Reduced left hemispheric white matter volume in twins with bipolar I disorder

Background Although the heritability of bipolar I disorder (BPI) is high, few magnetic resonance imaging (MRI) studies of siblings of bipolar patients exist. We performed MRI brain scans on a nationwide sample of twins with BPI, as well as on their co-twins and a demographically balanced sample of control twin subjects, to detect any structural alterations related to the disorder and to the increased genetic risk. Methods The National Hospital Discharge Register, National Population Register, and Finnish Twin Cohorts were used to identify bipolar twins. Structured diagnostic interviews and MRI scans were obtained for 24 twins with BPI, 15 healthy co-twins, and 27 control twin subjects. Results Patients and co-twins showed a significant decrease in left hemispheric white matter volume. The disparity in patients was −16.1 cm3 (95% confidence interval [CI] −26.6, −5.6) and in co-twins −11.3 cm3 (95% CI −22.1, −0.4) compared with control twin subjects. No gray matter decrease was seen in patients or co-twins. Conclusions The results of this first large-scale MRI study of twins with BPI, their co-twins, and appropriate control twin subjects, suggest that alterations of the left hemisphere white matter in BPI may reflect genetic factors predisposing to the disorder.