Schizophrenia is treated as a single (continuous) disorder diagnosed according to reliable, internationally-accepted criteria, despite contention that its optimal structure may comprise multiple distinct entities labelled ‘schizophrenia’.
Transcultural psychiatry studies that have examined the expression of schizophrenia across cultures have implicitly favoured the prevailing view that schizophrenia is universal, and consequently, that its structure is continuous.
Transethnic schizophrenia samples can inform debate at every level of the diagnostic spectrum: (1) broad theoretical (universalist vs. relativist); (2) diagnostic (nosological vs. dimensional); (3) structure of psychosis generally; and (4) structure of schizophrenia specifically. While transethnic samples have been used (primarily from an anthropological perspective) to challenge DSM-IV at the two broader levels, little work has elucidated the more specific levels. Few transethnic psychiatric studies have incorporated current diagnostic knowledge to explore the expression of demographic, clinical, and symptom variables and inform debate regarding schizophrenia’s classification.
This thesis aims to: (1) compare and contrast the way ‘schizophrenia’ is experienced by three ethnically different populations, (2) identify significant demographic, clinical and symptom differences in these populations, (3) examine these differences in the context of their relevance to the structure of schizophrenia, and (4) identify dimensions and/or clusters based on these differences, both within and across these populations, which may contribute to the discourse on the diagnostic classification of schizophrenia.
Demographic, clinical and symptom variables were analysed, and frequencies of core DSM-IV schizophrenia diagnostic criteria were contrasted in ethnically-distinct schizophrenia/schizoaffective samples from Australia (n=821), Chennai, India (n=520) and the Iban of Sarawak (n=298). Statistical methods used included χ2, T-Tests, General Linear Models and Logistic Regression. Exploratory Factor Analysis, Latent Class Analysis, and Factor Mixture Modeling were used to attempt to identify deficit schizophrenia (DS), which has been proposed as a stable, distinct schizophrenia subtype, in each sample, and the results were then tested using taxometric analyses.
Significant differences in demographic and clinical characteristics were identified between sites: (1) more individuals were living alone in Australia than India or Sarawak; (2) drug use was lower in India than Australia or Sarawak; (3) duration of untreated psychosis (DUP) was longer in India than Australia or Sarawak; (4) the rate of schizoaffective disorder was lower in India than Australia or Sarawak; and (5) mean age at psychosis onset (AAO) was approximately six years older in Sarawak than Australia or India.
More broadly, a distinct schizophrenia symptom profile was identified in the Sarawak sample. Compared with Australian and Indian populations, the Iban exhibit: low frequency of thought broadcast/insertion/withdrawal delusions, high frequency of auditory hallucinations and disorganized behaviour, with a comparatively short prodrome.
Diagnostically, differences in both DSM-IV ‘criterion A’ symptom composition and content were apparent between sites. Indian individuals with schizophrenia reported negative symptoms more frequently than those in Australia or Sarawak, whereas individuals from Sarawak reported disorganized symptoms more frequently than other sites, in particular India. Delusions of control and thought broadcast, insertion or withdrawal were less frequent in Sarawak than Australia. Curiously, a subgroup of 20 Indian individuals with schizophrenia reported no lifetime delusions or hallucinations. Nine members of this subgroup no longer meet the criteria for schizophrenia when diagnosed using DSM-5 criteria.
When the DS subtype was modelled in the three samples, there was broad consistency in the structural appearance of the best-fitting models, with both single class (no evidence of a distinct DS class) and two class (demarcation of a potential DS class) models performing well. A category was identified within the Indian and Sarawak samples that resembled DS, while the distinction in Australia was less clear-cut. Taxometric analyses suggested a two class distribution within each population, with a larger ‘deficit’ class at each site.
Overall, these results support schizophrenia not being a discrete, homogeneous condition. Some elements of individual experience and expression (individual socio-demographic and symptom variables, symptom profiles, diagnostic demarcations) differ between the three ethnically-distinct populations, whereas other elements (e.g. tentative evidence for a class resembling DS) appear stable across these samples. There is also evidence that some expressions are population-specific, for example the Indian subgroup (n=20) without positive symptoms.
Whereas many differences in clinical phenotype can be explained by cultural factors, the older AAO finding in the Iban is a promising candidate for genetic studies in ethnically-distinct populations, since the result is somewhat culturally counter-intuitive.
Evidence generally supporting the universality of DS, albeit with a hybrid structure across these three ethnically-distinct populations contributes to the discourse on the latent structure and diagnostic classification of ‘schizophrenia’.
Viewed in conjunction with evidence from recent genetic analyses, and mindful of the significant limitations on generalisability imposed by applying standardised assessment tools and diagnostic classifications across ethnic groups, these results expand our understanding of the nuances of schizophrenia and highlight the potential for comparing and contrasting transethnic schizophrenia samples to validate genetic clues, in order to better understand clinical heterogeneity.