Brain Magnetic Resonance Imaging of Structural Abnormalities in Bipolar Disorder

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Abstract
ALTHOUGH BIPOLAR disorder is a common psychiatric illness that causes considerable morbidity and mortality,1-3 its neuropathogenesis is poorly understood. Because mood dysregulation is the defining symptom of bipolar disorder, the neuroanatomic substrates of this illness likely include neural pathways that modulate emotional function. Indeed, numerous cases have been reported4-7 of affective syndromes developing following focal brain injuries. Specifically, lesions involving left prefrontal cortical or basal ganglial regions are associated with secondary depression, whereas secondary mania is more commonly associated with lesions of the orbitofrontal and basotemporal cortices, the head of the caudate, and the thalamus.4-7 Recent studies8-10 of healthy volunteers found activation in these same brain regions in response to induced mood states. Other brain regions, such as the amygdala-hippocampal complex, were also implicated8-10 in controlling induced emotion in these studies. Integrating human studies with studies of animals, investigators11-14 have proposed a neuroanatomic model of mood regulation involving 2 interconnected brain circuits: a limbic (amygdala)-thalamic-prefrontal cortical circuit and a limbic-striatal-pallidal-thalamic circuit. Disruptions in these pathways may contribute to the pathological mood states and neurovegetative symptoms of bipolar disorder.