Corticolimbic metabolic dysregulation in euthymic older adults with bipolar disorder

The corticolimbic dysregulation hypothesis of bipolar disorder suggests that depressive symptoms are related to dysregulation of components of an anterior paralimbic network (anterior cingulate, anterior temporal cortex, dorsolateral prefrontal cortex, parahippocampal gyrus, and amygdala) with excessive anterior limbic activity accompanied by diminished prefrontal activity. In younger patients, such abnormalities tend to resolve with remission of depression, but it remains to be established whether the same is true for older patients. This was a cross-sectional, between-subjects design conducted with 16 euthymic, medicated patients with bipolar disorder (10 type I, six type II) and 11 age-matched healthy controls. All participants were over age 50. Our main outcome measures were relative rates of cerebral metabolism derived from a resting 18flourodeoxyglucose positron emission tomography scan in specified regions of interest in the corticolimbic network. Resting metabolic rates in bipolar patients were significantly greater than in controls in bilateral amygdalae, bilateral parahippocampal gyri, and right anterior temporal cortex (BA 20, 38); they were significantly lower in bipolar patients than in controls in the bilateral dorsolateral prefrontal cortices (BA 9, 10, 46). The evidence of corticolimbic dysregulation observed is consistent with the hypothesis that bipolar disorder entails progressive, pernicious neurobiological disruptions that may eventually persist during euthymia. Persistent corticolimbic dysregulation may be related to residual affective, behavioral, and cognitive symptoms in older patients with bipolar disorder, even when not experiencing syndromal mood disturbance.