Cytokines and C-reactive protein alterations with respect to cognitive impairment in schizophrenia and bipolar disorder: A systematic review.

BACKGROUND

The aim of this article was to perform a systematic review of studies investigating the association between peripheral levels of cytokines and C-reactive protein (CRP), cytokine gene polymorphisms and cognition in patients with schizophrenia and bipolar disorder (BD).

METHODS

The following databases: PubMed, CINAHL Complete, Academic Search Complete, ERIC and Health Source: Nursing/Academic Edition databases were searched according to the PRISMA guidelines. We included studies that investigated the association between peripheral levels of CRP and cytokines, cytokine gene polymorphisms and cognitive performance in schizophrenia and/or BD patients. Subsequently, quality assessment of eligible publications was performed. Results were synthesized by discussing main findings around correlations between inflammatory markers and cognition.

RESULTS

Most consistent results indicate worse cognitive performance in schizophrenia patients with higher CRP levels. Less consistent evidence suggests better cognitive functioning of schizophrenia patients with higher levels of tumour necrosis factor-α (TNF-α). Evidence for the involvement of other cytokines in cognitive impairment in patients with schizophrenia is less convincing due to discordant results or scarcity of studies. Due to low number of studies, it is difficult to draw conclusions on the involvement of CRP and cytokine alterations in the development of cognitive deficits in BD. Single studies suggest the role of CRP, interleukin(IL)-1 receptor antagonist, IL-6 and TNF-α with its receptors in the development of cognitive impairment in BD.

CONCLUSIONS

Peripheral inflammation might be related to cognitive deficits in schizophrenia and BD. Unequivocal conclusions cannot be made due to methodological heterogeneity and low number of studies investigating particular cytokines.