An atypical case of Charles Bonnet syndrome secondary to advanced cataracts

Clinical record

A 73‐year‐old woman presented to her general practitioner with visual hallucinations. She described seeing cult and devil worshippers, and a big brown bear being patted by the devil. The hallucinations were vivid, not interactive, and she retained variable but present insight that they were not real. She had one previous admission to a mental health unit for attempted suicide in the context of domestic violence, and a previous dependence on benzodiazepine. Medical history was significant for ischaemic heart disease and restless leg syndrome.

The patient was referred to an outpatient psychiatrist. Pramipexole, recently prescribed for restless leg syndrome, was identified as a potential precipitant and ceased, and olanzapine 2.5 mg daily was commenced. Despite intervention, the hallucinations increased in frequency and she was brought to a rural emergency department by police. She was found wandering the streets, refusing to return home as this was where the hallucinations generally occurred.

The patient was then transferred to a mental health unit as an involuntary patient under the Mental Health Act 2007 (NSW). Inpatient psychiatric assessment found her cognition intact, with the absence of psychotic symptoms extending into other senses. Investigations including urine microscopy, baseline blood tests, electroencephalography and magnetic resonance imaging were all unremarkable. No apparent cause for the hallucinations could be found and the provisional diagnosis of delirium with prominent visual hallucinations was made. In order to extend admission, she was taken to the mental health tribunal, which was adjourned because of diagnostic uncertainty.

With no definitive cause for her visual hallucinations, broadening of potential differential diagnosis identified significant visual impairment. She had previously diagnosed bilateral nuclear sclerotic cataracts, with a best corrected visual acuity of 6/9 in the right eye and 6/6 in the left eye, and an unremarkable fundus examination. Ocular history was relevant for high myopia (– 8 dioptres). She had been lost to follow‐up before intervention, and her best corrected visual acuity had now declined to hand movements in the right eye, and 6/24 in the left. An intumescent right cataract was causing poor visual acuity with resultant exotropia but no relative afferent pupillary defect. Questioning revealed an abrupt decline in left visual acuity following loss of her spectacles 6 weeks before the onset of hallucinations.

It was concluded that Charles Bonnet syndrome resulting from advanced cataracts was the cause of her visual hallucinations, and urgent right cataract surgery was scheduled. Postoperatively, her uncorrected right visual acuity improved to 6/9. She experienced no further visual hallucinations and was discharged from the psychiatric unit the same day, and from community mental health 2 months later.

Discussion

Charles Bonnet syndrome refers to visual hallucinations resulting from the brain’s adaptation to significant ocular or cortical visual loss.1 Its reported prevalence varies, ranging from 0.4% up to 30% in the visually impaired.1 Prevalence increases as visual acuity declines, and affects young and older people equally.1 Age‐related macular degeneration is the most common cause.1

Charles Bonnet syndrome may be viewed as the visual analogue to phantom limb syndrome. The prevalent deafferentation hypothesis1 proposes that a loss of external or afferent visual input leads to a change in the excitability of the visual cortex, causing spontaneous neuronal discharge and resultant hallucinatory images.1 Discharge generally occurs in the visual association area, which comprises the greatest volume of grey matter in the visual cortex.1 As a result, the hallucinations are typically vivid, elaborate and recurrent.2 They can be classified as simple, such as patterns or lines, or complex, such as people, animals or landscapes.3

As there is no strict consensus on the definition of Charles Bonnet syndrome, diagnostic criteria vary. In general, diagnosis requires visual impairment with an otherwise intact cognition, absence of hallucinations extending into other senses, and insight that they are not real. Risk factors include worsening visual acuity, old age, hearing impairment, social isolation, and being female.2

Management is effective in reducing the rate of visual hallucinations.2 Maximising remaining visual function is essential. This can be done through visual rehabilitation or interventions treating the underlying cause. Our case was unusual because cataract surgery was able to abruptly reverse Charles Bonnet syndrome. Effectiveness of medical management is inconsistent, but includes antidopaminergic medications, selective serotonin reuptake inhibitors and anticonvulsants.1,2

Patient education is a simple and effective aspect of successful management. One‐third of patients find the hallucinations unpleasant, and 46% feel it affects their activities of daily living.4 This is concerning, as 75% of affected patients experience symptoms for over 5 years,4 and owing to fear of diagnoses such as dementia or mental illness, only 15% of patients will seek medical advice.1,3 Effective screening is therefore required, but is currently limited by lack of awareness, with less than 50% of family physicians aware of the syndrome.5 With appropriate counselling regarding the nature of the hallucinations, 63% of patients report being relieved or reassured, reducing fear association and encouraging habituation.4