Size and shape of the caudate nucleus in individuals with bipolar affective disorder

Objective: The caudate nucleus (CN) is a crucial component of the ventral striatum, which is part of a prefrontal-striatal-thalamic circuit that is modulated by limbic structures to subserve emotional processing. Bipolar disorder is thought to be underpinned by dysfunctional anterior limbic networks, although MRI studies examining the CN have shown equivocal results. As gross volumetric analyses may not detect subtle regional change, we aimed to clarify the role of the CN in bipolar disorder by undertaking shape analysis to detect regional reductions. Methods: The CN was manually traced on MRI scans from 27 patients with bipolar-I disorder and 24 matched controls. A non-parametric spherical harmonic shape analysis was undertaken using the SPHARM toolkit. Results: Whilst the left CN volume was consistently larger in the sample, there was no effect of group or gender or significant interactions between these variables. Volume did not correlate with illness duration or lithium dosage, but was larger in those with a history of psychosis at trend level. However, left caudate shape differed significantly between groups, with deflation in an area along the ventromedial surface (connecting to dorsolateral prefrontal regions) in bipolar patients. Psychotic patients showed increases in the dorsal head and body at trend level overall, in regions connecting to medial and orbitofrontal regions. Conclusions: These findings suggest that subtle rather than gross structural changes occur in the CN, which may not be detectable by volumetric analysis alone, and reflect alterations in specific frontostriatal circuitry in the disorder.