Brain morphological alterations in bipolar disorder are well documented, particularly in chronic cases. This study focuses on first-episode mania (FEM) to quantify neuroanatomical changes at early stages of the disorder.

To assess deviations from normative brain morphometry and age-related brain changes in patients with FEM.

Pretrained models, based on large, independent healthy samples, were applied to structural brain images from FEM patients (n = 83) and healthy individuals (n = 61). Normative deviation z-scores were computed for regional brain morphometry, along with global and voxel-level brain–age-gap estimates (G-brainAGE and L-brainAGE, respectively). The proportions of infranormal (z < −1.96) and supranormal (z > 1.96) deviations were measured for both groups. Ridge regression and support vector machine models were used to evaluate whether z-scores predicted symptom severity, IQ or diagnosis. Case-control differences in L-brainAGE and correlations between G-brainAGE and clinical features were analysed.

Both FEM and healthy individuals showed similar proportions of infra- and supranormal deviations in regional measures. Morphometric data, whether observed or normative, did not significantly predict clinical outcomes or diagnosis. Mean G-brainAGE in FEM was −1.04 (s.d. 3.26) years and negatively correlated with age of onset, while L-brainAGE did not differ significantly between groups.

Regional morphometry and local brain-ageing metrics in FEM patients aligned with normative ranges, suggesting minimal abnormalities in early bipolar disorder. However, subtle delays in global brain ageing may reflect variation based on the age of onset, highlighting a potential area for further exploration.

Two established staging models outline the longitudinal progression in bipolar disorder (BD) based on episode recurrence or inter-episodic functioning. However, underlying neurobiological mechanisms and corresponding biomarkers remain unexplored. This study aimed to investigate if global and (sub)cortical brain structures, along with brain-predicted age difference (brain-PAD) reflect illness progression as conceptualized in these staging models, potentially identifying brain-PAD as a biomarker for BD staging.

In total, 199 subjects with bipolar-I-disorder and 226 control subjects from the Dutch Bipolar Cohort with a high-quality T1-weighted magnetic resonance imaging scan were analyzed. Global and (sub)cortical brain measures and brain-PAD (the difference between biological and chronological age) were estimated. Associations between individual brain measures and the stages of both staging models were explored.

A higher brain-PAD (higher biological age than chronological age) correlated with an increased likelihood of being in a higher stage of the inter-episodic functioning model, but not in the model based on number of mood episodes. However, after correcting for the confounding factors lithium-use and comorbid anxiety, the association lost significance. Global and (sub)cortical brain measures showed no significant association with the stages.

These results suggest that brain-PAD may be associated with illness progression as defined by impaired inter-episodic functioning. Nevertheless, the significance of this association changed after considering lithium-use and comorbid anxiety disorders. Further research is required to disentangle the intricate relationship between brain-PAD, illness stages, and lithium intake or anxiety disorders. This study provides a foundation for potentially using brain-PAD as a biomarker for illness progression.