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Population-based questionnaires suggest that delusions and hallucinations are far more prevalent than was originally considered. They do not always imply that the patient has a mental illness. However, their presence has been noted to be higher in patients with a family history of mental disorders. Furthermore, delusions and hallucinations should always be reviewed in the context of the patient's ethnicity and social circumstances, although interpreting such symptoms in the context of ethnocultural diversity may be challenging. One study propounded that delusions may be caused by problems with the functioning of the orbitofrontal part of the brain, leading to difficulty in adapting to changing circumstances and external pressures.
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.
How common are delusions? (Epidemiology)
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. This is much higher than the percentage of the population diagnosed with a psychotic disorder, reported as being around 0.5% in England.
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- Capgras' delusion - belief that a close relative has been replaced by someone else who looks the same.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.
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.
How common are hallucinations? (Epidemiology)
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.
Further reading and references
Carter R, Ffytche DH; On visual hallucinations and cortical networks: a trans-diagnostic review. J Neurol. 2015 Jul262(7):1780-90. doi: 10.1007/s00415-015-7687-6. Epub 2015 Mar 13.
Corlett PR, Taylor JR, Wang XJ, et al; Toward a neurobiology of delusions. Prog Neurobiol. 2010 Nov92(3):345-69. doi: 10.1016/j.pneurobio.2010.06.007. Epub 2010 Jun 15.
Schlimme JE; Paranoid atmospheres: psychiatric knowledge and delusional realities. Philos Ethics Humanit Med. 2009 Sep 174:14.
Thornhill, C; Karl Jaspers,The Stanford Encyclopedia of Philosophy (Spring 2011 Edition), Edward N. Zalta (ed.)
Ian K, Jenner JA, Cannon M; Psychotic symptoms in the general population - an evolutionary perspective. Br J Psychiatry. 2010 Sep197(3):167-9.
Varghese D, Scott J, McGrath J; Correlates of delusion-like experiences in a non-psychotic community sample. Aust N Z J Psychiatry. 2008 Jun42(6):505-8.
Vega WA, Lewis-Fernandez R; Ethnicity and variability of psychotic symptoms. Curr Psychiatry Rep. 2008 Jun10(3):223-8.
Laws KR, Kondel TK, Clarke R, et al; Delusion-prone individuals: Stuck in their ways? Psychiatry Res. 2011 Apr 30186(2-3):219-24. Epub 2010 Oct 28.
Kiran C, Chaudhury S; Understanding delusions. Ind Psychiatry J. 2009 Jan18(1):3-18. doi: 10.4103/0972-6748.57851.
Pechey R, Halligan P; The prevalence of delusion-like beliefs relative to sociocultural beliefs in the Psychopathology. 201144(2):106-15. Epub 2010 Dec 24.
Warman DM, Lysaker PH, Luedtke B, et al; Self-esteem and delusion proneness. J Nerv Ment Dis. 2010 Jun198(6):455-7.
Psychosis and schizophrenia; NICE CKS, September 2021 (UK access only)
Garety P et al; Delusions: Investigations Into The Psychology Of Delusional Reasoning, 2013.
Cannon BJ, Kramer LM; Delusion content across the 20th century in an American psychiatric hospital. Int J Soc Psychiatry. 2011 Mar 18.
El-Mallakh RS, Walker KL; Hallucinations, psuedohallucinations, and parahallucinations. Psychiatry. 2010 Spring73(1):34-42.
Teeple RC, Caplan JP, Stern TA; Visual hallucinations: differential diagnosis and treatment. Prim Care Companion J Clin Psychiatry. 200911(1):26-32.
De Loore E, Gunther N, Drukker M, et al; Persistence and outcome of auditory hallucinations in adolescence: a longitudinal population study of 1800 individuals. Schizophr Res. 2011 Apr127(1-3):252-6. Epub 2011 Feb 18.
de Leede-Smith S, Barkus E; A comprehensive review of auditory verbal hallucinations: lifetime prevalence, correlates and mechanisms in healthy and clinical individuals. Front Hum Neurosci. 2013 Jul 167:367. doi: 10.3389/fnhum.2013.00367. eCollection 2013.
Hughes DF; Charles Bonnet syndrome: a literature review into diagnostic criteria, treatment and implications for nursing practice. J Psychiatr Ment Health Nurs. 2013 Mar20(2):169-75. doi: 10.1111/j.1365-2850.2012.01904.x. Epub 2012 Mar 27.
Pierre JM; Hallucinations in nonpsychotic disorders: toward a differential diagnosis of "hearing voices". Harv Rev Psychiatry. 2010 Jan-Feb18(1):22-35. doi: 10.3109/10673220903523706.
Shevlin M, Murphy J, Read J, et al; Childhood adversity and hallucinations: a community-based study using the National Comorbidity Survey Replication. Soc Psychiatry Psychiatr Epidemiol. 2010 Oct 8.