Inflammation as a neurobiological substrate of cognitive impairment in bipolar disorder: Evidence, pathophysiology and treatment implications

- Bipolar Disorder (BD) is associated with a chronic, low-grade inflammatory state.
- Elevated levels of inflammatory markers are associated with cognitive dysfunction in BD.
- Several biologically plausible pathways have been identified to explain this association.
- Anti-inflammatory agents may have potential for improving cognition in BD.
- Future studies are needed to determine the role of inflammation and anti-inflammatory agents in BD.

BackgroundBipolar disorder (BD) has been associated with cognitive impairment during depressed, manic and euthymic periods. Inflammation has been shown to be involved in the pathophysiology of BD and cognitive impairment.MethodsFor this systematic review, the MEDLINE/PubMed, Embase, Google Scholar and ClinicalTrials.gov databases were searched for relevant articles assessing the association between cognitive function and inflammatory markers in BD subjects. A discussion of potential mechanisms and therapeutic implications is also included to provide further context to the subject matter.ResultsEight studies, including a total of 555 BD subjects, assessing the association between cognitive function and inflammatory markers were identified. Cognitive dysfunction was associated with elevated levels of pro-inflammatory markers YKL40, IL-6, sCD40L, IL-1Ra, hsCRP and TNF-α. Mechanistically, elevation in inflammatory cytokines alters monoamine levels leading to cognitive and affective dysfunction. Neuro-inflammation, manifesting as microglial activation, leads to increased oxidative stress, pathologic synaptic pruning and impaired neuroplasticity in key brain regions sub-serving mood and cognition. Immune dysfunction also activates the hypothalamic-pituitary-adrenal (HPA) axis leading to hypercortisolemia and metabolic dysfunction, further promoting neuronal dysfunction. Anti-inflammatory agents are therefore currently being investigated in the treatment of BD and appear to exert an antidepressant effect; however, cognitive outcomes have yet to be reported.ConclusionSeveral studies suggest that immune dysfunction is associated with cognitive impairment in BD. Several neurobiological pathways have been identified whereby immune dysfunction may promote cognitive impairment in BD. Future investigations of anti-inflammatory agents targeting cognitive function as a treatment outcome are merited.