Comparisons of methamphetamine psychotic and schizophrenic symptoms: A differential item functioning analysis

Abstract

The concept of negative symptoms in methamphetamine (MA) psychosis (e.g., poverty of speech, flatten affect, and loss of drive) is still uncertain. This study aimed to use differential item functioning (DIF) statistical techniques to differentiate the severity of psychotic symptoms between MA psychotic and schizophrenic patients. Data of MA psychotic and schizophrenic patients were those of the participants in the WHO Multi-Site Project on Methamphetamine-Induced Psychosis (or WHO-MAIP study) and the Risperidone Long-Acting Injection in Thai Schizophrenic Patients (or RLAI-Thai study), respectively. To confirm the unidimensionality of psychotic syndromes, we applied the exploratory and confirmatory factor analyses (EFA and CFA) on the eight items of Manchester scale. We conducted the DIF analysis of psychotic symptoms observed in both groups by using nonparametric kernel-smoothing techniques of item response theory. A DIF composite index of 0.30 or greater indicated the difference of symptom severity. The analyses included the data of 168 MA psychotic participants and the baseline data of 169 schizophrenic patients. For both data sets, the EFA and CFA suggested a three-factor model of the psychotic symptoms, including negative syndrome (poverty of speech, psychomotor retardation and flatten/incongruous affect), positive syndrome (delusions, hallucinations and incoherent speech) and anxiety/depression syndrome (anxiety and depression). The DIF composite indexes comparing the severity differences of all eight psychotic symptoms were lower than 0.3. The results suggest that, at the same level of syndrome severity (i.e., negative, positive, and anxiety/depression syndromes), the severity of psychotic symptoms, including the negative ones, observed in MA psychotic and schizophrenic patients are almost the same.

Introduction

Methamphetamine (MA) psychosis is a state of MA intoxication with psychotic symptoms, commonly presented with delusions and hallucinations. This psychotic condition has been considered as a common and serious consequence of chronic, high dose, and/or continuous use of MA (Griffith et al., 1972). It is commonly described as closely simulating paranoid schizophrenia (Bell, 1965).

The similarities in many aspects of MA psychosis and schizophrenia have made amphetamine a primary psychotomimetic model agent in schizophrenia research. It is likely that the resemblance is caused by the altered function of mesolimbic dopamine systems and prefrontal cortical function (Robinson and Becker, 1986).

Although clinical research is necessary for the development of MA psychosis services, few studies in this area have been carried out. Of many respects, symptom studies may be a priority area because these results are basic knowledge for further studies of MA psychosis, e.g., etiology, course, prognosis, and treatment. So far, most of the findings on the resemblance between MA psychosis and schizophrenia have mainly focused on positive psychotic symptoms, in particular delusions and hallucinations.

The concept of negative symptoms in MA psychosis (e.g., poverty of speech, flatten affect, and loss of drive) is not new but still uncertain. Blunted affect and diminished spontaneity accompanied with paranoid-hallucinatory state were described in 1960s (Yui et al., 2000). In a study of 11 MA psychotic inpatients assessed by using the Scale for the Assessment of Negative Symptoms, Japanese investigators found that most patients showed a significant impairment on avolition-apathy, anhedonia-asociality, and attentional impairment (Tomiyama, 1990). Srisurapanont et al. (2003) found negative psychotic symptoms in 21.4% of 168 MA psychotic patients with various ethnicity (Srisurapanont et al., 2003) In this later study, the results of an exploratory factor analysis (EFA) also showed an independent syndrome of negative symptoms with a variance of 26.6%. Despite the above-mentioned evidence, negative symptoms of MA psychosis are still viewed as less severe and/or prevalent (Zorick et al., 2008).

Differential item functioning (DIF) statistical techniques are based on the principle that if different groups of patients (e.g., males vs. females) have the same level of disease severity (e.g., depression, psychosis), they should be rated or response similarly on an individual rating item of a measure, regardless of group membership. DIF assessment proceeds by controlling for an estimate of the underlying construct and then examining whether individuals in different groups have a similar distribution of responses to a particular item. The statistical techniques that directly compare item severity between groups are inferior to DIF analyses by the fact that the difference found by a direct comparison may not be a true phenomenological difference. It may be just reflective of greater overall symptom severity in one group versus another. In contrast, the existence of DIF between different groups on a symptom indicates the real difference of symptom severity because the overall disease or syndrome severity has been controlled.

Recently, we have finished two studies: (i) the WHO Multi-Site Project on Methamphetamine-Induced Psychosis (WHO-MAIP study), a cross-sectional observation study aiming to evaluate MA psychosis (Ali et al., 2006, Ali et al., 2010, Srisurapanont et al., 2003), and (ii) Risperidone Long-Acting Injection in Thai Schizophrenic Patients (RLAI-Thai study), a three-month, non-randomized, open-label, single-arm study of risperidone long-acting injection in Thai individuals with schizophrenia (Arunpongpaisal et al., 2010). The psychotic symptoms of participants in both studies were assessed using the Manchester scale (Krawiecka et al., 1977). Because the symptoms of these two studies were comparable, we proposed to apply the DIF statistical techniques to differentiate the severity of psychotic symptoms between these populations.