Refers to a firm and fixed belief based on inadequate grounds and not amenable to argument or evidence to contrary.



Delusions do not correlated with regional, cultural and educational background. 



It is distinct from a belief based on false or incomplete information, confabulation, dogma, illusion, or some other misleading effects of perception.



Delusions occur with many pathological states and are of diagnostic importance in psychotic disorders including schizophrenia, paraphrenia, manic episodes of bipolar disorder, and psychotic depression.



Delusions are categorized into four different groups:



Bizarre delusion



Non-bizarre delusion



Mood-congruent delusion



Mood-neutral delusion



Bizarre delusion: if they are clearly implausible and not understandable to same-culture peers and do not derive from ordinary life experiences.



Non-bizarre delusion refers to a delusion that, though false, is at least technically possible.



Mood-congruent delusions are consistent with either a depressive or manic state.



Mood-neutral delusions do  not relate to the sufferer’s emotional state.



Delusions often manifest according to a consistent theme. 



Common delusion themes: 



Delusion of the belief that another person, group of people, or external force controls one’s general thoughts, feelings, impulses, or behavior.



False belief that oneself does not exist or has died.



False belief that a spouse or lover is having an affair, with no proof to back up their claim.



Delusion of guilt or sin.



Delusion of mind being read.



Delusion of thought insertion.



Delusion of reference: False belief that insignificant remarks, events, or objects in one’s environment have personal meaning or significance. 



Erotomania: False belief that another person is in love with them.



Religious delusion:



Somatic delusion:



Delusion of poverty



Grandiose delusions 



Persecutory delusions



Persecutory delusions are the most common form of delusions in schizophrenia.



Close relatives of people with delusional disorder are at increased risk of delusional traits. 



Delusions may arise from distorted ways people have of explaining life to themselves. 



Delusions are more common among those  with poor hearing or sight. 



Increased stress is associated with a higher possibility of developing delusions. 



The two factors in developing  of delusions are: 



Disorder of brain functioning



 Background influences of temperament and personality. 



Higher levels of dopamine in the brain are needed to sustain certain delusions.



Cultural factors have influence in shaping delusions.



Delusions require dysfunction in both belief formation systems and belief evaluation systems.



Dysfunction in evaluations systems is localized to the right lateral prefrontal cortex, as demonstrated by neuroimaging.



The right lateral prefrontal cortex has 


abnormal activation and reduced volume in people with delusions, as well as in disorders associated with delusions such as frontotemporal dementia, psychosis and Lewy body dementia. 



Lesions in this region of the brain are associated with jumping to conclusions, post-stroke delusions, and hypometabolism



Delusions are thought to be a result of people assigning excessive importance to irrelevant stimuli. 



The neurotransmitter dopamine, is implicated in psychotic disorders.



Specific brain regions are associated with specific types of delusions: hippocampus and parahippocampus is related to paranoid delusions in Alzheimer’s disease.



Capgras delusions have been associated with occipito-temporal damage and may be related to failure to elicit normal emotions or memories in response to faces.



Delusions may vary in intensity and conviction over time.



Generally, to diagnose a belief as delusional: it is either patently bizarre, causing significant distress, or excessively pre-occupying, or if  the patient is subsequently unswayed in belief by counter-evidence or reasonable arguments.



Delusions must be distinguished from other symptoms such as anxiety, fear, or paranoia. 



A mental state examination is used to diagnose delusions and includes: evaluation of mood, affect, behavior, rate and continuity of speech, evidence of hallucinations or abnormal beliefs, thought content, orientation to time, place and person, attention and concentration, insight and judgment, as well as short-term memory.



Some suggest delusions are the natural consequence of failure to distinguish conceptual relevance. 



Irrelevant information is put in the form of disconnected experiences, taken to be relevant with false causal connections. 



Any relevant information is ignored.



Criteria for delusion are:









impossibility or falsity of content.



Mistaken beliefs may be mistaken for a delusion, such as when the belief in question is not demonstrably false but is nevertheless considered beyond the realm of possibility. 



A specific variant of delusion is gaslighting, when a person is fed lies in an attempt to convince them that they are delusional.



Psychotherapy, cognitive-behavioral therapy (CBT), and family therapy are treatments.




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