Individualized Structural Brain Deviations in Bipolar I Disorder: a Normative Modeling Study of 4 660 ENIGMA Participants

Bipolar disorder (BD) is a prevalent psychiatric condition characterized by marked clinical and neuroanatomical heterogeneity. Prior neuroimaging studies have used case-control comparisons yielding small to moderate effect sizes for group differences. Large and diverse samples are needed to map heterogeneity within BD. Here, we used individualized brain morphometry deviations derived from normative modeling for three MRI-based measures (regional cortical thickness, regional surface area, and subcortical volumes). We applied this approach to a large international sample of 4 660 individuals (1 145 with bipolar I disorder and 3 515 healthy controls) from the ENIGMA Bipolar Disorder Working Group consortium (age range = 8–80, female = 53.15%) and examined the distribution of deviations within the bipolar disorder group, differences between healthy controls and individuals with bipolar disorder, and associations between neuroanatomical deviations and clinical variables. We found that individuals with bipolar disorder showed a higher proportion of localized extreme deviations, notably negative cortical thickness deviations in regions consistently reported as thinner in prior studies (the left superior frontal gyrus, bilateral inferior frontal gyrus (pars opercularis), left precuneus) and subcortical volume negative deviations in the left hippocampus. In addition, ROI-level deviations in the left middle temporal gyrus and the left inferior temporal gyrus were associated with longer illness duration. This is the largest study to date exploring individualized deviations as neuroanatomical features associated with bipolar disorder. Our findings suggest that neuroanatomical heterogeneity is a core feature of bipolar disorder, with preferential left-lateralized cortical-hippocampal involvement pointing to spatially structured heterogeneity.