A delusion is a false belief which is firmly sustained and based on incorrect inference about reality. This belief is held despite evidence to the contrary and is not accounted for by the person’s culture or religion.
Karl Jaspers, a noted psychiatrist and philosopher, described the three main criteria required for a delusion:
- Certainty – the patient believes the delusion absolutely.
- Incorrigibility – the belief cannot be shaken.
- Impossibility – the delusion is without doubt untrue.
Since this original definition was published there has been lengthy discourse among psychiatrists about the criteria. Moreover, the strength of delusions can vary over time.
A UK study found that 39% of a sample of 1,000 randomly selected people completing a questionnaire (the Cardiff Beliefs Questionnaire) reported having at least one strong delusional-like belief. An American study of the general population reported that low self-esteem was associated with a proneness to develop delusions.
Types of delusions
- Monothematic – delusions are only relating to one particular topic.
- Polythematic – a range of delusional topics (seen in schizophrenia).
They can also be classified as:
- Primary – occurring in the mind, fully formed with no preceding reasons; strongly suggestive of schizophrenia.
- Secondary – eg, a depressed person feeling worthless.
- Delusional jealousy (Othello’s syndrome) – eg, believing a partner is being unfaithful.
- Capgras’ delusion – belief that a close relative has been replaced by someone else who looks the same.
- Unilateral neglect – belief that one limb or side does not exist.
- Thought insertion – belief that someone is putting thoughts into the brain.
- Grandiose delusion – belief of exaggerated self-worth.
One American study found that the most common delusion was persecutory, followed by religious, somatic and grandiose.
- Neurological diseases – eg, dementia, cerebral neoplasms.
- Psychiatric conditions – eg, schizophrenia, delusional disorder.
- Psychotic disorders.
A hallucination can be described as a sensory perception which is experienced despite there being no external stimulus. Hallucinations can occur with any sense and thus be visual, auditory, olfactory, gustatory or tactile.
In pseudohallucinations the patient is aware of a stimulus which they realise is in their mind – eg, hearing a voice. This differentiates them from hallucinations, which can be localised in a three-dimensional space outside the body. They are harmless, like hypnopompic and hypnagogic hallucinations.
Visual hallucinations have been reported in 16-72% of patients with schizophrenia and schizoaffective disorder.Auditory hallucinations in adolescence are usually transient but their persistence often suggests that the psychosis will deteriorate over time.
Many people experience hallucinations unrelated to mental illness. One study found that 75% of people experiencing auditory or visual hallucinations were otherwise healthy.Auditory hallucinations are a common feature of adolescent psychosis.
- Hypnagogic – occur on falling asleep and are harmless.
- Hypnopompic – occur on waking up and are harmless.
- Auditory – of one or more talking voices; seen commonly in schizophrenia.
- Charles Bonnet syndrome – visual hallucinations experienced by some people with severe sight impairment.
- Affective disorders.
- Conversion reactions.
- Parkinson’s disease – mainly visual, rarely auditory (usually voices).
- Lewy body dementia.
- Psychotic disorders.
- Delirium or acute confusion.
- Delirium tremens.
- Drug misuse – eg, alcohol, lysergic acid diethylamide (LSD), 3,4-methylenedioxymethamfetamine (MDMA), cannabis.
- Sleep deprivation.
- Neurological illness – eg hemispheric lesions, epilepsy, migraines.
- Ophthalmological disorders – eg, cataracts, retinal disease (causing visual hallucinations).
- Childhood adversity