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Charles Bonnet syndrome.

This article explores the features of Charles Bonnet syndrome and gives an account of its clinical presentation and the management strategies that should be considered by the practitioner.


It was the exceptional Swiss naturalist and philosopher Charles Bonnet (see Figure 1) who first described a case of elaborately formed visual hallucinations, which included people, buildings and birds, experienced by a mentally alert, elderly gentleman with impaired vision. (1) The eponym Charles Bonnet syndrome (CBS) was originally used by eminent Swiss neurologist Georges de Morsier in 1936 to describe the presence of visual hallucinations (VH) in elderly individuals with normal mental capacity as separate from those associated with neurodegenerative disease. (2) Visual impairment (VI) was merely recognised as an association. However, the most up to date definitions of CBS stipulate VI as being central to the condition, thereby reflecting Bonnet's original case study. In 2005, Menon, (4) an ophthalmologist, suggested the following criteria:

* Acquired VI

* Complex VH that are: persistent or recurrent, vivid and clear, pleasant or unpleasant, stereotyped or variable

* Preserved insight, cognition and intellectual function

* The absence of neurological and psychiatric disease

* The absence of hallucinations in other modalities.

Additionally, some authorities have included the presence of more basic hallucinations, such as spots, flashes of light and patterns, which are not attributable to entoptic phenomena. (5) Others recognise that individuals with neurological or psychiatric conditions in conjunction with visual impairment are more likely to experience VH, and this has been termed 'CBS plus.' (6)

The sudden development of unpleasant visual hallucinations is a disturbing proposition for most. Given the prevalence of CBS, it is essential that practitioners are able to identify patients who may be suffering from, or at risk of developing, this condition, and ensure they are managed appropriately.


In the general population, approximately 0.5% of people experience VH. However, studies indicate the prevalence of CBS at between 11 to 63% in those with VI, depending on the diagnostic and inclusion criteria used and the method of questioning. (7) It is important to note that those with CBS are often unwilling to disclose their symptoms, often telling no one, including close family members. They are highly unlikely to tell a medical practitioner and probably not their optometrist unless prompted gently. Not surprisingly, a sympathetic approach when dealing with these cases is much more likely to result in positive admissions and allow suitable advice to be given by the practitioner. (8) Although CBS seems to be more common in female and elderly patients, this appears to reflect the greater prevalence of VI in these populations. (9) Interestingly, there have been case reports of CBS in individuals as young as six years of age. (10)


CBS has been linked to social isolation, loneliness, isolation, fatigue and introversion, indeed factors much more likely to occur in the elderly. Other factors that seem to trigger CBS episodes include reduced sensory stimulation such as low light levels along with physical and mental inactivity. VH may disappear by themselves or from direct physical interventions such as blinking, turning on a light, looking away, or starting a new task. (10) CBS almost always occurs with bilateral visual impairment, (9) although unusual cases with unilateral visual loss have been described. (12) There appears to be a relationship between the likelihood of CBS and the extent of VI. In one study, CBS was more likely when bilateral VA dropped below 6/18 in those with a range of conditions affecting central vision. (12) When age-related macular degeneration (AMD) was specifically investigated, the threshold for VH was around 6/36.13 It has been suggested that contrast sensitivity may be a more appropriate way to locate a consistent threshold for CBS. (8)

A recent survey returned by 1,254 subjects with AMD revealed that 492 (39%) had CBS type hallucinations. (14) In this group, seeing patterns were the most commonly reported VH (63%), followed by faces (39%), objects (39%), figures (36%) and animals (22%). The survey revealed that episodes usually lasted for minutes (44%) or seconds (34%) and most frequently occurred weekly (30%), with a significant proportion experiencing hallucinations monthly (21%), daily (22%) and in some cases, present constantly (13%). Subjects with continuous experiences lasting hours with little respite found their hallucinations to be the most disruptive and distressing. (14) Hallucinations may be in colour or black and white, central or peripheral (16), are mostly unfamiliar, and may exhibit a range of movements. (17) The onset of hallucinations may be sudden or gradual, (18) but once established, the same hallucinations tend to recur. (13) Figure 2 depicts a selection of commonly reported images by CBS sufferers.

Insight has to present for a diagnosis of CBS, but may be delayed, especially if the hallucinations fit in well with surroundings. (4) Fortunately, patients with CBS do not differ from age-matched, non-hallucinating subjects when investigated using the Mini Mental State Exam and do not go on to develop additional cognitive loss. (19,20)

Although CBS is most common in patients with AMD due to its prevalence, it has been associated with numerous pathologies of the visual system such as cataract, diabetic retinopathy, glaucoma, optic nerve disease, occipital lobes strokes and arterio-venous malformations. (7)

The causes of CBS are not fully understood but appear to be due to the way the brain processes visual information. The leading theory proposes that reduced sensory input produces spontaneous independent activity. (21) Other theories include gap filling and the interplay between bottom up and top down image processing. (22) Modern neuroimaging confirms Bonnet's original theory that activity occurs in the parts of the brain responsible for the type of imagery seen. (23) Clearly, further research is needed to elicit the precise causes of CBS.

Management of CBS by optometrists

Screening for VH

Optometrists are ideally placed to screen for visual hallucinations. As awareness of CBS has traditionally been low and few patients mention their hallucinatory experiences, often a proactive approach by the practitioner is required. (7) A technique favoured and adopted by the author of this article is based upon the approach suggested by Menon. (4) When the main elements of the examination are complete and a rapport with the patient has been established, an initial screening question can be posed in a non-leading manner to patients with VI. For example, simply asking the patient if they have ever had any unusual experiences with their vision may be sufficient to elicit disclosure, although further enquiry is usually necessary. Advising patients that seeing objects which they know are not there, such as unusual patterns, faces and animals is a fairly common experience in those with VI is likely to result in a positive admission.

During the consultation, the clinician will have gained an overall impression of the patient's mental alertness in addition to the symptoms and history or information given by relatives who may be present. Once other causes have been excluded (see Table 1), a positive diagnosis of CBS can be made if it is certain that insight is present. An explanation of CBS should follow to provide reassurance and guidance to the patient and others that may be present at the consultation.

Where screening is negative, the explanation of CBS is still important as a warning that it may happen in the future, thus improving awareness and allaying the potential for distress in the future should VH transpire. In cases of CBS, which have still not been disclosed at this stage due to fear or embarrassment, this information serves as positive reassurance and the patient may admit to VH at a subsequent visit, when they feel ready.

Where there is coexisting pathology, which may also lead to VH, it is acceptable to explain to the patient that the VHs may be due to a combination of causes.

Treatment strategies

The majority of sufferers are relieved to know they have CBS and that it is not a sign of impending mental decline, so reassurance following diagnosis and careful explanation is very important and has a therapeutic effect.

Practical interventions should start with maximising vision by providing up to date spectacles if the refractive error has changed significantly and referring for low vision assessment and aids to increase visual input where necessary. Should the VH be bothersome, patients should be advised to try simple stopping measures such as blinking, moving the eyes, and increasing their level of physical or mental activity. Treatable pathology such as cataracts and wet AMD should be referred for appropriate intervention.

Although it may be tempting to tell patients that their hallucinations will go away, longer term investigation indicates that 75% of those with CBS experience VH for more than five years from onset and in a significant minority affect their quality of life (approximately 12% of 1,254 subjects). CBS can interfere with activities such as watching TV and affect mobility, particularly in those with persistent VH of long-duration as opposed to sporadic, transient episodes. (14)

Negative outcomes are also associated with those who have little or no prior knowledge of CBS and fear mental decline, highlighting the crucial role practitioners have to play in order to improve awareness. Patients with particularly persistent or disturbing hallucinations, which have proved resistant to the interventions mentioned earlier, could be offered referral for psychological or pharmaceutical intervention. As yet, there is no established medical treatment for CBS. (24) Nevertheless, case reports indicate that medicines used in the treatment of psychoses, depression and epilepsy have demonstrated effectiveness in the control of CBS-type hallucinations in a few patients. (25)


Irrespective of the diagnostic criteria, VH in those with eye disease is extremely common, and it is likely that increasing numbers of patients with acquired VI will experience CBS as the population ages. Awareness of CBS has traditionally been poor, leaving sufferers feeling isolated. Thankfully this is improving with increasing coverage in national newspapers and on television, along with continued publicity from the Macular Society, ongoing research efforts, and more recently, by the establishment of the CBS Foundation in Australia.

Optometrists encounter patients with VI on a regular basis and are ideally placed to screen for VH and diagnose CBS. Key aspects of management such as reassurance, education and maximising visual potential can be addressed during routine consultation.

Practitioners are encouraged to ask their next patient, and all subsequent patients, with acquired VI the appropriate screening questions highlighted in this article to identify those with CBS and to raise awareness of the condition more generally.

Course code: C-37888 | Deadline: October 17, 2014

Learning objectives

To be able to give appropriate advice to patients with Charles Bonnet syndrome (Group 1.2.4)

To be able to manage patients with Charles Bonnet syndrome in a caring and sensitive manner (Group 2.2.1)

To be able to investigate and interpret the presenting symptoms of patients with Charles Bonnet syndrome (Group 6.1.2)

Learning objectives

To be able to give factual information to patients about the clinical features of Charles Bonnet syndrome (Group 1.2.4)

To be able to manage patients with Charles Bonnet syndrome in a caring and sensitive manner (Group 2.2.1)

To be able to understand the clinical presentation of Charles Bonnet syndrome (Group 8.1.5)

Learning objectives

To be able to understand the presentation and natural progress of Charles Bonnet syndrome (Group 1.1.1)

To be able to take a comprehensive history in cases of Charles Bonnet syndrome (Group 2.1.1)

Reflective learning

Having completed this CET exam, consider whether you feel more confident in your clinical skills--how will you change the way you practice? How will you use this information to improve your work for patient benefit?

Exam questions

Under the enhanced CET rules of the GOC, MCQs for this exam appear online at Please complete online by midnight on October 17, 2014. You will be unable to submit exams after this date. Answers will be published on and CET points will be uploaded to the GOC every two weeks. You will then need to log into your CET portfolio by clicking on 'MyGOC' on the GOC website (www.optical. org) to confirm your points.


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Daniel Goh is an optometrist at Wycombe Hospital in Buckinghamshire and Luton and Dunstable University Hospital in Bedfordshire, with special interests in contact lenses and low vision. He is chairman of the Mid Thames AOP branch and is an AOP councillor. He completed a Master's degree in clinical optometry at City University London with a focus on Charles Bonnet syndrome.

Table 1 Causes of visual hallucinations (7)

Causes of visual hallucinations

Neurological disorders
  Parkinson's disease
  Lewy body dementia
  Brain stem lesions such as peduncular hallucinosis
  Migraine coma
  Narcolepsy-cataplexy syndrome

Psychiatric disorders
  Acute psychosis
  Affective disorder
  Conversion reaction

Toxic and metabolic
  Drug and alcohol withdrawal states
  Metabolic encephalopathies
  Hallucinogenic agents
  Medications or toxic side effects

  Intense emotional experiences such as bereavement
  Sensory and sleep deprivation
  Hypnopompic (sleep to waking)
  Hypnogogic (wake to sleep) transitional states
  Charles Bonnet syndrome
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Title Annotation:1 CET POINT
Author:Goh, Daniel
Publication:Optometry Today
Geographic Code:1USA
Date:Sep 19, 2014
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