Meta-analysis of magnetic resonance imaging studies of the corpus callosum in bipolar disorder

Reduced posterior corpus callosum area in suicidal and non-suicidal patients with bipolar disorder

BACKGROUND

Impulsivity is a characteristic of bipolar disorder (BD) that can contribute to the risk for suicidal behavior. Evidence suggests that gray and white matter abnormalities are linked with impulsivity, but little is known about the association between corpus callosum (CC) and impulsivity in BD. We examined the CC area and impulsivity in euthymic bipolar I patients, with and without lifetime history of suicide attempts, and in healthy controls.

METHODS

Nineteen bipolar patients with a suicide attempt history (BP-S), 21 bipolar patients without suicide attempt history (BP-NS), and 22 healthy controls (HC) underwent clinical assessment by the Structured Clinical Interview with the DSM-IV axis I (SCID-I), the Barratt Impulsiveness Scale (BIS-11), and MRI scan.

RESULTS

No differences were observed for any CC subregion between BP-S and BP-NS groups. There was a significant reduction in the genu (p=0.04) and isthmus areas (p=0.01), in bipolar patients compared with HC. In the BP-S group, the BIS-11 total (p=0.01), attention (p=0.001) and non-planning (p=0.02) impulsivity scores were significantly higher than in the BP-NS and HC groups.

LIMITATIONS

These results cannot establish causality because of the cross-sectional nature of the study.

CONCLUSION

This report potentially provides evidence that a reduction in the CC area is present even in non-symptomatic bipolar patients, which may be evidence of a biological trait marker for BD. Furthermore, the study demonstrated that BP-S group had higher impulsivity even during euthymia, which points to a sustained association between lifetime history of suicide attempts and impulsivity in BD.

Activity-dependent callosal axon projections in neonatal mouse cerebral cortex

Callosal axon projections are among the major long-range axonal projections in the mammalian brain. They are formed during the prenatal and early postnatal periods in the mouse, and their development relies on both activity-independent and -dependent mechanisms. In this paper, we review recent findings about the roles of neuronal activity in callosal axon projections. In addition to the well-documented role of sensory-driven neuronal activity, recent studies using in utero electroporation demonstrated an essential role of spontaneous neuronal activity generated in neonatal cortical circuits. Both presynaptic and postsynaptic neuronal activities are critically involved in the axon development. Studies have begun to reveal intracellular signaling pathway which works downstream of neuronal activity. We also review several distinct patterns of neuronal activity observed in the developing cerebral cortex, which might play roles in activity-dependent circuit construction. Such neuronal activity during the neonatal period can be disrupted by genetic factors, such as mutations in ion channels. It has been speculated that abnormal activity caused by such factors may affect activity-dependent circuit construction, leading to some developmental disorders. We discuss a possibility that genetic mutation in ion channels may impair callosal axon projections through an activity-dependent mechanism.

Effects of medication on neuroimaging findings in bipolar disorder: an updated review

OBJECTIVE

Neuroimaging is an important tool for better understanding the neurobiological underpinnings of bipolar disorder (BD). However, potential study participants are often receiving psychotropic medications which can possibly confound imaging data. To better interpret the results of neuroimaging studies in BD, it is important to understand the impact of medications on structural magnetic resonance imaging (sMRI), functional MRI (fMRI), and diffusion tensor imaging (DTI).

METHODS

To better understand the impact of medications on imaging data, we conducted a literature review and searched MEDLINE for papers that included the key words bipolar disorder and fMRI, sMRI, or DTI. The search was limited to papers that assessed medication effects and had not been included in a previous review by Phillips et al. (Medication effects in neuroimaging studies of bipolar disorder. Am J Psychiatry 2008; 165: 313-320). This search yielded 74 sMRI studies, 46 fMRI studies, and 15 DTI studies.

RESULTS

Medication appeared to influence many sMRI studies, but had limited impact on fMRI and DTI findings. From the structural studies, the most robust finding (20/45 studies) was that lithium was associated with increased volumes in areas important for mood regulation, while antipsychotic agents and anticonvulsants were generally not. Regarding secondary analysis of the medication effects of fMRI and DTI studies, few showed significant effects of medication, although rigorous analyses were typically not possible when the majority of subjects were medicated. Medication effects were more frequently observed in longitudinal studies designed to assess the impact of particular medications on the blood oxygen level-dependent (BOLD) signal. With a few exceptions, the observed effects were normalizing, meaning that the medicated individuals with BD were more similar than their unmedicated counterparts to healthy subjects.

CONCLUSIONS

The effects of psychotropic medications, when present, are predominantly normalizing and thus do not seem to provide an alternative explanation for differences in volume, white matter tracts, or BOLD signal between BD participants and healthy subjects. However, the normalizing effects of medication could obfuscate differences between BD and healthy subjects, and thus might lead to type II errors.

Genetic, morphometric, and behavioral factors linked to the midsagittal area of the corpus callosum

The corpus callosum is the main commissure connecting left and right cerebral hemispheres, and varies widely in size. Differences in the midsagittal area of the corpus callosum (MSACC) have been associated with a number of cognitive and behavioral phenotypes, including obsessive-compulsive disorders, psychopathy, suicidal tendencies, bipolar disorder, schizophrenia, autism, and attention deficit hyperactivity disorder. Although there is evidence to suggest that MSACC is heritable in normal human populations, there is surprisingly little evidence concerning the genetic modulation of this variation. Mice provide a potentially ideal tool to dissect the genetic modulation of MSACC. Here, we use a large genetic reference panel – the BXD recombinant inbred line – to dissect the natural variation of the MSACC. We estimated the MSACC in over 300 individuals from nearly 80 strains. We found a 4-fold difference in MSACC between individual mice, and a 2.5-fold difference among strains. MSACC is a highly heritable trait (h² = 0.60), and we mapped a suggestive QTL to the distal portion of Chr 14. Using sequence data and neocortical expression databases, we were able to identify eight positional and plausible biological candidate genes within this interval. Finally, we found that MSACC correlated with behavioral traits associated with anxiety and attention.

Grey matter differences in bipolar disorder: a meta-analysis of voxel-based morphometry studies

OBJECTIVE

Several neuroimaging studies have reported structural brain differences in bipolar disorder using automated methods. While these studies have several advantages over those using region of interest techniques, no study has yet estimated a summary effect size or tested for between-study heterogeneity. We sought to address this issue using meta-analytic techniques applied for the first time in bipolar disorder at the level of the individual voxel.

METHODS

A systematic review identified 16 voxel-based morphometry (VBM) studies comparing individuals with bipolar disorder with unaffected controls, of which eight were included in the meta-analysis. In order to take account of heterogeneity, summary effect sizes were computed using a random-effects model with appropriate correction for multiple testing.

RESULTS

Compared with controls, subjects with bipolar disorder had reduced grey matter in a single cluster encompassing the right ventral prefrontal cortex, insula, temporal cortex, and claustrum. Study heterogeneity was widespread throughout the brain; though the significant cluster of grey matter reduction remained once these extraneous voxels had been removed. We found no evidence of publication bias (Eggers p = 0.63).

CONCLUSIONS

Bipolar disorder is consistently associated with reductions in right prefrontal and temporal lobe grey matter. Reductions elsewhere may be obscured by clinical and methodological heterogeneity.

Interpreting magnetic resonance imaging findings in bipolar disorder

The episodic nature of bipolar disorder together with the ostensibly polar extremes of mania and depression have favored the acceptance of a functional model postulating regionally disturbed brain activity returning to normal with time or treatment. Seemingly contrary to that view, anatomical imaging studies have demonstrated abnormalities in brain structure which could reflect neurodegeneration or represent disturbed neuronal development. Resolution may come from an appreciation of adult neurogenesis, especially given the neuroprotective properties of drugs, such as lithium and their effects on brain volume. The brain regions vulnerable to structural changes also show evidence of dysfunction, giving rise to corticolimbic dysregulation interpretations of bipolar disorder. This article reviews the structural and functional magnetic resonance imaging data in bipolar disorder. Its focus is on the interpretation of findings in light of recent developments in the fields of neurobiology and image analysis, with particular attention paid to both the confounding effects of medication and the baseline energy state of the brain.

Magnetic resonance imaging studies in unipolar depression: systematic review and meta-regression analyses

Previous meta-analyses of structural MRI studies have shown diffuse cortical and sub-cortical abnormalities in unipolar depression. However, the presence of duplicate publications, recruitment of particular age groups and the selection of specific regions of interest means that there is uncertainty about the balance of current research. Moreover, the lack of systematic exploration of highly significant heterogeneity has prevented the generalisability of finding. A systematic review and random-effects meta-analysis was carried out to estimate effect sizes. Possible publication bias, and the impact of various study design characteristics on the magnitude of the observed effect size were systematically explored. The aim of this study was 1) to include structural MRI studies systematically comparing unipolar depression with bipolar disorder and healthy volunteers; 2) to consider all available structures of interest without specific age limits, avoiding data duplication, and 3) to explore the influence of factors contributing to the measured effect sizes systematically with meta-regression analyses. Unipolar depression was characterised by reduced brain volume in areas involved in emotional processing, including the frontal cortex, orbitofrontal cortex, cingulate cortex, hippocampus and striatum. There was also evidence of pituitary enlargement and an excess of white matter hyperintensity volume in unipolar depression. Factors which influenced the magnitude of the observed effect sizes were differences in methods, clinical variables, pharmacological interventions and sample age.

Suicidal behavior is associated with reduced corpus callosum area

BACKGROUND

Corpus callosum (CC) size has been associated with cognitive and emotional deficits in a range of neuropsychiatric and mood disorders. As such deficits are also found in suicidal behavior, we investigated specifically the association between CC atrophy and suicidal behavior.

METHODS

We studied 435 right-handed individuals without dementia from a cohort of community-dwelling persons aged 65 years and over (the ESPRIT study). They were divided in three groups: suicide attempters (n = 21), affective control subjects (AC) (n = 180) without history of suicide attempt but with a history of depression, and healthy control subjects (HC) (n = 234). T1-weighted magnetic resonance images were traced to measure the midsagittal areas of the anterior, mid, and posterior CC. Multivariate analysis of covariance was used to compare CC areas in the three groups.

RESULTS

Multivariate analyses adjusted for age, gender, childhood trauma, head trauma, and total brain volume showed that the area of the posterior third of CC was significantly smaller in suicide attempters than in AC (p = .020) and HC (p = .010) individuals. No significant differences were found between AC and HC. No differences were found for the anterior and mid thirds of the CC.

CONCLUSIONS

Our findings emphasize a reduced size of the posterior third of the CC in subjects with a history of suicide, suggesting a diminished interhemispheric connectivity and a possible role of CC in the pathophysiology of suicidal behavior. Further studies are needed to strengthen these results and clarify the underlying cellular changes leading to these morphometric differences.

Diffusion imaging studies of white matter integrity in bipolar disorder

Diffusion tensor imaging (DTI) is a neuroimaging technique with a potential to elucidate white matter integrity. Recently, it has been used in the field of psychiatry to further understand the pathophysiology of major diseases, including bipolar disorder (BD). This review sought to focus on existing DTI findings on white matter organization in BD.

Mapping corpus callosum morphology in twin pairs discordant for bipolar disorder

Callosal volume reduction has been observed in patients with bipolar disorder, but whether these deficits reflect genetic vulnerability to the illness remains unresolved. Here, we used computational methods to map corpus callosum abnormalities in a population-based sample of twin pairs discordant for bipolar disorder. Twenty-one probands with bipolar I disorder (mean age 44.4 ± 7.5 years; 48% female), 19 of their non-bipolar co-twins, and 34 demographically matched control twin individuals underwent magnetic resonance imaging. Three-dimensional callosal surface models were created to visualize its morphologic variability and to localize group differences. Neurocognitive correlates of callosal area differences were additionally investigated in a subsample of study participants. Bipolar (BPI) probands, but not their co-twins, showed significant callosal thinning and area reduction, most pronounced in the genu and splenium, relative to healthy twins. Altered callosal curvature was additionally observed in BPI probands. In bipolar probands and co-twins, genu and splenium midsagittal areas were significantly correlated with verbal processing speed and response inhibition. These findings suggest that aberrant connections between cortical regions--possibly reflecting decreased myelination of white matter tracts--may be involved in bipolar pathophysiology. However, findings of callosal thinning appear to be disease related, rather than reflecting genetic vulnerability to bipolar illness.

Disruption of white matter integrity in bipolar depression as a possible structural marker of illness

BACKGROUND

Diffusion tensor imaging allows the study of integrity of white matter (WM) tracts. Literature suggests that WM integrity could be altered in bipolar disorder. Heterogeneity of brain imaging methods, the studied samples, and drug treatments make localization, nature, and severity of the WM abnormalities unclear.

METHODS

We applied tract-based spatial statistics of diffusion tensor imaging measures to compare fractional anisotropy (FA), mean, and radial diffusivity of the WM skeleton in a group of 40 consecutively admitted inpatients affected by a major depressive episode without psychotic features with a diagnosis of bipolar disorder type I and 21 unrelated healthy volunteers from the general population.

RESULTS

Compared with control subjects, patients showed lower FA in the genu of the corpus callosum and in anterior and right superior-posterior corona radiata and higher values of radial diffusivity in WM tracts of splenium, genu and body of corpus callosum, right mid-dorsal part of the cingulum bundle, left anterior and bilateral superior and posterior corona radiata, bilateral superior longitudinal fasciculus, and right posterior thalamic radiation. Patients had no brain areas with higher FA or lower diffusivity values than control subjects.

CONCLUSIONS

Reduced FA with increased mean and radial diffusivity suggests significant demyelination and/or dysmyelination without axonal loss. Comparing our findings with other observations in homogeneous samples of euthymic and manic patients, it can be hypothesized that changes in measures of WM integrity might parallel illness phases of bipolar illness.

Structural brain and neuropsychometric changes associated with pediatric bipolar disorder with psychosis

OBJECTIVES

To identify neuropsychological and structural brain changes using a combination of high-resolution structural and diffusion tensor imaging in pediatric bipolar disorder (PBD) with psychosis (presence of delusions and or hallucinations).

METHODS

We recruited 15 patients and 20 euthymic age- and gender-matched healthy controls. All subjects underwent high-resolution structural and diffusion tensor imaging. Voxel-based morphometry (VBM), tract-based spatial statistics (TBSS), and probabilistic tractography were used to analyse magnetic resonance imaging data.

RESULTS

The PBD subjects had normal overall intelligence with specific impairments in working memory, executive function, language function, and verbal memory. Reduced gray matter (GM) density was found in the left orbitofrontal cortex, left pars triangularis, right premotor cortex, occipital cortex, right occipital fusiform gyrus, and right crus of the cerebellum. TBSS analysis showed reduced fractional anisotropy (FA) in the anterior corpus callosum. Probabilistic tractography from this cluster showed that this region of the corpus callosum is connected with the prefrontal cortices, including those regions whose density is decreased in PBD. In addition, FA change was correlated with verbal memory and working memory, while more widespread reductions in GM density correlated with working memory, executive function, language function, and verbal memory.

CONCLUSIONS

The findings suggest widespread cortical changes as well as specific involvement of interhemispheric prefrontal tracts in PBD, which may reflect delayed myelination in these tracts.

Neurocognitive and neuroimaging predictors of clinical outcome in bipolar disorder

Historically, bipolar disorder has been conceptualized as a disease involving episodic rather than chronic dysfunction. However, increasing evidence indicates that bipolar disorder is associated with substantial inter-episode psychosocial and vocational impairment. Here we review the contributions of neurocognitive deficits and structural and functional neuroanatomic alterations to the observed functional impairments. In particular, compelling evidence now suggests that neurocognitive impairments, particularly in the areas of attention, processing speed, and memory, are associated with functional outcome. Although investigation of the neural correlates of functional disability in bipolar disorder is only in its nascent stages, preliminary evidence suggests that white matter abnormalities may be predictive of poor outcome. A better understanding of the relationship between neurocognitive and neuroimaging assays and functional outcome has the potential to improve current treatment options and provide targets for new treatment strategies in bipolar disorder.

Volume reduction of the corpus callosum and its relationship with deficits in interhemispheric transfer of information in recent-onset psychosis

The present study aimed to investigate the presence of corpus callosum (CC) volume deficits in a population-based recent-onset psychosis (ROP) sample, and whether CC volume relates to interhemispheric communication deficits. For this purpose, we used voxel-based morphometry comparisons of magnetic resonance imaging data between ROP (n =122) and healthy control (n = 94) subjects. Subgroups (38 ROP and 39 controls) were investigated for correlations between CC volumes and performance on the Crossed Finger Localization Test (CFLT). Significant CC volume reductions in ROP subjects versus controls emerged after excluding substance misuse and non-right-handedness. CC reductions retained significance in the schizophrenia subgroup but not in affective psychoses subjects. There were significant positive correlations between CC volumes and CFLT scores in ROP subjects, specifically in subtasks involving interhemispheric communication. From these results, we can conclude that CC volume reductions are present in association with ROP. The relationship between such deficits and CFLT performance suggests that interhemispheric communication impairments are directly linked to CC abnormalities in ROP.

Brain networks: foundations and futures in bipolar disorder

BACKGROUND

Bipolar affective disorder is a common psychiatric illness with an often episodic nature, the neurobiological basis of which remains elusive. Symptom clusters in bipolar disorder can be conceptualized in terms of disordered brain networks, and doing so may aid our understanding of the varied presentations, differing illness courses and treatment responses.

AIMS

To review the rationale behind proposed disordered brain network function in bipolar disorder and the evidence of network dysfunction from imaging studies together with an overview of more novel techniques pertinent to this field.

METHODS

Medline databases were searched using the terms bipolar disorder, imaging, connectivity and brain networks. Relevant articles were reviewed and bibliographic cross-referencing was used to focus on key areas of interest, supplemented by additional Medline searches as required.

RESULTS

Structural and functional imaging studies support the concept of brain network dysfunction in bipolar disorder. Novel techniques such as diffusion tensor imaging and resting state network analysis can assess such dysfunction more directly, but there are few studies specific to bipolar disorder.

CONCLUSIONS

Brain network dysfunction is a useful framework for considering the varied presentations of bipolar disorder. Advanced imaging techniques are increasingly available, with the potential to provide insights into this important area.

White matter connectivity in bipolar disorder

There is evidence that intra- and inter-hemispheric white matter communication, mainly fronto-limbic and callosal connectivity, is impaired in bipolar disorder as reported in several magnetic resonance (MR) diffusion imaging studies. In this review we will discuss diffusion imaging studies that examined white matter integrity in patients with bipolar disorder, trying to outline future research strategies.

The integrity of corpus callosum and cluster B personality disorders: a quantitative MRI study in juvenile myoclonic epilepsy

Evidence suggests increased prevalence of cluster B personality disorders (PD) among patients with juvenile myoclonic epilepsy (JME), which has been associated with worse seizure control and more psychosocial dysfunctions. A preliminary voxel-based morphometry study demonstrated corpus callosum (CC) volume reduction in patients with JME and cluster B PD, particularly in the posterior midbody and isthmus. In this study we aimed to follow up these results with region of interest analysis. Sixteen JME patients with cluster B PD, 38 JME patients without any psychiatric disorder, and 30 demographically matched healthy controls submitted to a psychiatric evaluation and a magnetic resonance imaging scan. The total and regional callosal areas were obtained from the midsagittal slice using a semi-automated program. Psychiatric evaluation was performed through SCID-I and -II. Significant reductions in the posterior region of the CC were observed in the JME with PD group relative to the other groups. These data support previous findings of callosal reductions in cluster B PD, as well as a possible involvement of CC in patients with JME and such personality characteristics.

Meta-analysis of magnetic resonance imaging studies in chromosome 22q11.2 deletion syndrome (velocardiofacial syndrome)

OBJECTIVES

22q11.2 deletion syndrome (22q11.2DS), also known as velocardiofacial syndrome (VCFS) or DiGeorge Syndrome, is a genetic disorder due to a micro deletion on chromosome 22q11.2. VCFS is associated with abnormalities in brain structure and with an increased risk of psychiatric disorders, particularly schizophrenia. The aim of this review was to statistically summarize the structural imaging literature on VCFS which due to the relatively rarity of the disorder tends to consider small sample sizes.

METHOD

A systematic review and meta-analysis of region of interest (ROI) studies comparing VCFS with healthy controls was carried out. Significant heterogeneity was explored using meta-regression.

RESULTS

Subjects with VCFS were characterised by global brain volumetric reduction including several cortical regions, cerebellum and hippocampus. The area of the corpus callosum was increased.

CONCLUSIONS

Many regions extensively studied in schizophrenia were not covered in the existing VCFS literature. However, the studies considered support volumetric abnormalities which may help explain why VCFS is associated with a greatly increased risk of psychosis and other psychiatric disorders.

DTI studies of corpus callosum in bipolar disorder

Although the pathogenesis of bipolar disorder is still not completely understood, there is evidence from imaging studies that abnormalities in inter-hemispheric communication may play a major role in the pathophysiology of bipolar disorder. In the present review, we discuss the most consistent findings from diffusion imaging studies exploring corpus callosum integrity in bipolar disorder.

Anterior genu corpus callosum and impulsivity in suicidal patients with bipolar disorder

Suicidality is a life-threatening symptom in patients with bipolar disorder (BD). Impulsivity and mood instability are associated with suicidality in mood disorders. Evidence suggests that gray and white matter abnormalities are linked with impulsivity in mood disorders, but little is known about the association between corpus callosum (CC) and impulsivity in BD. We examined the relationship between CC areas, impulsivity and suicidality in BD patients. We studied 10 female BD patients with a history of suicide attempt (mean+/-SD age 36.2+/-10.1 years), 10 female BD patients without suicide attempt history (44.2+/-12.5 years) and 27 female healthy subjects (36.9+/-13.8 years). Impulsivity was evaluated by the Barratt Impulsivity Scale (BIS). We traced MR images to measure the areas of the CC genu, anterior body, posterior body, isthmus and splenium. The genu was divided into anterior, middle and posterior regions. The suicidal and non-suicidal BD patients had significantly higher BIS total, attention and non-planning scores than the healthy subjects (ps<0.01), and the suicidal BD patients had significantly higher BIS motor scores than the non-suicidal BD and healthy subjects (ps<0.01). There were no significant differences among the three groups on any regional CC areas, although the suicidal BD patients had the smallest areas. The suicidal BD patients showed a significant inverse correlation between anterior genu area and the BIS total (r=-0.75, p=0.04), motor (r=-0.79, p=0.02) and non-planning scores (r=-0.79, p=0.02). These correlations were not found in the non-suicidal BD patients or healthy subjects. The results suggest that the anterior medial frontal region may be involved in the pathophysiology of impulsive and suicidal behaviors in BD.

A role for white matter abnormalities in the pathophysiology of bipolar disorder

Bipolar disorder is a chronically disabling psychiatric disorder characterized by manic states that is often interspersed with periods of depression whose neurobiology remains largely unknown. There is, however, increasing evidence that white matter (WM) abnormalities may play an important role in the neurobiology of the disorder. In this review we critically evaluate evidence for WM abnormalities in bipolar disorder obtained from neuroimaging, neuropathological, and genetic research. Increased rates of white matter hyperintensities, regional volumetric abnormalities, abnormal water diffusion along prefrontal-subcortical tracts, fewer oligodendrocytes in prefrontal WM, and alterations in the expression of myelin- and oligodendrocyte-related genes are among the most consistent findings. Abnormalities converge in the prefrontal WM and, in particular, tracts that connect prefrontal regions and subcortical gray matter structures known to be involved in emotion. Taken together, the evidence supports and clarifies a model of BD that involves disconnectivity in regions implicated in emotion generation and regulation.

Laterality effects in schizophrenia and bipolar disorder

There are numerous reports in the literature of lateralised structural cerebral abnormalities and alterations of the corpus callosum in the major psychoses. In the light of these findings the purpose of this study was to directly compare hemispheric differences and callosal interhemispheric transmission (IT) in schizophrenia and bipolar disorder. To do that we tested schizophrenic (SCZ), bipolar disorder (BD) patients and controls in a simple manual reaction time (RT) task with lateralised visual stimuli (Poffenberger paradigm) which enables one to test both laterality effects and IT time. We found an overall slowing of responses with the right hand in schizophrenics but not in bipolar patients, who, like controls, showed no hand differences. This selective slowing down of the right hand is likely to be related to abnormalities of intrahemispheric cortico-cortical connections in the left hemisphere. In contrast, IT time was similar in SCZ and BD patients and did not differ with respect to controls. Two are the novel findings of the present study: first both SZC and BD share a normal IT of visuomotor information despite the presence of callosal abnormalities. Second, an impairment of intrahemispheric left hemispheric processing is present only in SCZ patients. This represents a potentially important clue to a further understanding of the pathogenetic differences between the two major psychoses.

Corpus callosum abnormalities in pediatric bipolar disorder

The corpus callosum (CC) is a midline white matter brain region that is important in interhemispheric communication and coordination. CC abnormalities are associated with a variety of psychiatric conditions, including increased vulnerability for psychotic illness, stressful early-life experiences, marijuana use, attention-deficit/hyperactivity disorder, obsessive-compulsive disorder, borderline personality disorder, dementia, schizophrenia and bipolar disorder. CC abnormalities in bipolar disorder have been identified in both pediatric and adult populations. In adults, a consistent finding has been a reduction in CC size, as well as abnormal axonal orientation or structure. Axonal abnormalities have also been noted in pediatric populations, but overall CC size reductions have not thus far been demonstrated. Furthermore, there are unique gender differences in the expression of CC abnormalities in pediatric populations, possibly related to androgen changes during puberty. The protean number of conditions in which the CC is involved is reflective of its central role in normal brain function and its potential as an early marker of neuropathology in psychiatric illness. Specifically, in bipolar disorder it has the potential to be useful as an early preclinical marker of disease or disease risk.

Corpus callosum size and shape alterations in individuals with bipolar disorder and their first-degree relatives

Reductions in the size of the corpus callosum (CC) have been described in patients with bipolar disorder (BD), although the contribution of genetic factors to these changes is unclear. We previously showed a global thinning of the CC in BD patients, and found those with a family history of affective disorders had a larger CC than those without. In this study, we compared callosal size and shape in 180 individuals: 70 with BD, 45 of their first-degree relatives, and 75 healthy controls. The callosum was extracted from a mid-sagittal slice from T1-weighted magnetic resonance images, and its total area, length and curvature were compared across groups. A non-parametric permutation method was used to examine for alterations in width of the callosum along 39 points. Validating our previous findings, a significant global reduction in callosal thickness was seen in BD patients, with a disproportionate thinning in the anterior body. First-degree relatives did not differ in callosal size or shape from controls. In BD patients, duration of illness and age were associated with thinning in the anterior body; BD patients on lithium treatment showed a thicker anterior mid-body than those on other psychotropics. Global and regional thinning of the callosum is seen in BD but not in their first-degree relatives. This suggests that CC abnormalities are linked to disease expression in BD and may not represent a marker of familial predisposition.

Bipolar and major depressive disorder: neuroimaging the developmental-degenerative divide

Both major depressive disorder and bipolar disorder are the subject of a voluminous imaging and genetics literature. Here, we attempt a comprehensive review of MRI and metabolic PET studies conducted to date on these two disorders, and interpret our findings from the perspective of developmental and degenerative models of illness. Elevated activity and volume loss of the hippocampus, orbital and ventral prefrontal cortex are recurrent themes in the literature. In contrast, dorsal aspects of the PFC tend to display hypometabolism. Ventriculomegaly and white matter hyperintensities are intimately associated with depression in elderly populations and likely have a vascular origin. Important confounding influences are medication, phenotypic and genetic heterogeneity, and technological limitations. We suggest that environmental stress and genetic risk variants interact with each other in a complex manner to alter neural circuitry and precipitate illness. Imaging genetic approaches hold out promise for advancing our understanding of affective illness.