Processing speed in bipolar disorder and relationship to peripheral inflammation: A 4-year longitudinal study

Bipolar disorder (BD) is associated with cognitive impairments, which can worsen with age. This study aimed to investigate longitudinal changes in processing speed among individuals with BD and the influence of peripheral inflammation on these trajectories. Data were collected from 69 BD participants and 101 age-matched healthy controls (HC) using a multi-cohort burst longitudinal design. Processing speed was assessed three times annually using the Brief Assessment of Cognition in Schizophrenia (BACS) Symbol Coding task. Baseline peripheral inflammation was measured via IL-6, TNF-α, and C-reactive protein. Mixed-effects models were used to examine the relationship between cognitive changes and inflammation. The BD group exhibited slower processing speeds compared to the HC group across all time points (β = -9.15, SE = 0.03, p < 0.001). There were no significant differences in the rate of cognitive change between the BD and HC groups; older age did not predict faster change, and age did not interact with group. Baseline levels of inflammation did not significantly predict cognitive change over time in the BD group, although interactions suggested that younger BD participants with higher inflammation might show more significant cognitive improvements than older or less inflamed counterparts (β = -0.03, SE = 0.01, p = 0.033). While individuals with BD show slower processing speeds relative to HC, this study did not find evidence to support accelerated cognitive decline in BD or a strong predictive role for peripheral inflammation. These findings suggest the need for further investigation into the complex relationships between inflammation and cognitive decline.