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Visual Hallucination and Illusion Disorders: A Clinical Guide

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Many patients who on some occasions ‘see things that are not really there’ will, on other occasions, ‘see real things incorrectly’. Furthermore, different classes of ‘incorrect seeing’ (polyopia and metamorphopsic distortions, for example) seem to apply equally to hallucinated things as real things. Put another way, patients describe illusions of hallucinations - a situation which amounts to a logical impossibility. How can something not actually there, be seen incorrectly?

Ghosting is a form of polyopia.

Termed pseudohallucinations in the European psychopathological tradition, vivid visual images carry very different aetiological implications to those of hallucinations and illusions, imagery pointing to a diagnostic spectrum which ranges from normal phenomena to clinical anxiety syndromes. Note that, confusingly, the American psychopathological tradition uses the term pseudohallucination in a different way, referring to hallucinations that are recognised for what they are (i.e. those for which the patient has insight).

HPPD symptoms can be considered pseudohallucinations in the US.

In one syndrome patients will describe hallucinations from a palette which ranges from simple unformed lines, dots, colours and flashes, through more complex grid patterns and lattices to distorted faces (grotesque or gargoyle-like), unfamiliar figures in bizarre costume (often wearing elaborate hats) and extended landscape scenes.
...those with Parkinson’s disease will have subtle visual deficits suggestive of visual pathway dysfunction.
Visual evoked potentials may help characterise a visual pathway lesion but there are, as yet, no known VEP markers for susceptibility to hallucinations.
For brainstem /cholinergic syndrome hallucinations that are distressing, multi-modality or with persistent delusions (or visual pathway syndrome hallucinations that are persistent and distressing) largely anecdotal treatment options include cholinesterase inhibitors, anti-convulsants and atypical antipsychotics. Which class of medication to use will depend on the clinical context, with cholinesterase inhibitors a logical first choice for patients with cognitive impairment and atypical antipsychotics for those with pronounced delusions.
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