The notion of a schizophrenia spectrum

The concept of a spectrum of schizophrenia-related phenotypes originates in the observation that several ostensibly different disorders tend to cluster among biological relatives of individuals with clinical schizophrenia.^{45, 79, 80} Epidemiological and family study data suggest that the genetic liability to schizophrenia is shared with liability to other related syndromes.^{81, 82} The term 'schizotypy', introduced by Rado⁸³ and Meehl,⁸⁴ describes a personality characterized by anhedonia, ambivalence, 'interpersonal aversiveness', body image distortion, 'cognitive slippage', and sensory, kinaesthetic or vestibular aberrations. Chapman et al.⁸⁵ designed scales to measure perceptual aberrations and 'magical ideation' as traits predicting 'psychosis proneness'. These constructs were amalgamated with clinical descriptions from the Danish-US adoptive study into the DSM-III diagnostic category of schizotypal personality disorder (SPD), which is now central to the spectrum notion.^{81, 86} The frequent occurrence of SPD among first-degree relatives of probands with schizophrenia has been replicated in the Roscommon epidemiological study,⁸⁷ which added to the schizophrenia spectrum further disorders cosegregating within families. The resulting 'continuum of liability' included: (i) 'typical' schizophrenia (ii) schizotypal and paranoid personality disorders; (iii) schizoaffective disorder, depressed type; (iv) other nonaffective psychotic disorders (schizophreniform, atypical psychosis); and (v) psychotic affective disorders. The correlation of liability to the five disorders between probands with schizophrenia and their first-degree relatives was 0.36.⁸⁷ Recent evidence from the Finnish family adoptive study of schizophrenia suggests more restrictive genetic boundaries of the spectrum, by excluding paranoid personality disorder and psychotic affective disorders.⁸⁸ In all its variations, however, the spectrum concept remains critically dependent on the validity of the SPD concept. Accumulating evidence from family and twin data indicates that SPD is multidimensional and may be genetically heterogeneous.^{89, 90, 91, 92} Its manifestations fall into two genetically independent clusters: a 'negative' cluster (odd speech and behaviour, inappropriate affect and social withdrawal), more common among relatives of schizophrenic probands, and a 'positive' cluster (magical ideation, brief quasipsychotic episodes), associated with increased incidence of affective disorders in relatives.⁹³ 'Negative' schizotypy may indeed represent a personality-based counterpart of schizophrenia,⁹⁴ manifesting attenuated cognitive deficits^{95, 96, 97} and brain structural abnormalities⁹⁸ characteristic of schizophrenia.

Positive–negative schizophrenia ('Type I' and 'Type II')

A general 'weakening' of mental processes resulting in a 'defect' was the cornerstone of Kraepelin's dementia praecox, who suggested that precursors of 'defect' could be detected early in the illness, coexisting with 'productive' or 'florid' symptoms. Since the 1970s, the terms 'defect' and 'productive' symptoms have been virtually replaced by 'negative' and 'positive' symptoms.⁹⁹ Crow¹⁰⁰ proposed a simple subclassification of schizophrenia, based on the predominance of either positive or negative symptomatology. 'Type I' (positive) schizophrenia was characterized by hallucinations, delusions, and formal thought disorder, with a presumed underlying dopaminergic dysfunction, while patients with 'Type II' (negative) schizophrenia displayed social withdrawal, loss of volition, affective flattening, and poverty of speech, presumed to be associated with structural brain abnormalities. Criteria and rating scales for positive (SAPS) and negative (SANS) schizophrenia were proposed by Andreasen and Olsen.¹⁰¹ The initial typology, implying pathogenetically discrete and mutually exclusive 'types', was later replaced by a negative and a positive dimension, allowing the two kinds of symptoms to co-occur in the same individual and share a common aetiology.¹⁰²

In essence, the positive–negative typology is a descriptive device without a strong theoretical basis.¹⁰³ Its construct validity and neurobiological correlates remain ambiguous.^{104, 105, 106} As no theory-based rule exists for classifying the symptoms of schizophrenia as negative or positive, the distinction is supported by their differential loadings on separate factors. The argument is somewhat circular, since cross-sectional symptomatology is typically assessed by rating scales designed a priori to reflect such distinction.¹⁰⁷

Date: 2016-04-22; view: 657