A Critical Review on Structural Neuroimaging Studies in BD: a Transdiagnostic Perspective from Psychosis to Fronto-Temporal Dementia

In the last decades, many neuroimaging studies have investigated the brain structural deficits associated with the pathophysiology of Bipolar Disorder (BD). Notably, the available literature identified the presence of selective brain structural abnormalities, in terms of both gray matter (GM) and white matter, known to be involved in emotion regulation and processing. Nonetheless, the complex and heterogeneous nature of BD has not yet allowed the identification of a clear biological signature of this disorder. In this context, this review aims to a) briefly summarize the evidence from structural Magnetic Resonance Imaging (MRI) studies on BD in order to provide a clearer picture of the neural circuits involved in this disabling mental illness, and b) describe some critical clinical issues that need to be taken into account when studying BD, especially the history of psychosis and the link between BD and behavioral variant fronto-temporal dementia (bvFTD). Evidence from structural MRI studies on BD suggests ventricular, prefrontal and temporal abnormalities but remain largely inconsistent, possibly reflecting the disorder complexity. Moreover, the reviewed evidence showed that psychotic BD might represent a subtype of the disorder with specific and enhanced alterations of GM morphology. Finally, from the available MRI evidence exploring the relationship between BD and bvFTD emerged the need of a differential identification of biological markers to enable the early and accurate detection of the two disorders, ultimately guaranteeing a better prognosis and the employment of more targeted and effective treatments.