Functional neuroimaging studies have implicated the hippocampus formation in the pathophysiology of bipolar disorder, but findings from volumetric studies have been less consistent. The authors aim to further investigate the existence of volumetric abnormalities in the hippocampus of individuals with bipolar disorder. In addition to methodological inconsistencies, many previous studies have been lacking clinical robustness with respect to characterizing bipolar patients and comparison subjects. Hence, the present study matched the groups closely across a number of demographic parameters. Using MRI, hippocampal volumes of 24 bipolar patients were compared to 24 sex-, age-, and education-matched comparison subjects, and these findings were further investigated in relation to both illness and treatment factors. A significantly larger (8.5%) right hippocampus was seen in bipolar patients than in comparison subjects, and this difference was not associated with a history of psychosis, familial illness, or lithium treatment, after controlling for potential confounds. Patients reporting fewer affective episodes did however have significantly larger left hippocampus volumes than comparison subjects. The authors found that the left hippocampus was larger in a group of adult bipolar subjects relative to the healthy comparison group. The reason for this is unclear, but in this sample, it was not associated with family history, psychotic features, or medication exposure. A negative association was found between left hippocampal volume and number of episodes or duration of illness, suggesting the hippocampus might be larger in the early phase of bipolar disorder but becomes smaller with time.
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