Psychosis is a syndrome (i.e., a set of symptoms) that may be associated with many different psychiatric disorders, but which is not in itself a specific disorder in diagnostic schemes, such as DSM-IV or ICD-10.
Psychosis causes a set of delusions and hallucinations, and in general it also includes symptoms such as disorganized speech and behaviour, as well as serious distortions of reality. Therefore, psychosis can be considered as a set of symptoms in which the mental abilities of a person, his emotional responses and ability to recognize reality, communicate and relate to others are compromised .
Schizophrenia is the most well-known psychotic disorder. It affects approximately 1% of the population, and another 2-3% suffer from less severe schizophrenic symptoms that do not meet the criteria for the diagnosis of this disease.
Epidemiology and Aetiology
The age of onset of psychosis is variable, but in the case of childhood psychosis, in the early years of life there may be behaviours that indicate the onset of the syndrome. Psychosis has an incidence of 1.5 to 4.2 / 100,000 people between 15 and 54 years of age. The disorder varies in severity and prognosis depending on the characteristics of the disorder, and on the characteristics of the environment in which the person lives. Studies by the World Health Organization, the International Pilot Study of Schizophrenia and the Collaborative Study on Determinants of Outcome of Severe Mental Disorders , conducted on 1400 individuals observed in a period of more than 20 years, show that schizophrenia is ubiquitous, and that different social contexts determine different social outcomes. There are no geographic areas with particularly high incidence of psychotic disorders. A much better prognosis was evident to persons belonging to countries in the developing world, both for the clinical acute onset, and for those with progressive onset.
Symptoms and neuropsychological profile
Psychotic symptoms can be attributed to disorder of form of thought (conceptual flow alterations to the “flight of ideas” and incoherence; alterations of associative links, such as paralogy, tangentiality, speeches without aim and fast exchange of arguments), disorders of thought content (predominant delusional ideation: delusions, ideas of interpretation), as well as sensory and perceptual disorders (auditory hallucinations, visual, olfactory, tactile, kinaesthetic).
Schizophrenia, the most common psychotic disorder, involves delusions and auditory hallucinations, loss of logic and consistency in thought and speech, and reduction of emotional expression or inappropriate emotion to the social context .
According to the classification of the DSM-IV-TR, it is possible to diagnose a schizophrenic disorder when there is:
A) The persistent presence of two or more of the symptoms that follow, for a significant period that is considered at least a month (it is noted that the duration may be lower if the symptom recedes following treatment): delusions, hallucinations, disorganized verbal discourse (e.g., lose the thread, incoherence, digression and expression too abstract); severe disorganization of behaviour (e.g., in clothing, in the daytime habits, sleep disturbances, dysphoria, crying or laughing frequently and inappropriately), the presence of negative symptoms, e.g. a strong sense of disinterest, distance or absence of the subject: affective flattening (lack or strong reduction of emotional responses), alogia (lack of speech), avolition (lack of motivation), deficit of attention and intellectual capacity, lack of eye contact.
B) Deficit or social dysfunction and/or lack of employment: for a significant period of time, one or more of the main areas of the subject’s life are severely compromised, compared to the time before the onset of the disorder (work, interpersonal relationships, body care, food, etc.)
C) Duration: persistence of symptoms “B” for at least six months, including at least a month when symptoms “A” persist.
Auditory hallucinations is a primary symptom found in over 60% of people diagnosed with schizophrenia, and also common in people with bipolar disorder and/or major depression . Bipolar disorder and major depression can often involve other psychotic symptoms such as delusions and symptoms related to emotional dysregulation. 
Therefore, schizophrenia, bipolar disorder and major depression show extensive phenotypic overlap, and their polygenic bases also appear to be partially overlapping   . In conclusion, it was conceptualized that schizophrenia, bipolar disorder and major depression overlap in their behavioural manifestations and, at the centre of this group of disorders, problems arise from psychosis and deficits in “mentalistic” cognition and behaviour, which lead to an increased and especially dysfunctional “hyper-mentalism”, meaning that individuals suffering from these disorders give too much importance to their own or to other people’s thoughts .
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