The key question is what defines which cortical regions are affected in a given patient, and hence what defines
which syndrome they will have. While as yet unanswered,an important clue seems to be where their primary
pathology lies. Table 2 sets out the hallucination-related disorders likely to be encountered clinically, arranged into
two pathophysiological groupings, one in which the primary pathology lies in the visual pathways or higher visual areas (prototypical disorder, Macular disease), the other in which it lies in the brainstem and/or cholinergic system (prototypical disorder, Parkinson’s disease). The choice of terminology in the second group is not intended to diminish the possible role of other brainstem neurotransmitter systems in visual hallucinations (see for review), it serves merely to emphasise that role played by the cholinergic system. Of course many hallucination related disorders fail to fit neatly into this dichotomous classification, either because their pathology is unclear (e.g. the psychoses) or because they contain elements of both. For example, visually hallucinating patients with Lewy body dementia will have both brainstem/cholinergic pathology and Lewy bodies in higher visual areas while those with Parkinson’s disease will have subtle visual deficits suggestive of visual pathway dysfunction. However, despite its obvious shortcomings, the classification is worth pursuing as it neatly divides clinical conditions into those associated with one or other of the syndrome palettes. Thus the spectrum of visual pathway disorders outlined in Table 1 are associated with the syndrome shown on the left of Figure 1, while the spectrum of brainstem/cholinergic disorders (as well as those of mixed and unknown aetiology) are associated with those on the right of Figure 1. One might wish to refer to the visual pathway disorder palette as the Charles Bonnet syndrome, although it is important to realise that different clinical specialties use the term in different ways.The hallucinations associated with visual pathway lesions and those associated with brainstem/cholinergic lesions also differ in another important respect: their prognosis. Visual pathway hallucinations tend to be self limiting and resolve without pharmacotherapy (60% of patients with hallucinations secondary to Macular disease are hallucination free at 18 months). In contrast,brainstem/ cholinergic hallucinations (at least those
found in Parkinson’s disease) tend to persist and progress, even acting to precipitate institutional care