Charles Bonnet syndrome: a great mimicker of late-life psychosis and neurodegeneration

Introduction

Visual hallucinations in older adults are often attributed to psychosis or neurodegenerative disease, frequently leading to misdiagnosis and unnecessary antipsychotic treatment [1]. However, hallucinations in late life are clinically heterogeneous and may arise from non-psychiatric mechanisms. Charles Bonnet syndrome (CBS), characterised by complex visual hallucinations in the context of visual impairment with preserved insight, remains underrecognised in psychiatric practice [2]. Failure to identify CBS may result in inappropriate diagnostic labelling and delays in addressing underlying sensory causes. Distinguishing CBS from primary psychotic disorders and neurodegenerative conditions is therefore clinically important, particularly in geriatric populations. In this context, multimodal assessment—including neuropsychological testing, neuroimaging, and emerging plasma biomarkers—may improve diagnostic accuracy and support appropriate management.

Aims

We report a case of CBS initially misdiagnosed as psychosis, aiming to illustrate the role of multimodal assessment in diagnostic reclassification and exclusion of neurodegenerative aetiology.

Methods

A comprehensive multimodal evaluation was conducted, including neuropsychological testing, structural and functional neuroimaging, ophthalmological assessment, and plasma biomarkers.

Results

A 73-year-old female patient was referred to our geriatric psychiatry outpatient clinic by a geriatrics department with a working diagnosis of psychosis following a two-year history of complex visual hallucinations consisting of human silhouettes. She consistently maintained full insight, recognising the figures as unreal. There were no auditory hallucinations, paranoid ideation, or motor symptoms. Her medical history was significant for type 2 diabetes mellitus and gout. Prior to referral, duloxetine had been initiated for comorbid depressive symptoms; visual hallucinations predated this treatment and therefore pharmacological aetiology was considered unlikely. Following our assessment, ophthalmological evaluation was arranged and revealed bilateral cataracts and diabetic retinopathy, both contributing to significant visual impairment. On neuropsychological assessment, the Mini-Mental State Examination score was 28 out of 30, consistent with intact global cognition. Clock Drawing Test performance was reduced, attributed to visual impairment rather than primary visuospatial or executive dysfunction. Magnetic resonance imaging demonstrated mild white matter changes (Fazekas grade 1), mild medial temporal atrophy (Medial Temporal Atrophy score 1), and mild global cortical atrophy (Global Cortical Atrophy grade 1), without focal cortical pathology. Fluorodeoxyglucose positron emission tomography with magnetic resonance imaging was negative for neurodegenerative pathology. Plasma phosphorylated tau 217, a non-invasive biomarker for Alzheimer pathology, was within normal limits, further excluding neurodegenerative aetiology. A diagnosis of Charles Bonnet syndrome secondary to diabetes and cataract related visual deprivation was established in accordance with published criteria [1, 2]. Cataract surgery was planned. At the time of reporting, visual hallucinations had not resolved, pending surgical intervention.

Conclusions

This case highlights the risk of misclassifying sensory deprivation-related hallucinations as psychosis in older adults. Multimodal assessment—including neuroimaging and plasma biomarkers—can support diagnostic reclassification by excluding neurodegenerative and primary psychiatric aetiologies. These findings emphasise the importance of recognising CBS as a distinct, non-psychotic condition and avoiding unnecessary antipsychotic treatment, while supporting targeted ophthalmological management.

References

1. Teunisse RJ, Cruysberg JR, Hoefnagels WH, Verbeek AL, Zitman FG. Visual hallucinations in psychologically normal people: Charles Bonnet’s syndrome. Lancet. 1996;347(9004):794–797.
2. Hamedani AG, Pelak VS. The Charles Bonnet syndrome: a systematic review of diagnostic criteria. Current Treatment Options in Neurology. 2019;21(9):41.