Persistence of the extended psychosis phenotype in young people: Link between vulnerability and clinical need

Psychosis is one of the most severe psychiatric conditions, in terms of both individual and societal burden. The pathway from the earliest and mildest expressions of psychosis to clinical disorder is highly variable and heterogeneous. A better understanding of the psychosis phenotype and its development into clinical states is important, since this offers opportunities for early intervention of psychotic disorders, with the ultimate goal to delay, attenuate or even prevent transition to clinical disorder. Psychosis is currently conceptualized as an extended, or continuous, phenotype, ranging from normal functioning at one end, through eccentricity and subclinical psychotic experiences, to florid psychotic disorder at the other end. The manifestation of this (liability to) psychotic psychopathology is represented by a continuous distribution of psychotic symptom severity/intensity in the general population. The studies in this thesis showed that many adolescents reported incidental subclinical psychotic experiences, underlining the high level of psychosis proneness in this developmental life phase. This psychosis proneness decreased with age, demonstrated by the lower levels of psychotic experiences reported by the young adult female sample compared to the adolescent samples. A model with five subdimensions, labeled Hallucinations, Delusions, Paranoia, Grandiosity and Paranormal beliefs, was shown to describe these experiences well in two large adolescent samples from the general population and was furthermore replicated in young adulthood, suggesting life-long stability of this underlying structure. Only some of these subdimensions (i.e. Hallucinations, Delusions and Paranoia) may tap into the extended psychosis phenotype in their continuity with mental illness. Psychosis proneness was shown to be very dynamic, particularly during adolescence. Many (biological, psychological and sociological) factors play a role in its development, such as trauma, cannabis use, depression, coping and genetic liability for psychosis; thus, it demonstrates that the development of psychosis over time takes place in a developmental and psychopathological context. Subclinical psychotic experiences are thought to be predictive for later clinical psychosis, as is, more strongly, Ultra High Risk (UHR) status. In other words, it is assumed that there is a dose-response function of risk between psychotic experiences/symptoms and later psychotic disorder. There is, however, a gap in the conceptualization of the psychosis continuum between subclinical psychosis at the level of the general population and UHR status for psychosis: there is no clear phenotype on the hypothesized psychosis continuum connecting individuals from the general population and individuals who are seeking help for their psychotic experiences. Studying the persistence of subclinical psychotic experiences over time may form the bridge between general population samples and UHR samples in the study of (the development of) psychosis, because this paradigm offers the possibility to study the extended psychosis phenotype at the level of the general population while focusing on a more specific phenotype that may be indicative for later psychotic development. Studying this phenotype of persisting subclinical psychotic experiences that can be seen as intermediate between general population and clinical population may thus be a fruitful paradigm for identifying individuals at increased risk for the development of psychosis