but normal visual sensation prevents the awareness of them. If normal vision is disturbed and sensation reaches a lower limit the images are no longer inhibited and are then conscious perceptions.21

There is also a theory that draws similarities between CBS hallucinations and dreams. It suggests that dreams are just images that are impeded by wakeful visual activity. When this activity is reduced, for example with visual loss, the 'dreams' are brought into consciousness. This may be linked to the fact that CBS hallucinations are often reported to occur when the subject is drowsy.22,23

Differential diagnosis

Visual hallucinations can arise from a number of conditions. Examples include psychiatric disease, such as depression, schizophrenia, organic brain disease, dementia, Alzheimer's disease as well as side effects of medication and even migraine. The main difference between CBS and other conditions is that subjects with CBS appreciate the illusionary nature of the images and hallucinations have no associated tactile, auditory or olfactory features.9,20,23

Prevalence and awareness

Estimates of prevalence of CBS are wide-ranging (10-52 per cent),20,24 and thought to be underestimated. This lack of accuracy in estimating the number of sufferers is partly owing to patients' fear of being thought mad or foolish when admitting to symptoms. There is also a lack of awareness in primary care professionals, such as optometrists and general practitioners, and the condition often goes unrecognised and misdiagnosed.25

CBS can be extremely distressing for some patients. It must also be remembered that the syndrome often occurs in those who are isolated or even depressed for whom this distress may be more detrimental. Menon26 showed that patients with the syndrome had admitted to being concerned, seeing disturbing hallucinations, were frightened of being thought insane or they believed themselves that they were becoming insane. Menon's study also showed that none of those patients with the syndrome freely admitted to having the hallucinations and only 6.5 per cent admitted it on non-leading questioning and it took direct questioning for the other 93.5 per cent to admit they had seen any images.

With the currently ageing population and therefore the inevitable rise in acquired visual loss, it is essential that awareness of CBS within primary care is improved. Case studies show that patients' worries can be relieved easily by the reassurance of a healthcare professional that the hallucinations they are experiencing are not signs of impending illness, mental or otherwise.3,25,26 Reports of patients going through unnecessary extensive psychiatric evaluation are not uncommon, where simple cognitive ability tests are often adequate to diagnose CBS or establish the need for further testing.

Primary care

History and symptoms

Primarily information needs to be elicited from the patients during history and symptoms. The advised approach to this is to directly but sympathetically question the patients. For example, 'It is common for people with low vision to see strange things that they know aren't real, have you experienced this?'

It has been shown that patients are most likely to admit having visual hallucinations to sensitive, impartial, professionals.24 If the patient reports hallucinations, practitioners must determine whether or not the patient has insight, that is, are they aware that the images are not there? Does the patient find the images distressing?

General health and medication information is also important as this may give practitioners insight into any possible underlying neuropsychiatric disorder. The possibility of depression and associated symptoms such as poor sleep patterns or loss of appetite should also be investigated.27 As mentioned, social isolation and loneliness are risk factors for CBS,7 therefore questioning on the patient's social situation is also key, for example, whether they live alone and whether they have regular help/visitors.9,11

Advice to patients

Patients experiencing CBS must be relieved of any concerns that they are 'going mad' and be sufficiently reassured that these images are a common symptom of visual loss and that they are very likely to cease with time.9

Patients can be informed of distraction techniques to reduce the frequency and duration of the hallucinations. These include increasing social interaction, blinking, closing and opening eyes, turning the light on and off, focusing attention on something else or even hitting or shouting at the image. The patient must be made aware that techniques work for some people and not for others.6

Patients can also be directed to the RNIB website where there is information on CBS and opportunities to join groups for visually impaired people, providing the opportunity to get help where needed and to meet people in similar situations.

Low vision patients who do not report visual hallucinations must be made aware of the possibility of encountering images and again reassured as to their likely benign nature.9,11,28

Referral

Where the patient has no insight into the illusory nature of their hallucinations or the hallucinations have an auditory or olfactory nature, the patient should be referred to their GP for cognitive testing. If the practitioner is concerned that a psychiatric problem may be contributing to the hallucinations - for example where patients show signs of depression - Alzheimer's or dementia, referral to their GP for investigation, diagnosis and possible help or treatment is recommended.27

If ever hallucinations cause the patient distress (despite reassurance from the optometrist), or are impacting on the patient's quality of life, referral to the patient's GP can lead to help in the form of psychological rehabilitation, for example hypnosis and relaxation techniques. These techniques help patients even when there is no psychiatric element involved in the formation of hallucinations.28 Patients must be referred to ophthalmology departments where improvement in vision is possible. Improving vision can reduce or abolish hallucinations.29 Examples of this are cataract operations or laser treatment for diabetic retinopathy. The introduction of low vision aids has also been shown to reduce CBS symptoms, therefore referral to low vision clinics where applicable is also recommended.9,11,30

Increasing awareness among primary care optometrists and low vision optometrists is fundamental in improving patient care. This could decrease patient anxiety, and save time and money by stopping needless, extensive psychiatric investigation.

References

1. Bonnet C. Essai Analytique sur les Faculties de l'Ame, Copenhagen: Philibert, 1760:176-177.
2. Hedges T. Charles Bonnet, His Life and His Syndrome, Survey of Ophthalmology, 2007 52:111-114.
3. Plummer C, Kleinitz A, Vroomen P, Watts R. Of Roman Chariots and goats in overcoats: The syndrome of Charles Bonnet, Journal of Clinical Neuroscience, 2007 14:709-714.
4. Teunisse R, Zitman F, Reas D. Clinical Evaluation of 14 Patients with the Charles Bonnet syndrome (Isolated Visual Hallucinations), Comprehensive Psychiatry, 1994 35:70-75.
5. Ffytche D, Howard R, Brammer M, David A, Woodruff P, Williams S. The anatomy of conscious vision: An fMRI study of visual hallucinations, Nature Neuroscience, 1998 1:738-742.
6. Teunisse R, Cruysberg J, Hoefnagels W, Verbeek A, Zitman F. Visual Hallucinations in psycologically normal people : Charles Bonnets syndrome, Lancet, 1996 347:794-797.
7. Jackson M, Bassett K, Nirmalan P. Charles Bonnet hallucinations: Natural history and risk factors, International Congress Series, 2005 1282:592-595.
8. De Morsier G. Le syndrome de Charles Bonnet: hallucinations visuelles des vieillards sans deficience mentale, Annales médico-psychologiques, 1967 2:677-702.
9. Menon G, Rahman I, Menon S, Dutton G. Complex visual hallucinations in the visually impaired. The Charles Bonnet syndrome, Survey of Ophthalmology, 2003 48:58-72.
10. Teunisse R. Charles Bonnet syndrome, insight and cognitive impairment, Journal of the American Geriatrics society, 1997 45:892-893.
11. Eperjesi F, Akbarali N. Rehabilitation in Charles Bonnet Syndrome : a review of treatment options, Clinical and Experimental Optometry, 2004 87:149-152.
12. Schultz G, Melzack R. Visual hallucinations and mental state. A study of 14 Charles Bonnet Hallucinators, The Journal of Nervous and Mental Disease, 1993 181:715-726.
13. Foerster O. The cerebral cortex in man, Lancet, 1931 2:309-312.
14. Price J, Whitlock F, Hall R. The psychiatry of vetrebrobasilar insufficiency in the report of a case, Psychiatria Clinica (Basel), 1983 16:26-44.
15. Chatterjee A, Southwood M. Cortical blindness and visual imagry, Neurology, 1999 45:2189-2195.
16. Santhouse A, Howard R, Ffytche D. Visual hallucinatory syndromes and the anatomy of the visual brain, Brain, 2000 123:2055-2064.
17. Burke W. The Neural basis of Charles Bonnet hallucinations: a hypothesis, Journal of Neurology, Neurosurgery, and Psychiatry, 2002 73:535-541.
18. Asaad G, Shapiro B. Hallucinations: theoretical and clinical overview, The American Journal of Psychiatry, 1986 143:1088-1097.
19. Bartlette J. A case of organised visual hallucinations in an old man with cataract, and their relationship to the phenomena of the phantom limb, Brain, 1951 74:363-373.
20. Biousse V, Skibell B, Watts R, Loupe D, Drews-Botsch C, Newman N. Ophthalmologic features of Parkinson's disease, Neurology, 2004 62 :177-180.
21. West L. A clinical and theoretical overview of hallucinatory phenomena. In: Hallucinations: Behaviour, Experience and Theory, [Eds.]:Siegal R, West L, New York: John Wiley and Sons, 1975:287-311.
22. Hartmann P. A clinical and theoretical overview of hallucinatory phenomena. In: Hallucinations: Behaviour, Experience and Theory, [Eds.]:Siegal R, West L,New York: John Wiley and Sons, 1975:287-311.
23. Manford M, Anderman F. Complex visual hallucinations. Clinical and Neurological insights, Brain, 1998 121:1819-1840.
24. Crane W, Fletcher D, Schuchard R. Prevalence of photopsias and Charles Bonnet syndrome in a low vision population, Ophthalmology Clinics of North America, 1994 7:143-149.
25. Nixon P, Mason J. Visual Hallucinations from age-related macular degeneration, The American Journal of Medicine, 2006 119:e1-e2.
26. Menon G. Complex visual hallucinations in the visually impaired: a structured history taking approach, Archives of Ophthalmology, 2005 123:349-355.
27. Varstraten P. Experiences with a protocol for the Charles Bonnet syndrome. In: On the special Needs of Blind and Low Vision Seniors: Research and Practice Concepts, [Eds]: Whal H, Schulze H, Amsterdam: IOS Press, 2001:209-213.
28. Needham W, Taylor R. Benign visual hallucinations, or 'phantom vision' in visually impaired and blind persons, Journal of Visual Impairment and Blindness, 1992 86:245-248.
29. Fernendez A, Lichtshein G, Vieweg W. The Charles Bonnet Syndrome : A review, The journal of Nervous and Mental Disease, 1997 185:195-200.
30. Pankow L, Pliskin N, Luchins D. An optical intervention for visual hallucinations associated with visual impairment and dementia in elderly patients, The Journal of Neuropsychiatry and Clinical Neurosciences, 1996 8:88-92.

● Letitia Lamdin is a pre-registration optometrist at Boots Opticians, Newport and Andrew Feyi-Waboso is a consultant ophthalmologist at Royal Gwent Hospital, Newport

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