Brain structure in schizophrenia vs. psychotic bipolar I disorder: A VBM study
Introduction
Brain structural changes have been demonstrated in schizophrenia and bipolar disorder and interest has grown regarding areas of spatial overlap, in particular as both disorders share some clinical features and risk genes (Thaker, 2008). Given that brain structural changes are a putative endophenotype of schizophrenia and possibly bipolar disorder as well, the comparison of changes occurring in both disorders is of particular interest in understanding putative biomarker characteristics, for example with respect to specificity to a particular disorder or symptom characteristics.
Initial comparative studies have suggested that patients with schizophrenia might show volume loss in middle prefrontal and thalamic regions (McIntosh et al., 2004), and in total hippocampal volume (McDonald et al., 2006). Another study suggested more widespread prefrontal and temporal grey matter loss in schizophrenia, but not in bipolar disorder, for which sparing of cortical changes was observed (McDonald et al., 2005).
Subsequently, studies applying voxel-based morphometry (VBM) have been conducted to compare patients with schizophrenia and bipolar disorder. There has been some support from the notion of these initial studies that fronto-temporal grey matter deficits are more extensive in schizophrenia than in (psychotic) bipolar disorder (Brown et al., 2011, Molina et al., 2011, Ivleva et al., 2012, Yuksel et al., 2012), but results have been rather inconclusive for the thalamus and hippocampus, which had been as further focus of earlier studies.
Another approach to compare brain structural changes has been to conduct meta-analyses of VBM studies in these disorders (Bora et al., 2012): the results indicate that the observed changes within the prefrontal areas may differ in location, with schizophrenia showing reductions in dorsolateral prefrontal cortex (DLPFC) and anterior cingulate cortex (ACC) vs. fronto-insular reductions in bipolar disorder. Another aspect of this meta-analysis, which has been highlighted by a subsequent review of meta-analyses by Crow et al., is the issue of laterality (Crow et al., 2013): these authors identified a pattern of diverging laterality, which might be explained by the gender ratio across studies. Indeed, the meta-analysis by Bora et al. shows that the changes in bipolar disorder mostly manifest in the right hemisphere (e.g. right fronto-insular cortex, right subgenual/medial prefrontal, and right ACC/medial prefrontal cortex), while changes in schizophrenia studies more frequently show changes in right hemisphere structures (Bora et al., 2012). Differences in gender composition, esp. higher rates of male patients with schizophrenia, who might show more severe illness, might be a contributing factor.
However, such meta-analyses are limited as they normally do not include direct comparison between schizophrenia and bipolar disorder groups, and because it is also difficult to control for the numerous smaller systematic differences across studies, such as gender imbalance, details of VBM pre-processing and analyses. In fact, the current meta-analyses (Ellison-Wright and Bullmore, 2010, Yu et al., 2010, Bora et al., 2012) do not take into account those original studies (mostly published after these meta-analyses), which directly compare schizophrenia vs. bipolar disorder cohorts, and they do not take into account the heterogeneity within the patient samples (esp. the bipolar cohorts), which often include both bipolar I and II patients, as well as those with and without psychotic symptoms.
In this study, we aimed to add new insight to the ongoing study of brain structural differences between schizophrenia and bipolar disorder. We focused on comparing schizophrenia patients with a subgroup of bipolar disorder patients with (previous) psychotic symptoms, using an updated version of VBM, as implemented in the VBM8 toolbox, and restricting recruitment of patient to those in remission. Based on the cited previous studies, we hypothesised that schizophrenia patients would show volume reductions in DLPFC, thalamus, and hippocampus, exceeding those seen in (euthymic) bipolar disorder patients.
Section snippets
Subjects
We included a total of 85 subjects, who had provided written informed consent to study protocols approved by the Ethics Committee of the Friedrich-Schiller-University Medical School and in accordance with the current version of the Declaration of Helsinki. We only included subjects able to provide consent to participate in compliance with the Declaration of Helsinki, as well as local and national regulations. Our study included three groups recruited from in-patient, day clinic, and out-patient
Results
The analysis of main effect of diagnosis, as shown in Fig. 1, Table 2 (A) revealed wide-spread effects in multiple frontal, temporal, and thalamic areas.
Contrasting Sz vs. healthy controls, the schizophrenia group showed several areas of cortical deficits, including right dorsolateral prefrontal, bilateral medial prefrontal, bilateral ventrolateral prefrontal and insular cortical areas, left medial temporal, as well as thalamus (bilateral), left superior temporal cortex, and minor right
Discussion
In this VBM study, we compared remitted schizophrenia patients with euthymic bipolar I disorder patients (with previous psychotic symptoms), and healthy controls using VBM. Our findings corroborate the assumption of widespread medial and lateral prefrontal grey matter losses in schizophrenia, but not in bipolar patients. In addition, we provide evidence for insular and thalamic reductions compared to controls, but only for schizophrenia and not bipolar patients.
As mentioned, there is
Role of funding source
The authors declare that the funding institutions had no influence on the analyses carried out and presented here.
Contributors
I.N. and C.G. designed the study.
I.N., K.L., M.D., St.S., and H.S., contributed to patient recruitment and scanning.
I.N., R.M., K.L., M.D., C.L., J.R.R., St.S., and C.G. contributed to data collection, processing, and pre-processing.
I.N., R.M., C.L., and C.G. contributed to implementation of the image processing pipeline and imaging data analysis.
I.N. wrote the first drafts of the manuscript and all authors commented on/approved the final version.
Conflicts of interest statement
The authors declare that they have no conflicts of interest, in particular no relevant financial interests. The funding institutions had no influence on the analyses carried out and presented here.
Acknowledgments
This study was in part supported by grants from the European Union (FP6; RTN “EUTwinsS”; local PIs: I.N. and H.S.), the IZKF Jena (project Nenadic J33), BMBF grants 01EV0709 and 01GW0740 (to C.G.), and a Junior Scientist Grant of the Friedrich-Schiller-University of Jena (to I.N.; DRMF 21007087).
We are grateful to all study participants, as well as to our student research assistants for their support in recruitment and scanning.
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