The global cognitive impairment in schizophrenia: consistent over decades and around the world

Abstract

Objective: Schizophrenia results in cognitive impairments as well as positive, negative, and disorganized symptomatology. The present study examines the extent to which these cognitive deficits are generalized across domains, potential moderator variables, and whether the pattern of cognitive findings reported in schizophrenia has remained consistent over time and across cultural and geographic variation.

Method: Relevant publications from 2006 to 2011 were identified through keyword searches in PubMed and an examination of reference lists. Studies were included if they (1) compared the cognitive performance of adult schizophrenia patients and healthy controls, (2) based schizophrenia diagnoses on contemporary diagnostic criteria, (3) reported information sufficient to permit effect size calculation, (4) were reported in English, and (5) reported data for neuropsychological tests falling into at least 3 distinct cognitive domains. A set of 100 non-overlapping studies was identified, and effect sizes (Hedge's g) were calculated for each cognitive variable.

Results: Consistent with earlier analyses, patients with schizophrenia scored significantly lower than controls across all cognitive tests and domains (grand mean effect size, g=-1.03). Patients showed somewhat larger impairments in the domains of processing speed (g=-1.25) and episodic memory (g=-1.23). Our results also showed few inconsistencies when grouped by geographic region.

Conclusions: The present study extends findings from 1980 to 2006 of a substantial, generalized cognitive impairment in schizophrenia, demonstrating that this finding has remained robust over time despite changes in assessment instruments and alterations in diagnostic criteria, and that it manifests similarly in different regions of the world despite linguistic and cultural differences.

Keywords: Cognition; Memory; Meta-analysis; Neuropsychology; Processing speed; Schizophrenia.

Figure 1

Selected effect sizes for impairment in schizophrenia relative to control performance. Neuropsychological domain names are labeled in capital letters with black diamonds as effect size markers. Selected individual variables in each domain are marked with small black circles.