Charles Bonnet Syndrome

DAVID Y. KIM, MD. HOWARD F. FINE, MD, MHSc

When a patient presents with vivid visual hallucinations, most physicians will consider common neurologic or psychiatric diagnoses, such as dementia, psychosis, or a drug-related condition.¹

However, such hallucinations are experienced in the absence of neurologic or psychiatric disorder by a considerable proportion of patients who have deteriorating vision. This often underappreciated condition, which can lead to great distress in patients, is known as Charles Bonnet syndrome (CBS).² The providing of proper information to these patients may avoid unnecessary concern about a neurologic or psychiatric origin of symptoms.

Charles Bonnet syndrome is the occurrence of recurrent or persistent complex visual hallucinations (see Table), which may be considered more specifically as pseudohallucinations, in individuals with normal mental health without evidence of altered consciousness, delirium, dementia, metabolic derangement, or focal neurological disease. Loss of vision as a consequence of ophthalmic pathology is found in most cases.^2,3

Georges de Morsier coined the eponym in 1936 in honor of Charles Bonnet, an 18th century Swiss naturalist and philosopher who, in 1769, described complex visual hallucinations experienced by his cognitively sound 89-year-old grandfather, who had developed visual impairment subsequent to bilateral cataracts. These hallucinations were described as silent visions of people, birds, carriages, and buildings varying in size, shape, and place. Bonnet's grandfather was otherwise healthy, maintained intact cognition, and was fully aware that the hallucinations were "fictions" of his mind.⁴ Bonnet would himself go on to develop deteriorating vision and similar symptoms later in life.⁵

Figure 1. Charles Bonnet (1720-1793)

EPIDEMIOLOGY

Reports of the prevalence of CBS in patients with severe visual impairment vary widely, from 1% to 21% depending on the population studied. A prevalence in the range of 1% to 2% has been reported in studies of consecutive patients referred to psychogeriatric units and patients having elderly psychiatric outpatient visits.^6-8 However, when examining low-vision patient populations, studies found higher prevalence rates of 12%,^9,10 13%,¹¹ 15%,¹² and 21%.¹³ Still other studies have reported CBS in 12%¹⁴ of patients with macular choroidal neovascularization, 13%¹⁵ with macular degeneration, 5%¹⁶ after photodynamic therapy, and 16.6%¹⁷ after macular photocoagulation. A recent report looking at 1000 consecutive ophthalmologic and optometric outpatients at a Japanese university hospital found a low prevalence of CBS (<1%), but the patients had relatively good vision.¹⁸ Thus, the variation in prevalence rates of CBS among studies appears to be associated with the characteristics of the different populations examined — especially degree of visual impairment.

Charles Bonnet syndrome has also been reported in individual cases associated with macular degeneration,¹⁹ central retinal artery occlusion,²⁰ macular translocation surgery,²¹ and enucleation.²² The most common ophthalmic pathologies associated with CBS are age-related macular degeneration, followed by glaucoma and cataract. The syndrome occurs most commonly among the elderly, secondary to low vision with advancing age. A clear gender bias does not seem to occur.²³

IMPACT ON PATIENTS

Charles Bonnet syndrome often goes undiagnosed in the clinical setting. Patients are reluctant to voluntarily share their symptoms, fearing they might be labeled as insane or malingering. In a study by Teunisse and colleagues,¹⁰ of 60 patients with CBS, the emotional response of the patients to their hallucinations ranged from anxiety or irritation in 32%, mixed emotions in 18%, joy or amusement in 13%, and neutrality in 37%. Notably, 73% of these patients had not voluntarily mentioned their hallucinations to their physicians, many fearing their doctors would not take them seriously or would think them mentally ill. Of the 16 patients who did mention their symptoms to a general practitioner or an ophthalmologist, only 1 was informed of the proper diagnosis of CBS. Seven of the 16 experienced their physician's reaction as negative, with 1 patient reporting that her physician had responded by advising her "to not talk about such silly things."

In cases where the diagnosis of CBS was correctly made, patients reported feeling great relief to hear that CBS is a known phenomenon and that it is not thought to be related to mental disease.^11,24 Failure to recognize CBS may cause unnecessary distress for patients or prevent treatment of a potentially treatable causative factor of visual hallucinations. Therefore, awareness of CBS among physicians is important, especially those who treat patients with visual impairment. Clinicians should inform those patients who are susceptible to CBS about this condition and ask direct questions about the appearance of hallucinations if CBS is suspected.

CLINICAL FEATURES

There is no universally approved set of diagnostic criteria for CBS, but the core features are the occurrence of well-formed, vivid, elaborate, often stereotyped visual hallucinations in a partially sighted person who has insight into the unreality of what they are seeing. These hallucinations are always perceived as outside the body and can variably persist from several seconds to hours and change in frequency and complexity over time.¹

The most common image is that of a person, but the content of hallucinations varies and reports have included disembodied faces, animals, and complex figures. The images can be in black and white or color, with the latter being more common. The images are well organized, defined, and brilliantly clear. In the context of coexistent visual impairment, the clarity of hallucinatory images contrasts sharply with the blurred perception of real objects. Participation of another sense (eg, auditory hallucination) is generally considered an exclusion criterion for CBS. With regard to movement, images have been described as static, moving en bloc without internal change, or dynamic with internal movement. Triggering factors seem to include conditions of general sensory reduction, fatigue, stress, or low levels of illumination, as well as bright lights. Eye closure may trigger or terminate hallucinations. Elementary visual hallucinations may progressively evolve into the complex visual hallucinations that define CBS.²

DIFFERENTIAL DIAGNOSIS

The differential diagnosis of complex hallucinations, similar to those seen in CBS, includes: delirium tremens, dementia, Parkinson's disease, complex partial seizures, schizophrenia, and drug-induced hallucinations, as well as more uncommon conditions. Patients' awareness of the unreality of their hallucinations varies among these conditions, with CBS patients being the most aware. In general, once CBS patients are familiar with their hallucinations and are informed of their benign nature, they find them nonthreatening and have good insight into their condition, in contrast to hallucinations of a psychiatric etiology.²⁵

PATHOGENESIS

In 1973, Cogan classified visual hallucinations into irritative and release variants.²⁶ He suggested that the irritative variant is typically brief, intermittent, repetitive, and sometimes associated with discharge activity in other parts of the motor or sensory nervous system. The release variant, in contrast, is relatively continuous, complex, and not associated with other motor or sensory discharges. Cogan believed the latter complex hallucinations were due to a release phenomenon secondary to normal sensory input attenuation with subsequent release of indigenous cerebral activity of the visual system, somewhat analogous to the "phantom limb syndrome."²⁶

As mentioned above, there remains a strong correlation between the incidence of CBS and the degree of visual acuity (VA) loss. Current theories on the pathogenesis of CBS continue to emphasize the importance of deafferentation (both "physiological," through ganglion cell loss, and "functional," related, for example, to blindfolding or cataract), with the loss of visual input resulting in a change in cortical excitability leading to hallucinations. Recent findings indicate that reduced VA is not a necessary prerequisite for CBS, and those patients with preserved acuity but significant deafferenting ocular disease (eg, advanced glaucoma) are at risk for developing CBS.²⁷ Furthermore, it has been suggested that reduction of VA (dynamic or acute) has a greater impact on the onset of CBS than low VA (static or chronic) per se in some patients, which may also explain why low-VA patients do not always suffer from CBS and why hallucinations often resolve after the patient loses sight completely.¹⁸

MANAGEMENT

The course, prognosis, and treatment of CBS vary with the etiology of the visual deterioration. Improvement of vision leads to resolution of symptoms as reported in cataract removal, surgery for diabetic retinopathy, and laser photocoagulation for subretinal hemorrhage. Pharmacologic interventions using antiseizure or antipsychotic medications have been reported to both alleviate and increase the hallucinations of CBS.²⁴ However, most reports offer only anecdotal evidence, and efficacy has not been established in clinical trials.³

Physician awareness and compassion are the mainstays of management of CBS.^2,24 Several reports state that the best form of rehabilitation for CBS consists of patients discussing their symptoms with a sympathetic physician and reassurance by the physician of the benign nature of the hallucinations. Explanation of the cause of hallucinations using the phantom limb analogy may be helpful, as well as recommendations to optimize the visual environment using lighting and low-vision aids.

Because most patients do not mention their symptoms unless asked, comprehensive history taking is essential to ascertain the existence of visual hallucinations in elderly visually impaired patients. Once hallucinations have been documented, ophthalmic and neurologic examination is recommended to determine if there is any potentially treatable causal pathology. In an alert, well-oriented patient, a simple test of cognitive function may be adequate, with psychiatric evaluation only in the context of cognitive impairment.²

CONCLUSION

With a growing elderly population and a concomitant increased prevalence of visual impairment, it is important for physicians who see patients with vision loss to be familiar with CBS. Relief of unnecessary suffering may be as simple as patient education and counseling, especially in an elderly patient already burdened with failing vision. RP

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