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Charles Bonnet syndrome: an example of cortical dissociation syndrome affecting vision?
  1. Department of Neurology, University of Glasgow, UK

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    Although Cole's article was published sometime ago,1 it was brought to our attention only recently during the weekly journal review by one of our senior house officers. Myself and my other neurological colleagues are left with little doubt that the symptoms of visual hallucinations experienced by Cole is due to the Charles Bonnet syndrome (CBS).

    This syndrome comprises the triad of visual hallucinations, visual sensory deprivation, and preserved cognitive status. The visual hallucinations in CBS may persist on eye closure (unlike the visual hallucinations of hemianopia) and are often complex, vivid, and elaborate consisting of moving and colourful images. In epidemiological studies, two common factors for CBS were visual sensory deprivation typically affecting the anterior visual pathway (due to cataract or senile macular degeneration) and advanced age ( older than 60 years). As the hallucinatory symptoms in CBS occur with preserved insight, these are not true but pseudohallucinations.

    The syndrome occurs as a result of the dissociation between visual perception and visual sensory input in psychologically normal aged people. Functional neuroimaging (fMRI) has shown that the hallucinations of colour, faces, textures, and objects in CBS correlate with the cerebral activity in the ventral extrastriate visual cortex whereas the contents of hallucinations reflect the functional specialisation of this region.2 In our experience, carbamazepine has been partially effective in suppressing the visual pseudohallucinations of CBS, presumably because it might inhibit the increased ventral extrastriate neuronal activity in patients with CBS that persists between the attacks of hallucinatory symptoms.2

    In Cole's case,1 the occipital infarct in the dominant (left) hemisphere led to the dissociation between the visual sensory input (now limited to the right striate cortex) and the visual perception sense of the dominant extrastriate and the visual association cortex spared by the ischaemic event. Collicular vision (“blindsight”) plays no part in the symptoms of visual hallucination. In the light of the recent fMRI data, CBS may be considered as a visual dissociation syndrome similar to the cortical dissociation syndromes well recognised in the Geschwind model of language function. Cole gives one of the finest clinical examples to illustrate this phenomenon. Why CBS should occur with visual sensory deprivation exclusively in elderly people and not in young people is unknown but it might reflect the nature of neural plasticity in the visual cortex as opposed to the other cortical functions.

    We were, however, a little surprised that CBS did not feature even once in the otherwise erudite discussion of the case report.1Did Cole and the reviewers of the Journalnot consider this common diagnostic possibility?