White matter microstructural abnormalities in families multiply affected with bipolar I disorder: a diffusion tensor tractography study

White matter modifications of corpus callosum in bipolar disorder: A DTI tractography review

BACKGROUND

The recent widespread use of diffusion tensor imaging (DTI) tractography allowed researchers to investigate the diffusivity modifications and neuroanatomical changes of white matter (WM) fascicles in major psychiatric disorders, including bipolar disorder (BD). In BD, corpus callosum (CC) seems to have a crucial role in explaining the pathophysiology and cognitive impairment of this psychiatric disorder. This review aims to provide an overview on the latest results emerging from studies that investigated neuroanatomical changes of CC in BD using DTI tractography.

METHODS

Bibliographic research was conducted on PubMed, Scopus and Web of Science datasets until March 2022. Ten studies fulfilled our inclusion criteria.

RESULTS

From the reviewed DTI tractography studies a significant decrease of fractional anisotropy emerged in the genu, body and splenium of CC of BD patients compared to controls. This finding is coupled with reduction of fiber density and modification in fiber tract length. Finally, an increase of radial and mean diffusivity in forceps minor and in the entire CC was also reported.

LIMITATIONS

Small sample size, heterogeneity in terms of methodological (diffusion gradient) and clinical (lifetime comorbidity, BD status, pharmacological treatments) characteristics.

CONCLUSIONS

Overall, these findings suggest the presence of structural modifications in CC in BD patients, which may in turn explain the cognitive impairments often observed in this psychiatric disorder, especially in executive processing, motor control and visual memory. Finally, structural modifications may suggest an impairment in the amount of functional information and a morphological impact within those brain regions connected by CC.

Is structural connectivity different in child and adolescent relatives of patients with bipolar disorder? A narrative review according to studies with DTI

INTRODUCTION

Bipolar disorder (BD) has been associated with a decrease in white matter integrity. Diffusion tensor imaging (DTI) studies have enabled these changes to be elucidated with higher quality. Due to BD's high heritability, some studies have been conducted in relatives of BD patients looking at white matter integrity, and have found that structural connectivity may also be affected. This alteration has been proposed as a potential BD biomarker of vulnerability. However, there are few studies in children and adolescents.

OBJECTIVE

To conduct a review of the literature on changes in white matter integrity determined by DTI in high-risk children and adolescents.

RESULTS

Brain structural connectivity in the paediatric population is described in studies using DTI. Changes in the myelination process from its evolution within normal neurodevelopment to the findings in fractional anisotropy (FA) in BD patients and their high-risk relatives are also described.

CONCLUSIONS

Studies show that both BD patients and their at-risk relatives present a decrease in FA in specific brain regions. Studies in children and adolescents with a high risk of BD, indicate a reduced FA in axonal tracts involved in emotional and cognitive functions. Decreased FA can be considered as a vulnerability biomarker for BD.

The inferior frontal gyrus and familial risk for bipolar disorder

Bipolar disorder (BD) is a familial disorder with high heritability. Genetic factors have been linked to the pathogenesis of BD. Relatives of probands with BD who are at familial risk can exhibit brain abnormalities prior to illness onset. Given its involvement in prefrontal cognitive control and in frontolimbic circuitry that regulates emotional reactivity, the inferior frontal gyrus (IFG) has been a focus of research in studies of BD-related pathology and BD-risk mechanism. In this review, we discuss multimodal neuroimaging findings of the IFG based on studies comparing at-risk relatives and low-risk controls. Review of these studies in at-risk cases suggests the presence of both risk and resilience markers related to the IFG. At-risk individuals exhibited larger gray matter volume and increased functional activities in IFG compared with low-risk controls, which might result from an adaptive brain compensation to support emotion regulation as an aspect of psychological resilience. Functional connectivity between IFG and downstream limbic or striatal areas was typically decreased in at-risk individuals relative to controls, which could contribute to risk-related problems of cognitive and emotional control. Large-scale and longitudinal investigations on at-risk individuals will further elucidate the role of IFG and other brain regions in relation to familial risk for BD, and together guide identification of at-risk individuals for primary prevention.

Investigation of endophenotype potential of decreased fractional anisotropy in pediatric bipolar disorder patients and unrelated offspring of bipolar disorder patients

BACKGROUND

Bipolar disorder (BD) is a severe psychiatric disorder associated with structural and functional brain abnormalities, some of which have been found in unaffected relatives as well. In this study, we examined the potential role of decreased fractional anisotropy (FA) as a BD endophenotype, in adolescents at high risk for BD.

METHODS

We included 15 offspring of patients with BD, 16 pediatric BD patients, and 16 matched controls. Diffusion weighted scans were obtained on a 3T scanner using an echo-planar sequence. Scans were segmented using FreeSurfer.

RESULTS

Our results showed significantly decreased FA in six brain areas of offspring group; left superior temporal gyrus (LSTG; P < .0001), left transverse temporal gyrus (LTTG; P = .002), left banks of the superior temporal sulcus (LBSTS; P = .002), left anterior cingulum (LAC; P = .003), right temporal pole (RTP; P = .004) and left frontal pole (LFP; P = .017). On analysis, LSTG, LAC, and RTP demonstrated a potential to be an endophenotype when comparing all three groups. FA values in three regions, LBSTS, LTTG, and LFP were increased only in controls.

CONCLUSION

Our findings point at decreased FA as a possible endophenotype for BD, as they were found in children of patients with BD. Most of these areas were previously found to have morphological and functional changes in adult and pediatric BD, and are thought to play important roles in affected domains of functioning. Prospective follow up studies should be performed to detect reliability of decreased FA as an endophenotype and effects of treatment on FA.

Hippocampal cingulum white matter increases over time in young people at high genetic risk for bipolar disorder

BACKGROUND

Bipolar disorder (BD) is a strongly familial psychiatric disorder associated with white matter (WM) brain abnormalities. It is unclear whether such abnormalities are present in relatives without BD, and little is known about WM trajectories in those at increased genetic risk.

METHODS

Diffusion magnetic resonance imaging (dMRI) data were acquired at baseline and after two years in 91 unaffected individuals with a first-degree relative with bipolar disorder (HR), and 85 individuals with no family history of mental illness (CON). All participants were aged between 12 and 30 years at baseline. We examined longitudinal change in Fractional Anisotropy (FA) using tract-based spatial statistics (TBSS).

RESULTS

Compared to the CON group, HR participants showed a significant increase in FA in the right cingulum (hippocampus) (CGH) over a two-year period (p < .05, FDR corrected). This effect was more pronounced in HR individuals without a lifetime diagnosis of a mood disorder than those with a mood disorder.

LIMITATIONS

While our study is well powered to achieve the primary objectives, our sub-group analyses were under powered.

CONCLUSIONS

In one of the very few longitudinal neuroimaging studies of young people at high risk for BD, this study reports novel evidence of atypical white matter development in HR individuals in a key cortico-limbic tract involved in emotion regulation. Our findings also suggest that this different white matter developmental trajectory may be stronger in HR individuals without affective psychopathology. As such, increases in FA in the right CGH of HR participants may be a biomarker of resilience to mood disorders.

White matter characteristics in the early and late stages of bipolar disorder: A diffusion tensor imaging study

BACKGROUND

Bipolar disorder (BD) is characterized by recurrent mood episodes that may progress over time. Staging models may be used to follow the long-term course of BD. BD is associated with microstructural changes in white matter (WM). This study aims to compare the WM integrity within patients groups who are in different stages of BD and healthy controls and investigate whether WM integrity changes may be a biomarker that can be used in the clinical staging of BD.

METHODS

The study sample included euthymic 54 patients diagnosed with BD according to Diagnostic and Statistical Manual of Mental Disorders-IV (DSM-IV) and 27 healthy volunteers. Early-stage patients (n = 26) were determined as patients who have not had any mood episodes after the first manic episode, and late-stage patients (n = 28) determined as patients with recurrent mood episodes. MRI was performed using a 1.5 Tesla MR system and DTI sequences were acquired.

RESULTS

Region of interest (ROI) analyses showed that late-stage patients had significantly reduced fractional anisotropy (FA) in the right sagittal stratum and genu of the corpus callosum compared with healthy controls and early-stage patients. Regression models show that corpus callosum genu and right sagittal stratum FA values are predictive for the late-stage patient group.

LIMITATIONS

There are some limitations of the ROI method. The cross-sectional design is another limitation of this study.

CONCLUSIONS

WM integrity of corpus callosum genu and right sagittal stratum may be a biomarker for clinical staging of BD. Identifying stage-specific biomarkers may help us predict the neuroprogressive course of BD. Longitudinal studies would be required to detect stage-specific biomarkers.

Longitudinal Changes in Structural Connectivity in Young People at High Genetic Risk for Bipolar Disorder

OBJECTIVE

Recent studies of patients with bipolar disorder or at high genetic risk reveal structural dysconnections among key brain networks supporting cognitive and affective processes. Understanding the longitudinal trajectories of these networks across the peak age range of bipolar disorder onset could inform mechanisms of illness onset or resilience.

METHODS

Longitudinal diffusion-weighted MRI and phenotypic data were acquired at baseline and after 2 years in 183 individuals ages 12-30 years in two cohorts: 97 unaffected individuals with a first-degree relative with bipolar disorder (the high-risk group) and 86 individuals with no family history of mental illness (the control group). Whole-brain structural networks were derived using tractography, and longitudinal changes in these networks were studied using network-based statistics and mixed linear models.

RESULTS

Both groups showed widespread longitudinal changes, comprising both increases and decreases in structural connectivity, consistent with a shared neurodevelopmental process. On top of these shared changes, high-risk participants showed weakening of connectivity in a network encompassing the left inferior and middle frontal areas, left striatal and thalamic structures, the left fusiform, and right parietal and occipital regions. Connections among these regions strengthened in the control group, whereas they weakened in the high-risk group, shifting toward a cohort with established bipolar disorder. There was marginal evidence for even greater network weakening in those who had their first manic or hypomanic episode before follow-up.

CONCLUSIONS

Neurodevelopment from adolescence into early adulthood is associated with a substantial reorganization of structural brain networks. Differences in these maturational processes occur in a multisystem network in individuals at high genetic risk of bipolar disorder. This may represent a novel candidate to understand resilience and predict conversion to bipolar disorder.

Transdiagnostic role of glutamate and white matter damage in neuropsychiatric disorders: A Systematic Review

Neuropsychiatric disorders including generalized anxiety disorder (GAD), obsessive-compulsive disorder (OCD), major depressive disorder (MDD), bipolar disorder (BD), and schizophrenia (SZ) have been considered distinct categories of diseases despite their overlapping characteristics and symptomatology. We aimed to provide an in-depth review elucidating the role of glutamate/Glx and white matter (WM) abnormalities in these disorders from a transdiagnostic perspective. The PubMed online database was searched for studies published between 2010 and 2021. After careful screening, 401 studies were included. The findings point to decreased levels of glutamate in the Anterior Cingulate Cortex in both SZ and BD, whereas Glx is elevated in the Hippocampus in SZ and MDD. With regard to WM abnormalities, the Corpus Callosum and superior Longitudinal Fascicle were the most consistently identified brain regions showing decreased fractional anisotropy (FA) across all the reviewed disorders, except GAD. Additionally, the Uncinate Fasciculus displayed decreased FA in all disorders, except OCD. Decreased FA was also found in the inferior Longitudinal Fasciculus, inferior Fronto-Occipital Fasciculus, Thalamic Radiation, and Corona Radiata in SZ, BD, and MDD. Decreased FA in the Fornix and Corticospinal Tract were found in BD and SZ patients. The Cingulum and Anterior Limb of Internal Capsule exhibited decreased FA in MDD and SZ patients. The results suggest a gradual increase in severity from GAD to SZ defined by the number of brain regions with WM abnormality which may be partially caused by abnormal glutamate levels. WM damage could thus be considered a potential marker of some of the main neuropsychiatric disorders.

The uncinate fasciculus in individuals with and at risk for bipolar disorder: A meta-analysis

BACKGROUND

Bipolar disorder (BD) is a severe mental disorder, characterized by prominent mood swings and emotion regulation (ER) deficits. The uncinate fasciculus (UF), a white matter tract connecting the amygdala and the ventral prefrontal cortex, has been implicated in ER. Aberrancies in UF microstructure may be an endophenotype associated with increased risk for BD. However, studies in individuals with BD and their first-degree relatives (REL) have yielded inconsistent findings. This meta-analysis takes a region-of-interest approach to consolidate the available evidence and elucidate the role of the UF in the risk-architecture of BD.

METHODS

Using web-based search engines, we identified diffusion tensor imaging (DTI) studies focusing on the left and right UF and conducted meta-analyses comparing fractional anisotropy (FA) and radial diffusivity (RD) between BD or REL and healthy control participants (HC).

RESULTS

We included 32 studies (n_BD=1186, n_REL=289, n_HC=2315). Compared to HC, individuals with BD showed lower FA in the right (WMD=-0.31, p<0.0001) and left UF (WMD=-0.21, p = 0.010), and higher RD in the right UF (WMD=0.32, p = 0.009). We found no significant differences between REL and HC. In the right but not left UF, REL showed higher FA than BD (p = 0.043).

CONCLUSION

Our findings support aberrant UF microstructure, potentially related to alterations in myelination, as a mechanism, but not as an endophenotype of BD. However, given the limited power in the REL subsample, the latter finding must be considered preliminary. Studies examining the role of the UF in individuals at familial risk for BD are warranted.

White Matter Abnormalities in Late Onset First Episode Mania: A Diffusion Tensor Imaging Study

INTRODUCTION

A first manic episode after 50 years of age is uncommon. Late Onset Mania might be indicative of abnormalities in white matter, probably related to vascular, degenerative, or inflammatory processes.

OBJECTIVE

To determine if patients with late onset mania have reduced white matter integrity according to Magnetic Resonance Diffusion Tensor Imaging (DTI) and structural MRI.

METHODS

Twenty-two patients with late onset mania (>50 years old) and 22 age-paired healthy subjects were included in the study. Fractional anisotropy (FA) was used as a quantitative measure of white matter integrity. Fazekas scale was assessed also to measure white matter abnormalities in the FLAIR sequence. The Frontal Assessment Battery, COGNISTAT and Trail making test A and B were used as cognitive measurements.

RESULTS

According to DTI, commissural connections (left corpus callosum), and limbic connections (right and left uncinate fasciculus) were different between the patients and the comparison group. Fractional anisotropy values in the left corpus callosum showed significant correlations with neuropsychological measures, and with the Fazekas scale score. According to Fazekas scale, a pathological score in the FLAIR sequence was significantly more frequent in the patients as compared to the comparison group.

CONCLUSIONS

Patients with first episode mania in late life have relevant white matter abnormalities not explained by age, affecting interhemispheric and fronto-limbic networks probably related to executive functioning and emotional processing, at the level of the corpus callosum and the uncinate fasciculus. The etiology of this white matter loss of integrity in patients with late-onset mania is yet to be explored.

Aberrant Subnetwork and Hub Dysconnectivity in Adult Bipolar Disorder: A Multicenter Graph Theory Analysis

Neuroimaging evidence implicates structural network-level abnormalities in bipolar disorder (BD); however, there remain conflicting results in the current literature hampered by sample size limitations and clinical heterogeneity. Here, we set out to perform a multisite graph theory analysis to assess the extent of neuroanatomical dysconnectivity in a large representative study of individuals with BD. This cross-sectional multicenter international study assessed structural and diffusion-weighted magnetic resonance imaging data obtained from 109 subjects with BD type 1 and 103 psychiatrically healthy volunteers. Whole-brain metrics, permutation-based statistics, and connectivity of highly connected nodes were used to compare network-level connectivity patterns in individuals with BD compared with controls. The BD group displayed longer characteristic path length, a weakly connected left frontotemporal network, and increased rich-club dysconnectivity compared with healthy controls. Our multisite findings implicate emotion and reward networks dysconnectivity in bipolar illness and may guide larger scale global efforts in understanding how human brain architecture impacts mood regulation in BD.

Widespread Disruptions of White Matter in Familial Multiple Sclerosis: DTI and NODDI Study

Diffusion tensor imaging (DTI) is a noninvasive, quantitative MRI technique that measures white matter (WM) integrity. Many brain dimensions are heritable, including white matter integrity measured with DTI. Family studies are valuable to provide insights into the interactive effects of non-environmental factors on multiple sclerosis (MS). To examine the contribution of familial factors to the diffusion signals across WM microstructure, we performed DTI and calculated neurite orientation dispersion plus density imaging (NODDI) diffusion parameters in two patient groups comprising familial and sporadic forms of multiple sclerosis and their unaffected relatives. We divided 111 subjects (49 men and 62 women: age range 19-60) into three groups conforming to their MS history. The familial MS group included 30 participants (patients; n = 16, healthy relatives; n = 14). The sporadic group included 41 participants (patients; n = 10, healthy relatives; n = 31). Forty age-matched subjects with no history of MS in their families were defined as the control group. To study white matter integrity, two methods were employed: one for calculating the mean of DTI, FA, and MD parameters on 18 tracts using Tracts Constrained by Underlying Anatomy (TRACULA) and the other for whole brain voxel-based analysis using tract-based spatial statistics (TBSS) on NDI and ODI parameters derived from NODDI and DTI parameters. Voxel-based analysis showed considerable changes in FA, MD, NDI, and ODI in the familial group when compared with the control group, reflecting widespread impairment of white matter in this group. The analysis of 18 tracts with TRACULA revealed increased MD and FA reduction in more tracts (left and right ILF, UNC, and SLFT, forceps major and minor) in familial MS patients vs. the control group. There were no significant differences between the patient groups. We found no consequential changes in healthy relatives of both patient groups in voxel-based and tract analyses. Considering the multifactorial etiology of MS, familial studies are of great importance to clarify the effects of certain predisposing factors on demyelinating brain pathology.

Is Structural Connectivity Different in Child and Adolescent Relatives of Patients with Bipolar Disorder? A Narrative Review According to Studies with DTI

INTRODUCTION

Bipolar disorder (BD) has been associated with a decrease in white matter integrity. Diffusion tensor imaging (DTI) studies have enabled these changes to be elucidated with higher quality. Due to BD's high heritability, some studies have been conducted in relatives of BD patients looking at white matter integrity, and have found that structural connectivity may also be affected. This alteration has been proposed as a potential BD biomarker of vulnerability. However, there are few studies in children and adolescents.

OBJECTIVE

To conduct a review of the literature on changes in white matter integrity determined by DTI in high-risk children and adolescents.

RESULTS

Brain structural connectivity in the paediatric population is described in studies using DTI. Changes in the myelination process from its evolution within normal neurodevelopment to the findings in fractional anisotropy (FA) in BD patients and their high-risk relatives are also described.

CONCLUSIONS

Studies show that both BD patients and their at-risk relatives present a decrease in FA in specific brain regions. Studies in children and adolescents with a high risk of BD, indicate a reduced FA in axonal tracts involved in emotional and cognitive functions. Decreased FA can be considered as a vulnerability biomarker for BD.

The Amygdala in Schizophrenia and Bipolar Disorder: A Synthesis of Structural MRI, Diffusion Tensor Imaging, and Resting-State Functional Connectivity Findings

Frequently implicated in psychotic spectrum disorders, the amygdala serves as an important hub for elucidating the convergent and divergent neural substrates in schizophrenia and bipolar disorder, the two most studied groups of psychotic spectrum conditions. A systematic search of electronic databases through December 2017 was conducted to identify neuroimaging studies of the amygdala in schizophrenia and bipolar disorder, focusing on structural MRI, diffusion tensor imaging (DTI), and resting-state functional connectivity studies, with an emphasis on cross-diagnostic studies. Ninety-four independent studies were selected for the present review (49 structural MRI, 27 DTI, and 18 resting-state functional MRI studies). Also selected, and analyzed in a separate meta-analysis, were 33 volumetric studies with the amygdala as the region-of-interest. Reduced left, right, and total amygdala volumes were found in schizophrenia, relative to both healthy controls and bipolar subjects, even when restricted to cohorts in the early stages of illness. No volume abnormalities were observed in bipolar subjects relative to healthy controls. Shape morphometry studies showed either amygdala deformity or no differences in schizophrenia, and no abnormalities in bipolar disorder. In contrast to the volumetric findings, DTI studies of the uncinate fasciculus tract (connecting the amygdala with the medial- and orbitofrontal cortices) largely showed reduced fractional anisotropy (a marker of white matter microstructure abnormality) in both schizophrenia and bipolar patients, with no cross-diagnostic differences. While decreased amygdalar-orbitofrontal functional connectivity was generally observed in schizophrenia, varying patterns of amygdalar-orbitofrontal connectivity in bipolar disorder were found. Future studies can consider adopting longitudinal approaches with multimodal imaging and more extensive clinical subtyping to probe amygdalar subregional changes and their relationship to the sequelae of psychotic disorders.

The anterior limb of the internal capsule: Anatomy, function, and dysfunction

The last two decades have seen a re-emergence of neurosurgery for severe, refractory psychiatric diseases, largely due to the advent of more precise and safe operative techniques. Nevertheless, the optimal targets for these surgeries remain a matter of debate, and are often grandfathered from experiences in the late 20th century. To better explore the rationale for one target in particular - the anterior limb of the internal capsule (ALIC) - we comprehensively reviewed all available literature on its role in the pathophysiology and treatment of mental illness. We first provide an overview of its functional anatomy, followed by a discussion on its role in several prevalent psychiatric diseases. Given its structural integration into the limbic system and involvement in a number of cognitive and emotional processes, the ALIC is a robust target for surgical treatment of refractory psychiatric diseases. The advent of novel neuroimaging techniques, coupled with image-guided therapeutics and neuromodulatory treatments, will continue to enable study on the ALIC in mental illness.

Fractional anisotropy of the uncinate fasciculus and cingulum in bipolar disorder type I, type II, unaffected siblings and healthy controls

BACKGROUND

Fractional anisotropy in the uncinate fasciculus and the cingulum may be biomarkers for bipolar disorder and may even be distinctly affected in different subtypes of bipolar disorder, an area in need of further research.AimsThis study aims to establish if fractional anisotropy in the uncinate fasciculus and cingulum shows differences between healthy controls, patients with bipolar disorder type I (BD-I) and type II (BD-II), and their unaffected siblings.

METHOD

Fractional anisotropy measures from the uncinate fasciculus, cingulum body and parahippocampal cingulum were compared with tractography methods in 40 healthy controls, 32 patients with BD-I, 34 patients with BD-II, 17 siblings of patients with BD-I and 14 siblings of patients with BD-II.

RESULTS

The main effects were found in both the right and left uncinate fasciculus, with patients with BD-I showing significantly lower fractional anisotropy than both patients with BD-II and healthy controls. Participants with BD-II did not differ from healthy controls. Siblings showed similar effects in the left uncinate fasciculus. In a subsequent complementary analysis, we investigated the association between fractional anisotropy in the uncinate fasciculus and polygenic risk for bipolar disorder and psychosis in a large cohort (n = 570) of healthy participants. However, we found no significant association.

CONCLUSIONS

Fractional anisotropy in the uncinate fasciculus differs significantly between patients with BD-I and patients with BD-II and healthy controls. This supports the hypothesis of differences in the physiological sub-tract between bipolar disorder subtypes. Similar results were found in unaffected siblings, suggesting the potential for this biomarker to represent an endophenotype for BD-I. However, fractional anisotropy in the uncinate fasciculus seems unrelated to polygenic risk for bipolar disorder or psychosis.Declaration of interestNone.

Genetic polymorphisms associated with circadian rhythm dysregulation provide new perspectives on bipolar disorder

OBJECTIVES

The objective of this study was to present a broad view of how genetic polymorphisms in genes that control the rhythmicity and function of circadian rhythm may influence the etiology, pathophysiology and treatment of bipolar disorder (BD).

METHODS

A bibliographic search was performed to identify and select papers reporting studies on variations in circadian genes and BD. A search of Medline, Google Scholar, Scopus, and Web of Science was carried out to review the literature.

RESULTS

Several studies provide evidence of contributions of variations in circadian genes to disease etiology, pathophysiological variations and lithium drug response. Dysfunction of the sleep-wake cycle, an important brain function regulator, is indicated as the primary means by which circadian gene variations act in mood disorders.

CONCLUSIONS

Investigations of the effects of circadian genes have suggested that the chronotype offers hope for guiding and improving management of patients with BD. However, BD is a disease of a complex nature and presents multiple endophenotypes determined by different associations between genetics and the environment. Thus, new genomic studies to delimit variations that may help improve the clinical condition of these patients are extremely important.

Differential effect of disease-associated ST8SIA2 haplotype on cerebral white matter diffusion properties in schizophrenia and healthy controls

Brain white matter abnormalities are evident in individuals with schizophrenia, and also their first-degree relatives, suggesting that some alterations may relate to underlying genetic risk. The ST8 alpha-N-acetyl-neuraminide alpha-2,8-sialyltransferase 2 (ST8SIA2) gene, which encodes the alpha-2,8-sialyltransferase 8B enzyme that aids neuronal migration and synaptic plasticity, was previously implicated as a schizophrenia susceptibility gene. This study examined the extent to which specific haplotypes in ST8SIA2 influence white matter microstructure using diffusion-weighted imaging of individuals with schizophrenia (n = 281) and healthy controls (n = 172), recruited across five Australian sites. Interactions between diagnostic status and the number of haplotype copies (0 or ≥1) were tested across all white matter voxels with cluster-based statistics. Fractional anisotropy (FA) in the right parietal lobe was found to show a significant interaction between diagnosis and ST8SIA2 protective haplotype (p < 0.05, family-wise error rate (FWER) cluster-corrected). The protective haplotype was associated with increased FA in controls, but this effect was reversed in people with schizophrenia. White matter fiber tracking revealed that the region-of-interest was traversed by portions of the superior longitudinal fasciculus, corona radiata, and posterior limb of internal capsule. Post hoc analysis revealed that reduced FA in this regional juncture correlated with reduced IQ in people with schizophrenia. The ST8SIA2 risk haplotype copy number did not show any differential effects on white matter. This study provides a link between a common disease-associated haplotype and specific changes in white matter microstructure, which may relate to resilience or risk for mental illness, providing further compelling evidence for involvement of ST8SIA2 in the pathophysiology of schizophrenia.

Diffusion tensor imaging tractography study in bipolar disorder patients compared to first-degree relatives and healthy controls

AIM

We aimed to compare white matter structural changes in specific tracts by diffusion tensor imaging (DTI) tractography in patients with bipolar disorder (BD) I, non-ill first-degree relatives (FDR) of the patients, and healthy controls (HC).

METHODS

In a cross-sectional study, we studied right-handed subjects consisting of 16 euthymic BD I patients, 15 FDR, and 15 HC. The anterior thalamic radiation, uncinate fasciculus, corpus callosum, and cingulum bundle were reconstructed by DTI tractography. Mean fractional anisotropy (FA) and apparent diffusion coefficient (ADC) values were compared for group differences followed by post-hoc analysis.

RESULTS

The three groups did not differ in terms of sociodemographic variables. There were significant group differences in the FA values among the BD I patients, their FDR, and the HC for the corpus callosum, the dorsal part of the right cingulum bundle, the hippocampal part of the cingulum bundle bilaterally, and the uncinate fasciculus (P < 0.001). The FA values in the patients were significantly lower than in controls, and FDR also showed similar differences; however, they were smaller than those in patients. No significant difference was found between the groups for FA values of the dorsal part of the left cingulum bundle and anterior thalamic radiation. Significant differences were present for ADC values among the groups for the corpus callosum, the dorsal and hippocampal parts of the cingulum, anterior thalamic radiation, and uncinate fasciculus bilaterally (P < 0.01). The FA and ADC values did not correlate significantly with age or any clinical variables.

CONCLUSION

These findings suggest that BD patients and their FDR show alterations in microstructural integrity of white matter tracts, compared to the healthy population.

Effects of ANK3 variation on gray and white matter in bipolar disorder

The single-nucleotide polymorphism rs9804190 in the Ankyrin G (ANK3) gene has been reported in genome-wide association studies to be associated with bipolar disorder (BD). However, the neural system effects of rs9804190 in BD are not known. We investigated associations between rs9804190 and gray and white matter (GM and WM, respectively) structure within a frontotemporal neural system implicated in BD. A total of 187 adolescent and adult European Americans were studied: a group homozygous for the C allele (52 individuals with BD and 56 controls) and a T-carrier group, carrying the high-risk T allele (38 BD and 41 controls). Subjects participated in high-resolution structural magnetic resonance imaging and diffusion tensor imaging (DTI) scanning. Frontotemporal region of interest (ROI) and whole-brain exploratory analyses were conducted. DTI ROI-based analysis revealed a significant diagnosis by genotype interaction within the uncinate fasciculus (P⩽0.05), with BD subjects carrying the T (risk) allele showing decreased fractional anisotropy compared with other subgroups, independent of age. Genotype effects were not observed in frontotemporal GM volume. These findings support effects of rs9804190 on frontotemporal WM in adolescents and adults with BD and suggest a mechanism contributing to WM pathology in BD.

Steeper Slope of Age-Related Changes in White Matter Microstructure and Processing Speed in Bipolar Disorder

OBJECTIVES

Bipolar disorder (BD) is associated with compromised white matter (WM) integrity and deficits in processing speed (PS). Few studies, however, have investigated age relationships with WM structure and cognition to understand possible changes in brain health over the lifespan. This investigation explored whether BD and healthy counterpart (HC) participants exhibited differential age-related associations with WM and cognition, which may be suggestive of accelerated brain and cognitive aging.

DESIGN

Cross-sectional study.

SETTING

University of California San Diego and the Veterans Administration San Diego Healthcare System.

PARTICIPANTS

33 euthymic BD and 38 HC participants.

MEASUREMENTS

Diffusion tensor imaging was acquired as a measure of WM integrity, and tract-specific fractional anisotropy (FA) was extracted utilizing the Johns Hopkins University probability atlas. PS was assessed with the Number and Letter Sequencing conditions of the Delis-Kaplan Executive Function System Trail Making Test.

RESULTS

BD participants demonstrated slower PS compared with the HC group, but no group differences were found in FA across tracts. Multiple linear regressions revealed a significant group-by-age interaction for the right uncinate fasciculus, the left hippocampal portion of the cingulum, and for PS, such that older age was associated with lower FA values and slower PS in the BD group only. The relationship between age and PS did not significantly change after accounting for uncinate FA, suggesting that the observed age associations occur independently.

CONCLUSIONS

Results provide support for future study of the accelerated aging hypothesis by identifying markers of brain health that demonstrate a differential age association in BD.

Neurobiology of Risk for Bipolar Disorder

Bipolar disorder (BD) is a chronic mental illness which follows a relapsing and remitting course and requires lifetime treatment. The lack of biological markers for BD is a major difficulty in clinical practice. Exploring multiple endophenotypes to fit in multivariate genetic models for BD is an important element in the process of finding tools to facilitate early diagnosis, early intervention, prevention of new episodes, and follow-up of treatment response in BD. Reviewing of studies on neuroimaging, neurocognition, and biochemical parameters in populations with high genetic risk for the illness can yield an integrative perspective on the neurobiology of risk for BD. The most up-to-date data reveals consistent deficits in executive function, response inhibition, verbal memory/learning, verbal fluency, and processing speed in risk groups for BD. Functional magnetic resonance imaging (fMRI) studies report alterations in the activity of the inferior frontal gyrus, medial prefrontal cortex, and limbic areas, particularly in the amygdala in unaffected first-degree relatives (FDR) of BD compared to healthy controls. Risk groups for BD also present altered immune and neurochemical modulation. Despite inconsistencies, accumulating data reveals cognitive and imaging markers for risk and to a less extent resilience of BD. Findings on neural modulation markers are preliminary and require further studies. Although the knowledge on the neurobiology of risk for BD has been inadequate to provide benefits for clinical practice, further studies on structural and functional changes in the brain, neurocognitive functioning, and neurochemical modulation have a potential to reveal biomarkers for risk and resilience for BD. Multimodal, multicenter, population-based studies with large sample size allowing for homogeneous subgroup analyses will immensely contribute to the elucidation of biological markers for risk for BD in an integrative model.

White matter abnormalities in the fornix are linked to cognitive performance in SZ but not in BD disorder: An exploratory analysis with DTI deterministic tractography

BACKGROUND

In psychosis, white matter (WM) microstructural changes have been detected previously; however, direct comparisons of findings between bipolar (BD) and schizophrenia (SZ) patients are scarce. In this study, we employed deterministic tractography to reconstruct WM tracts in BD and SZ patients.

METHODS

Diffusion tensor imaging (DTI) data was carried out with n=32 euthymic BD type I patients, n=26 SZ patients and 30 matched healthy controls. Deterministic tractography using multiple indices of diffusion (fractional anisotropy (FA), tract volume (Vol), tract length (Le) and number of tracts (NofT)) were obtained from the fornix, the cingulum, the anterior thalamic radiation, and the corpus callosum bilaterally.

RESULTS

We showed widespread WM microstructural changes in SZ, and changes in the corpus callosum, the left cingulum and the fornix in BD. Fornix fiber tracking scores were associated with cognitive performance in SZ, and with age and age at disease onset in the BD patient group.

LIMITATIONS

Although the influence of psychopharmacological drugs as biasing variables on morphological alterations has been discussed for SZ and BD, we did not observe a clear influence of drug exposure on our findings.

CONCLUSIONS

These results confirm the assumption that SZ patients have more severe WM changes than BD patients. The findings also suggest a major role of WM changes in the fornix as important fronto-limbic connections in the etiology of cognitive symptoms in SZ, but not in BD.

Interhemispheric white matter integrity in young people with bipolar disorder and at high genetic risk

BACKGROUND

White matter (WM) impairments have been reported in patients with bipolar disorder (BD) and those at high familial risk of developing BD. However, the distribution of these impairments has not been well characterized. Few studies have examined WM integrity in young people early in the course of illness and in individuals at familial risk who have not yet passed the peak age of onset.

METHOD

WM integrity was examined in 63 BD subjects, 150 high-risk (HR) individuals and 111 participants with no family history of mental illness (CON). All subjects were aged 12 to 30 years.

RESULTS

This young BD group had significantly lower fractional anisotropy within the genu of the corpus callosum (CC) compared with the CON and HR groups. Moreover, the abnormality in the genu of the CC was also present in HR participants with recurrent major depressive disorder (MDD) (n = 16) compared with CON participants.

CONCLUSIONS

Our findings provide important validation of interhemispheric abnormalities in BD patients. The novel finding in HR subjects with recurrent MDD - a group at particular risk of future hypo/manic episodes - suggests that this may potentially represent a trait marker for BD, though this will need to be confirmed in longitudinal follow-up studies.

A comprehensive tractography study of patients with bipolar disorder and their unaffected siblings

BACKGROUND

Diffusion tensor imaging studies show reductions in fractional anisotropy (FA) in individuals with bipolar disorder and their unaffected siblings. However, the use of various analysis methods is an important source of between-study heterogeneity. Using tract-based spatial statistics, we previously demonstrated widespread FA reductions in patients and unaffected relatives. To better interpret the neuroanatomical pattern of this previous finding and to assess the influence of methodological heterogeneity, we here applied tractography to the same sample.

METHODS

Diffusion-weighted images were acquired for 96 patients, 69 unaffected siblings and 56 controls. We applied TRACULA, an extension of a global probabilistic tractography algorithm, to automatically segment 18 major fiber tracts. Average FA within each tract and at each cross-section along each tract was compared between groups.

RESULTS

Patients had reduced FA compared to healthy controls and their unaffected siblings in general, and in particular in the parietal part of the superior longitudinal fasciculus. In unaffected siblings, FA was nominally reduced compared to controls in the corpus callosum. Point-wise analyses indicated that similar effects were present along extended sections, but with variable effect sizes. Current symptom severity negatively correlated with FA in several fronto-limbic association tracts.

CONCLUSIONS

The differential sensitivity of analysis techniques likely explains between-study heterogeneity in anatomical localization of FA reductions. The present tractography analysis confirms the presence of overall FA reductions in patients with bipolar disorder, which are most pronounced in the superior longitudinal fasciculus. Unaffected siblings may display similar, albeit more subtle and anatomically restricted FA reductions. Hum Brain Mapp 37:3474-3485, 2016. © 2016 Wiley Periodicals, Inc.

Abnormal white matter integrity as a structural endophenotype for bipolar disorder

BACKGROUND

Several lines of evidence suggest that bipolar disorder (BD) is associated with white matter (WM) pathology. Investigation of unaffected first-degree relatives of BD patients may help to distinguish structural biomarkers of genetic risk without the confounding effects of burden of illness, medication or clinical state. In the present study, we applied tract-based spatial statistics to study WM changes in patients with BD, unaffected siblings and controls.

METHOD

A total of 27 euthymic patients with BD type I, 20 unaffected siblings of bipolar patients and 29 healthy controls who did not have any current or past diagnosis of Axis I psychiatric disorders were enrolled in the study.

RESULTS

Fractional anisotropy (FA) was significantly lower in BD patients than in the control group in the corpus callosum, fornix, bilateral superior longitudinal fasciculus, inferior longitudinal fasciculus, inferior fronto-occipital fasciculus, anterior thalamic radiation, posterior thalamic radiation, cingulum, uncinate fasciculus, superior corona radiata, anterior corona radiata and left external capsule. In region-of-interest (ROI) analyses, we found that both unaffected siblings and bipolar patients had significantly reduced FA in the left posterior thalamic radiation, the left sagittal stratum, and the fornix compared with healthy controls. Average FA for unaffected siblings was intermediate between the healthy controls and bipolar patients within these ROIs.

CONCLUSIONS

Decreased FA in the fornix, left posterior thalamic radiation and left sagittal stratum in both bipolar patients and unaffected siblings may represent a potential structural endophenotype or a trait-based marker for BD.

Genome-wide discovered psychosis-risk gene ZNF804A impacts on white matter microstructure in health, schizophrenia and bipolar disorder

Background. Schizophrenia (SZ) and bipolar disorder (BD) have both been associated with reduced microstructural white matter integrity using, as a proxy, fractional anisotropy (FA) detected using diffusion tensor imaging (DTI). Genetic susceptibility for both illnesses has also been positively correlated in recent genome-wide association studies with allele A (adenine) of single nucleotide polymorphism (SNP) rs1344706 of the ZNF804A gene. However, little is known about how the genomic linkage disequilibrium region tagged by this SNP impacts on the brain to increase risk for psychosis. This study aimed to assess the impact of this risk variant on FA in patients with SZ, in those with BD and in healthy controls. Methods. 230 individuals were genotyped for the rs1344706 SNP and underwent DTI. We used tract-based spatial statistics (TBSS) followed by an analysis of variance, with threshold-free cluster enhancement (TFCE), to assess underlying effects of genotype, diagnosis and their interaction, on FA. Results. As predicted, statistically significant reductions in FA across a widely distributed brain network (p < 0.05, TFCE-corrected) were positively associated both with a diagnosis of SZ or BD and with the double (homozygous) presence of the ZNF804A rs1344706 risk variant (A). The main effect of genotype was medium (d = 0.48 in a 44,054-voxel cluster) and the effect in the SZ group alone was large (d = 1.01 in a 51,260-voxel cluster), with no significant effects in BD or controls, in isolation. No areas under a significant diagnosis by genotype interaction were found. Discussion. We provide the first evidence in a predominantly Caucasian clinical sample, of an association between ZNF804A rs1344706 A-homozygosity and reduced FA, both irrespective of diagnosis and particularly in SZ (in overlapping brain areas). This suggests that the previously observed involvement of this genomic region in psychosis susceptibility, and in impaired functional connectivity, may be conferred through it inducing abnormalities in white matter microstructure.

Anterior Cortical Development During Adolescence in Bipolar Disorder

BACKGROUND

Increasing evidence supports a neurodevelopmental model for bipolar disorder (BD), with adolescence as a critical period in its development. Developmental abnormalities of anterior paralimbic and heteromodal frontal cortices, key structures in emotional regulation processes and central in BD, are implicated. However, few longitudinal studies have been conducted, limiting understanding of trajectory alterations in BD. In this study, we performed longitudinal neuroimaging of adolescents with and without BD and assessed volume changes over time, including changes in tissue overall and within gray and white matter. Larger decreases over time in anterior cortical volumes in the adolescents with BD were hypothesized. Gray matter decreases and white matter increases are typically observed during adolescence in anterior cortices. It was hypothesized that volume decreases over time in BD would reflect alterations in those processes, showing larger gray matter contraction and decreased white matter expansion.

METHODS

Two high-resolution magnetic resonance imaging scans were obtained approximately 2 years apart for 35 adolescents with bipolar I disorder (BDI) and 37 healthy adolescents. Differences over time between groups were investigated for volume overall and specifically for gray and white matter.

RESULTS

Relative to healthy adolescents, adolescents with BDI showed greater volume contraction over time in a region including insula and orbitofrontal, rostral, and dorsolateral prefrontal cortices (p < .05, corrected), including greater gray matter contraction and decreased white matter expansion over time, in the BD compared with the healthy group.

CONCLUSIONS

The findings support neurodevelopmental abnormalities during adolescence in BDI in anterior cortices, including altered developmental trajectories of anterior gray and white matter.

DTI and Myelin Plasticity in Bipolar Disorder: Integrating Neuroimaging and Neuropathological Findings

Bipolar disorder (BD) is a major psychiatric illness with a chronic recurrent course, ranked among the worldwide leading disabling diseases. Its pathophysiology is still not completely understood and findings are still inconclusive, though a great interest on the topic has been constantly raised by magnetic resonance imaging, genetic and neuropathological studies. In recent years, diffusion tensor imaging (DTI) investigations have prompted interest in the key role of white matter (WM) abnormalities in BD. In this report, we summarize and comment recent findings from DTI studies in BD, reporting fractional anisotropy as putative measure of WM integrity, as well as recent data from neuropathological studies focusing on oligodendrocyte involvement in WM alterations in BD. DTI research indicates that BD is most commonly associated with a WM disruption within the fronto-limbic network, which may be accompanied by other WM changes spread throughout temporal and parietal regions. Neuropathological studies, mainly focused on the fronto-limbic network, have repeatedly shown a loss in cortical and subcortical oligodendrocyte cell count, although an increased subcortical oligodendrocyte density has been also documented suggesting a putative role in remyelination processes for oligodendrocytes in BD. According to our review, a greater integration between DTI and morphological findings is needed in order to elucidate processes affecting WM, either glial loss or myelin plasticity, on the basis of a more targeted research in BD.

Structural brain network analysis in families multiply affected with bipolar I disorder

Disrupted structural connectivity is associated with psychiatric illnesses including bipolar disorder (BP). Here we use structural brain network analysis to investigate connectivity abnormalities in multiply affected BP type I families, to assess the utility of dysconnectivity as a biomarker and its endophenotypic potential. Magnetic resonance diffusion images for 19 BP type I patients in remission, 21 of their first degree unaffected relatives, and 18 unrelated healthy controls underwent tractography. With the automated anatomical labelling atlas being used to define nodes, a connectivity matrix was generated for each subject. Network metrics were extracted with the Brain Connectivity Toolbox and then analysed for group differences, accounting for potential confounding effects of age, gender and familial association. Whole brain analysis revealed no differences between groups. Analysis of specific mainly frontal regions, previously implicated as potentially endophenotypic by functional magnetic resonance imaging analysis of the same cohort, revealed a significant effect of group in the right medial superior frontal gyrus and left middle frontal gyrus driven by reduced organisation in patients compared with controls. The organisation of whole brain networks of those affected with BP I does not differ from their unaffected relatives or healthy controls. In discreet frontal regions, however, anatomical connectivity is disrupted in patients but not in their unaffected relatives.

Corpus callosum area in patients with bipolar disorder with and without psychotic features: an international multicentre study

BACKGROUND

Previous studies have reported MRI abnormalities of the corpus callosum (CC) in patients with bipolar disorder (BD), although only a few studies have directly compared callosal areas in psychotic versus nonpsychotic patients with this disorder. We sought to compare regional callosal areas in a large international multicentre sample of patients with BD and healthy controls.

METHODS

We analyzed anatomic T1 MRI data of patients with BD-I and healthy controls recruited from 4 sites (France, Germany, Ireland and the United States). We obtained the mid-sagittal areas of 7 CC subregions using an automatic CC delineation. Differences in regional callosal areas between patients and controls were compared using linear mixed models (adjusting for age, sex, handedness, brain volume, history of alcohol abuse/dependence, lithium or antipsychotic medication status, symptomatic status and site) and multiple comparisons correction. We also compared regional areas of the CC between patients with BD with and without a history of psychotic features.

RESULTS

We included 172 patients and 146 controls in our study. Patients with BD had smaller adjusted mid-sagittal CC areas than controls along the posterior body, the isthmus and the splenium of the CC. Patients with a positive history of psychotic features had greater adjusted area of the rostral CC region than those without a history of psychotic features.

LIMITATIONS

We found small to medium effect sizes, and there was no calibration technique among the sites.

CONCLUSION

Our results suggest that BD with psychosis is associated with a different pattern of interhemispheric connectivity than BD without psychosis and could be considered a relevant neuroimaging subtype of BD.

Widespread white matter tract aberrations in youth with familial risk for bipolar disorder

Few studies have examined multiple measures of white matter (WM) differences in youth with familial risk for bipolar disorder (FR-BD). To investigate WM in the FR-BD group, we used three measures of WM structure and two methods of analysis. We used fractional anisotropy (FA), axial diffusivity (AD), and radial diffusivity (RD) to analyze diffusion tensor imaging (DTI) findings in 25 youth with familial risk for bipolar disorder, defined as having both a parent with BD and mood dysregulation, and 16 sex-, age-, and IQ-matched healthy controls. We conducted a whole brain voxelwise analysis using tract based spatial statistics (TBSS). Subsequently, we conducted a complementary atlas-based, region-of-interest analysis using Diffeomap to confirm results seen in TBSS. When TBSS was used, significant widespread between-group differences were found showing increased FA, increased AD, and decreased RD in the FR-BD group in the bilateral uncinate fasciculus, cingulum, cingulate, superior fronto-occipital fasciculus (SFOF), superior longitudinal fasciculus (SLF), inferior longitudinal fasciculus, and corpus callosum. Atlas-based analysis confirmed significant between-group differences, with increased FA and decreased RD in the FR-BD group in the SLF, cingulum, and SFOF. We found significant widespread WM tract aberrations in youth with familial risk for BD using two complementary methods of DTI analysis.

Fusing Functional MRI and Diffusion Tensor Imaging Measures of Brain Function and Structure to Predict Working Memory and Processing Speed Performance among Inter-episode Bipolar Patients

Evidence for abnormal brain function as measured with diffusion tensor imaging (DTI) and functional magnetic resonance imaging (fMRI) and cognitive dysfunction have been observed in inter-episode bipolar disorder (BD) patients. We aimed to create a joint statistical model of white matter integrity and functional response measures in explaining differences in working memory and processing speed among BD patients. Medicated inter-episode BD (n=26; age=45.2±10.1 years) and healthy comparison (HC; n=36; age=46.3±11.5 years) participants completed 51-direction DTI and fMRI while performing a working memory task. Participants also completed a processing speed test. Tract-based spatial statistics identified common white matter tracts where fractional anisotropy was calculated from atlas-defined regions of interest. Brain responses within regions of interest activation clusters were also calculated. Least angle regression was used to fuse fMRI and DTI data to select the best joint neuroimaging predictors of cognitive performance for each group. While there was overlap between groups in which regions were most related to cognitive performance, some relationships differed between groups. For working memory accuracy, BD-specific predictors included bilateral dorsolateral prefrontal cortex from fMRI, splenium of the corpus callosum, left uncinate fasciculus, and bilateral superior longitudinal fasciculi from DTI. For processing speed, the genu and splenium of the corpus callosum and right superior longitudinal fasciculus from DTI were significant predictors of cognitive performance selectively for BD patients. BD patients demonstrated unique brain-cognition relationships compared to HC. These findings are a first step in discovering how interactions of structural and functional brain abnormalities contribute to cognitive impairments in BD.

Diffusion tensor imaging in first degree relatives of schizophrenia and bipolar disorder patients

OBJECTIVES

White matter (WM) abnormalities are one of the most widely and consistently reported findings in schizophrenia (SZ) and bipolar disorder (BD). If these abnormalities are inherited determinants of illness, suitable to be classified as an endophenotype, relatives of patients must also have them at higher rate compared to the general population. In this review, we evaluate published diffusion tensor imaging (DTI) studies comparing first degree relatives of SZ and BD patients and healthy control subjects.

METHODS

We searched PubMed, Embase and PsychInfo for DTI studies which included an unaffected relative and a healthy comparison group.

RESULTS

22 studies fulfilled the inclusion criteria. WM abnormalities were found in many diverse regions in relatives of SZ patients. Although the findings were not completely consistent across studies, the most implicated areas were the frontal and temporal WM regions and the corpus callosum. Studies in relatives of BD patients were fewer in number with less consistent findings reported across studies.

CONCLUSIONS

Our review supports the concept of WM abnormalities as an endophenotype in SZ, with somewhat weaker evidence in BD, but larger and higher quality studies are needed to make a definitive comment.

Frontotemporal White Matter in Adolescents with, and at-Risk for, Bipolar Disorder

Frontotemporal neural systems are highly implicated in the emotional dysregulation characteristic of bipolar disorder (BD). Convergent genetic, postmortem, behavioral and neuroimaging evidence suggests abnormalities in the development of frontotemporal white matter (WM) in the pathophysiology of BD. This review discusses evidence for the involvement of abnormal WM development in BD during adolescence, with a focus on frontotemporal WM. Findings from diffusion tensor imaging (DTI) studies in adults and adolescents are reviewed to explore possible progressive WM abnormalities in the disorder. Intra- and interhemispheric frontotemporal abnormalities were reported in adults with BD. Although evidence in children and adolescents with BD to date has been limited, similar intrahemispheric and interhemispheric findings have also been reported. The findings in youths suggest that these abnormalities may represent a trait marker present early in the course of BD. Functional connectivity studies, demonstrating a relationship between WM abnormalities and frontotemporal dysfunction in BD, and DTI studies of vulnerability in first-degree relatives of individuals with BD, are discussed. Together, findings suggest the involvement of abnormal frontotemporal WM development in the pathophysiology of BD and that these abnormalities may be early trait markers of vulnerability; however, more studies are critically needed.