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Schizophrenia is associated with significant cognitive impairment. However, the pathophysiological mechanisms underlying cognitive dysfunction in schizophrenia remain unclear. Brain-derived neurotrophic factor (BDNF) and C-reactive protein (CRP) are among the most commonly investigated peripheral markers of cognition in schizophrenia.
A systematic review in PubMed and Scopus databases was performed until 31 January 2019 to assess the relationship between cognitive impairment, CRP and BDNF levels in schizophrenia. A random-effects meta-analysis was conducted.
Current meta-analysis included 21 studies including 2449 patients with schizophrenia-spectrum disorders. Overall, both BDNF [r = 0.12, confidence interval (CI) 0.04–0.19] and CRP (r = −0.13, CI 0.08–0.18) levels were very modestly but significantly related to cognitive functioning in schizophrenia (r = 0.12, CI 0.04–0.19). In meta-analyses of cognitive domains, BDNF levels were significantly associated with verbal memory (r = 0.16, CI 0.09–0.23), working memory (r = 0.14, CI 0.06–0.22), processing speed (r = 0.18, CI 0.10–0.26) and verbal fluency (r = 0.09, CI 0–0.18) performances. Elevated CRP levels were related to all cognitive domains (r = −0.09 to −0.13) except for fluency. Subgroup analyses suggested that the relationship between cognitive and BDNF levels were more pronounced in chronic samples.
Current findings suggest that cognitive impairment in schizophrenia is significantly related to elevated CRP and reduced BDNF levels in schizophrenia, particularly in chronic samples. However, small effect sizes of these correlations suggest that inflammation and decreased BDNF levels do not play a major role in cognitive dysfunction in most patients with schizophrenia. Further studies are needed to investigate the potential intermediating and confounding factors which can influence the level of relationship between inflammation, neurotrophic factors and cognition in schizophrenia.
Individuals with bipolar disorder (BD) have a higher prevalence of obesity and metabolic syndrome (MetS) compared with the general population. Obesity and MetS are associated with cognitive deficits and brain imaging abnormalities in the general population. Obesity and components of MetS might potentially associate with neuroimaging and neurocognitive findings in BD.
A literature search of studies investigating the association between obesity (and other components of MetS) and neurocognitive and neuroimaging findings in BD was conducted. In addition to a systematic review, a random-effects meta-analysis was conducted when sufficient data were available.
Twenty-three studies were included in the current systematic review. Overweight/obese patients were significantly associated with impaired neurocognition compared normal weight individuals with BD (d = 0.37). The most robust association between obesity and cognitive deficits in BD was observed in the cognitive subdomain of executive functions (d = 0.61). There was also evidence for a significant relationship between cognitive impairment in BD and other components of MetS including hypertension, dyslipidemia, and diabetes. Overweight/obese individuals with BD had more pronounced brain imaging abnormalities than normal weight individuals with BD.
Obesity and related cardiovascular risk factors significantly are associated with more severe cognitive and brain imaging abnormalities in BD. Medical co-morbidities can potentially contribute to functional decline observed in some patients throughout the course of BD.
Cognitive functioning in affective psychosis and schizoaffective disorder is much less studied compared with schizophrenia.
To quantitatively undertake a meta-analysis of the available data that directly compares cognitive functioning across schizophrenia, schizoaffective disorder and affective psychosis.
Following a thorough literature review, 31 studies that compared the performances of people with schizophrenia (1979 participants) with that of those with affective psychosis or schizoaffective disorder (1314 participants) were included. To determine the effect of demographic and clinical confounders, meta-regression and subgroup analyses were conducted.
In 6 of 12 cognitive domains, people with schizophrenia performed worse than people with schizoaffective disorder or affective psychosis. However, the between-group differences were small and the distribution of effect sizes showed substantial heterogeneity. The between-group differences were driven by a higher percentage of males, more severe negative symptoms and younger age at onset of illness in the schizophrenia samples.
Neuropsychological data do not provide evidence for categorical differences between schizophrenia and other groups. However, a subgroup of individuals with schizophrenia who have more severe negative symptoms may be cognitively more impaired than those with affective psychosis/schizoaffective disorder.
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