The specificity of morphological changes of the corpus callosum in schizophrenia and related major mental disorders

Schizophrenia is a disabling major mental illness associated with marked impairments in reality testing, organization of speech and behaviour and cognition. Significant evidence points to functional dysconnectivity between cortical and subcortical regions as the major pathophysiological underpinning of the symptoms and disability associated with schizophrenia. Modern neuroimaging techniques have suggested that this dysconnectivity is driven, at least partially, by neuroanatomical changes to connectivity in the brain at the level of white matter tracts, the main connecting “organs” in the brain. This thesis describes the analysis of the structure of the corpus callosum, the brain’s largest white matter fibre tract, with the aim of determining if changes to anatomical connectivity in schizophrenia are associated with a unique callosal shape “signature”. This was undertaken by using a shape analysis methodology that examined regional callosal thickness, using a non-parametric permutation method to determine between-group differences and the relationship between illness variables and callosal shape. This methodology was applied to multiple illness stages: established illness, first-episode psychosis and pre-psychotic patients. It was then applied to other major mental disorders, including multiple cohorts of patients with bipolar disorder and patients with major depression, to determine if any changes seen in schizophrenia patients were specific to schizophrenia-spectrum illness or were more general markers of major mental illness. The results suggest that patients with schizophrenia-illness show specific thickness reductions at the level of the anterior callosum, connecting frontal cortical regions, that are present during the pre-psychotic phase and with first-episode illness. Furthermore, with established illness, these changes are accompanied by additional changes in the callosum connecting cingulate, temporal and parietal regions. Changes seen in healthy individuals as part of the normal ageing process appeared to be disrupted in schizophrenia patients. In bipolar patients, a very different pattern of results emerged, with more global thickness reductions and disproportionate thinning at the level of the posterior callosum. Depressed patients, by contrast, showed state-specific posterior expansions, which bore some homology to changes seen in patients with depressed first-episode psychotic patients and patients with schizoaffective disorder. Furthermore, in the schizophrenia-spectrum group, changes at the level of the genu were strongly predictive of transition to psychosis in those individuals at high-risk for psychosis, and in first-episode individuals were highly predictive of long-term outcome of their psychotic illness. These changes suggest that there are schizophrenia-specific changes at the level of the callosum, marking a unique callosal “signature” for schizophrenia-spectrum illness. These changes show predictive validity for outcome at the earliest stages of illness, and are distinct from changes seen in major affective disorders. These findings suggest that shape changes to white matter structures may be a useful marker to aid diagnosis, in the identification of individuals who may develop a psychotic illness, and in defining the nature of their future illness course.