Individuals with a family history of bipolar disorder are at increased risk of developing affective psychopathology. Longitudinal imaging studies in young people with familial risk have been limited, and cortical developmental trajectories in the progression towards illness remain obscure.

To establish high-resolution longitudinal differences in cortical structure that are associated with risk of bipolar disorder.

Using structural magnetic resonance imaging data from 217 unrelated ‘Bipolar Kids and Sibs study’ participants (baseline n = 217, follow-up n = 152), we examined changes over a 2-year period in cortical area, thickness and volume, measured at each vertex across the cortical surface. Groups comprised 105 ‘high-risk’ participants with a first-degree relative with bipolar disorder (female n = 64; age in years: M (mean) = 20.9, s.d. = 5.5) and 112 controls with no familial psychiatric history (females n = 60; age in years: M = 22.4, s.d. = 3.7).

Accelerated thickness and volume reductions over time were observed in ‘high-risk’ individuals across multiple cortical regions, relative to controls, including right lateral orbitofrontal thickness (β = 0.033, P < 0.001) and inferior frontal volume (β = 0.021, P < 0.001). These differences were observed after controlling for age, sex, ancestry, current medication status, lifetime psychiatric diagnoses and measures of gross brain morphology.

Longitudinal group differences suggest the presence of thicker cortex in familial ‘high-risk’ individuals at earlier developmental stages, followed by accelerated thinning towards the typical age of bipolar disorder onset. Future examination of genetic and environmental components of familial risk and the mechanistic nature (pathological or protective) of cortical-trajectory differences over time may facilitate the identification of prodromal biomarkers and opportunities for early clinical intervention.

Obesity is highly prevalent and disabling, especially in individuals with severe mental illness including bipolar disorders (BD). The brain is a target organ for both obesity and BD. Yet, we do not understand how cortical brain alterations in BD and obesity interact.

We obtained body mass index (BMI) and MRI-derived regional cortical thickness, surface area from 1231 BD and 1601 control individuals from 13 countries within the ENIGMA-BD Working Group. We jointly modeled the statistical effects of BD and BMI on brain structure using mixed effects and tested for interaction and mediation. We also investigated the impact of medications on the BMI-related associations.

BMI and BD additively impacted the structure of many of the same brain regions. Both BMI and BD were negatively associated with cortical thickness, but not surface area. In most regions the number of jointly used psychiatric medication classes remained associated with lower cortical thickness when controlling for BMI. In a single region, fusiform gyrus, about a third of the negative association between number of jointly used psychiatric medications and cortical thickness was mediated by association between the number of medications and higher BMI.

We confirmed consistent associations between higher BMI and lower cortical thickness, but not surface area, across the cerebral mantle, in regions which were also associated with BD. Higher BMI in people with BD indicated more pronounced brain alterations. BMI is important for understanding the neuroanatomical changes in BD and the effects of psychiatric medications on the brain.