Processing Speed Impairment in Schizophrenia: An Updated Systematic Review and Meta-Analysis.

Importance: Cognition is impaired in people with schizophrenia, affecting quality of life and functioning. Therefore, it is important to understand and characterize this impairment. Objective: To update and revisit the evidence for a central processing speed impairment in people with schizophrenia and examine the factors that moderate this impairment. Data Sources: Articles were identified through the PubMed and PsycINFO databases from February 1, 2009, through November 2, 2023. Study Selection: Studies were included if they reported on a symbol coding test and at least 2 additional cognitive tests from 2 other cognitive domains, contrasted people with schizophrenia to controls, used contemporary diagnostic criteria, included sufficient detail to calculate Hedges g effect sizes, and were reported in English. Of 4530 identified articles, 115 studies met inclusion criteria. Data Extraction and Synthesis: This study followed the Preferred Reporting Items for Systematic Review and Meta-analyses (PRISMA) and Meta-Analysis of Observational Studies in Epidemiology (MOOSE) reporting guidelines. Means, SDs, and sample sizes were extracted for all cognitive tests that appeared in at least 3 of the 115 studies. Data were entered and visually checked by independent extractors. Data were generally pooled using random-effects models, except when specified. Measures of homogeneity (Q and I2) and publication bias (fail-safe N and funnel plots) were also examined. Main Outcomes and Measures: The primary outcome was the degree of cognitive impairment (Hedges g) observed for people with schizophrenia in 50 cognitive tests, focusing on symbol coding tests of processing speed. Further, this study aimed to identify clinical and study characteristics that moderate the degree of symbol coding impairment. Results: Data were available for 10 114 people with schizophrenia and 13 235 controls from 115 studies. Symbol coding tasks were among the most impaired (g = -1.52; 95% CI, -1.65 to -1.40) but did not reliably differ from 15 other tests. Intelligence quotient and age difference from controls, composition of sex assigned at birth, inpatient status, and whether the sample included schizoaffective and schizophreniform diagnoses all moderated the degree of symbol coding impairment. Conclusions and Relevance: This meta-analysis provides insight into the consistency of the processing speed impairment for people with schizophrenia. Findings support that this impairment may be central to global cognitive impairments, which might be a consequence of altered brain connectivity.