Table 1.

Visual hallucinations in wider clinical and non-clinical context

Condition	Key features
Parkinson’s disease	Occurs throughout PD from early stage disease without cognitive impairment to PDD (see above). Other hallucination modalities can be involved in later stages.
Charles Bonnet syndrome	Eye or visual pathway disease (see above).
Dementia	Includes AD, DLB, PDD, AD, VaD (see above). Other hallucination modalities can be involved.
Comorbid disease	Eye and neurodegenerative disease combined (see above).
Schizophrenia/bipolar disorder	Visual hallucinations are less prevalent than auditory hallucinations in schizophrenia and other psychoses. VH in these conditions rarely occur without auditory hallucinations during the course of the illness and are typically interspersed with unimodal auditory hallucinations.
Bereavement	VH of the deceased can occur as part of normal grief reaction but are less frequent than sensed presence of the deceased.
Delirium	VH are the most common modality of hallucination in delirium where they occur in the context of clouded consciousness, sleep dysregulation and affective symptoms.
Sleep-related	Occasional VH can be normal experiences at the margins of sleep (hypnagogic/hypnopompic hallucinations). They may also present as part of a sleep-disorder (eg, narcolepsy).
Medication side effects	PD medication can precipitate VH but the exact mechanism and its relation to PD neurodegeneration is unclear. Medication with anti-muscarinic effects and opiates are particularly implicated in VH.
Hallucinogen use	Visual perceptual phenomena including visual snow (see below) afterimages, palinopsia and flashback VH may persist after hallucinogen exposure (hallucinogen persisting perception disorder).
Peduncular hallucinations	Complex visual hallucinations caused by brainstem or thalamic lesions. When caused by brainstem lesions, VH are associated with sleep disturbance and eye movement dysfunction. Hallucinations in other modalities can occur.
Occipital/temporal seizures	Ictal phenomenology is based on location of seizure. Simple VH are associated with occipital foci. Complex VH imply involvement of the temporal lobe and limbic cortex.
Migraine	Teichopsia in classical migraine aura and other visual perceptual phenomena.
Visual snow syndrome	A syndrome characterised by persistent dynamic visual noise (snow), palinopsia, entopic phenomena, photophobia and nyctalopia. Associated with migraine.

AD, Alzheimer’s disease; DLB, dementia with Lewy bodies; PD, Parkinson’s disease; PDD, Parkinson’s disease dementia; VaD, vascular dementia; VH, visual hallucinations.