Age at onset and cognition in schizophrenia: meta-analysis.

BACKGROUND The relationship between cognition and age at onset of schizophrenia is largely unknown. AIMS To compare cognitive deficits in individuals with youth-onset and late-onset schizophrenia with those in adults with first-episode schizophrenia. METHOD Twenty-nine databases (including EMBASE, MEDLINE and PsycINFO) were searched from 1980 to 2008. Selected publications had to include healthy controls and analyse separately individuals diagnosed with schizophrenia or a related disorder and individuals with first-episode, youth-onset or late-onset schizophrenia. Descriptive and cognitive data were extracted and the latter aggregated into 22 cognitive measures. Cohen's effect size raw and weighted means of cognitive deficits were generated and compared in the three groups. RESULTS Individuals with youth-onset and first-episode schizophrenia demonstrate large deficits (mean effect size >or=0.8) on almost all cognitive measures. Individuals with youth-onset schizophrenia demonstrate larger deficits than those with first-episode schizophrenia on arithmetic, executive function, IQ, psychomotor speed of processing and verbal memory. In contrast, those with late-onset schizophrenia demonstrate minimal deficits on arithmetic, digit symbol coding and vocabulary, but larger ones on attention, fluency, global cognition, IQ and visuospatial construction. CONCLUSIONS Individuals with youth-onset schizophrenia have severe cognitive deficits, whereas those with late-onset schizophrenia have some relatively preserved cognitive functions. This finding supports the view that severity of the disease process is associated with different ages at onset. In addition, the cognitive pattern of people with late-onset schizophrenia suggests that their deficits are specific rather than solely as a result of ageing and related factors.