Trait- and state-dependent cortical inhibitory deficits in bipolar disorder

Objectives Euthymic patients with bipolar disorder (BD) have deficits in cortical inhibition. However, whether cortical inhibitory deficits are trait‐ or state‐dependent impairments is not yet known and their relationship with psychiatric symptoms is not yet understood. In the present study, we examined trait‐ and state‐dependent cortical inhibitory deficits and evaluated the potential clinical significance of these deficits. Methods Nineteen patients with bipolar I disorder were evaluated using the paired‐pulse transcranial stimulation protocol, which assessed cortical inhibition during an acute manic episode. Cortical inhibition measures were compared with those obtained in 28 demographically matched healthy controls. A follow‐up assessment was performed in 15 of these patients three months later, when there was remission from their mood and psychotic symptoms. The association between cortical inhibitory measures and severity of psychiatric symptoms was also studied. Results During mania, patients showed decreased short‐interval intracortical and transcallosal inhibition, as well as a normal cortical silent period and long‐interval cortical inhibition. These findings were the same during euthymia. Symptoms associated with motor hyperactivity were correlated negatively with the degree of cortical inhibition. These correlations were not significant when a Bonferroni correction was applied. Conclusions The present longitudinal study showed cortical inhibitory deficits in patients with BD, and supports the hypothesis that cortical inhibitory deficits in BD are trait dependent. Further research is necessary to confirm the clinical significance of these deficits.