Corpus callosum in neurodegenerative diseases: findings in Parkinson's disease

Long-term cognitive outcomes in Susac syndrome: A case series

Susac syndrome (SuS) presents with encephalopathy, visual disturbances, and hearing loss from immune-mediated microvascular occlusion. While acute SuS is well-described, long-term cognitive outcomes with current treatments are underknown. We assessed ten SuS patients treated in accordance with evidence-based guidelines using immunotherapies targeting humoral and cell-mediated pathways. Patients were followed for a median 3.6 years. Initially, cognition inversely correlated with corpus callosum lesions on MRI. All reported cognitive improvement; 5/10 patients had residual deficits in visual attention and executive function. Early, aggressive treatment was associated with good outcomes; extensive early corpus callosum lesions may identify patients at-risk of persistent cognitive deficits.

A PET-CT study on neuroinflammation in Huntington's disease patients participating in a randomized trial with laquinimod

Microglia activation, an indicator of central nervous system inflammation, is believed to contribute to the pathology of Huntington's disease. Laquinimod is capable of regulating microglia. By targeting the translocator protein, 11C-PBR28 PET-CT imaging can be used to assess the state of regional gliosis in vivo and explore the effects of laquinimod treatment. This study relates to the LEGATO-HD, multi-centre, double-blinded, Phase 2 clinical trial with laquinimod (US National Registration: NCT02215616). Fifteen patients of the UK LEGATO-HD cohort (mean age: 45.2 ± 7.4 years; disease duration: 5.6 ± 3.0 years) were treated with laquinimod (0.5 mg, N = 4; 1.0 mg, N = 6) or placebo (N = 5) daily. All participants had one 11C-PBR28 PET-CT and one brain MRI scan before laquinimod (or placebo) and at the end of treatment (12 months apart). PET imaging data were quantified to produce 11C-PBR28 distribution volume ratios. These ratios were calculated for the caudate and putamen using the reference Logan plot with the corpus callosum as the reference region. Partial volume effect corrections (Müller-Gartner algorithm) were applied. Differences were sought in Unified Huntington's Disease Rating Scale scores and regional distribution volume ratios between baseline and follow-up and between the two treatment groups (laquinimod versus placebo). No significant change in 11C-PBR28 distribution volume ratios was found post treatment in the caudate and putamen for both those treated with laquinimod (N = 10) and those treated with placebo (N = 5). Over time, the patients treated with laquinimod did not show a significant clinical improvement. Data from the 11C-PBR28 PET-CT study indicate that laquinimod may not have affected regional translocator protein expression and clinical performance over the studied period.

Midsagittal corpus callosal thickness and cognitive impairment in Parkinson's disease

People diagnosed with Parkinson's disease (PD) can experience significant neuropsychiatric symptoms, including cognitive impairment and dementia, the neuroanatomical substrates of which are not fully characterised. Symptoms associated with cognitive impairment and dementia in PD may relate to direct structural changes to the corpus callosum via primary white matter pathology, or as a secondary outcome due to the degeneration of cortical regions. Using magnetic resonance imaging, the corpus callosum can be investigated at the midsagittal plane, where it converges to a contiguous mass and is not intertwined with other tracts. The objective of this project was thus twofold; first, we investigated possible changes in the thickness of the midsagittal callosum and cortex in patients with PD with varying levels of cognitive impairment; and secondly, we investigated the relationship between the thickness of the midsagittal corpus callosum and the thickness of the cortex. Study participants included cognitively unimpaired PD participants (n = 35), PD participants with mild cognitive impairment (n = 22), PD participants with dementia (n = 17) and healthy controls (n = 27). We found thinning of the callosum in PD-related dementia compared to PD-related mild cognitive impairment and cognitively unimpaired PD participants. Regression analyses found thickness of the left medial orbitofrontal cortex to be positively correlated with thickness of the anterior callosum in PD-related mild cognitive impairment. This study suggests that a midsagittal thickness model can uncover changes to the corpus callosum in PD-related dementia, which occur in line with changes to the cortex in this advanced disease stage.

Comparative morphology of the corpus callosum across the adult lifespan in chimpanzees (Pan troglodytes) and humans

The human corpus callosum exhibits substantial atrophy in old age, which is stronger than what would be predicted from parallel changes in overall brain anatomy. To date, however, it has not been conclusively established whether this accentuated decline represents a common feature of brain aging across species, or whether it is a specific characteristic of the aging human brain. In the present cross-sectional study, we address this question by comparing age-related difference in corpus callosum morphology of chimpanzees and humans. For this purpose, we measured total midsagittal area and regional thickness of the corpus callosum from T1-weighted MRI data from 213 chimpanzees, aged between 9 and 54 years. The results were compared with data drawn from a large-scale human sample which was age-range matched using two strategies: (a) matching by chronological age (human sample size: n = 562), or (b) matching by accounting for differences in longevity and various maturational events between the species (i.e., adjusted human age range: 13.6 to 80.9 years; n = 664). Using generalized additive modeling to fit and compare aging trajectories, we found significant differences between the two species. The chimpanzee aging trajectory compared with the human trajectory was characterized by a slower increase from adolescence to middle adulthood, and by a lack of substantial decline from middle to old adulthood, which, however, was present in humans. Thus, the accentuated decline of the corpus callosum found in aging humans is not a universal characteristic of the aging brain, and appears to be human-specific.

Corpus callosum morphology across the lifespan in baboons (Papio anubis): A cross-sectional study of relative mid-sagittal surface area and thickness

The corpus callosum enables integration and coordination of cognitive processing between the cerebral hemispheres. In the aging human brain, these functions are affected by progressive axon and myelin deteriorations, reflected as atrophy of the midsagittal corpus callosum in old age. In non-human primates, these degenerative processes are less pronounced as previous morphometric studies on capuchin monkey, rhesus monkeys, and chimpanzees do not find old-age callosal atrophy. In the present study, we extend these previous findings by studying callosal development of the olive baboon (Papio anubis) across the lifespan and compare it to chimpanzee and human data. For this purpose, total relative (to forebrain volume) midsagittal area, subsectional area, and regional thickness of the corpus callosum were assessed in 91 male and female baboons using non-invasive MRI-based morphometry. The studied age range was 2.5-26.6 years and lifespan trajectories were fitted using general additive modelling. Relative area of the total and anterior corpus callosum showed a positive linear trajectory. That is, both measures increased slowly but continuously from childhood into old age, and no decline was observed in old age. Thus, comparable with all other non-human primates studied to-date, baboons do not show callosal atrophy in old age. This observation lends supports to the notion that atrophy of the corpus callosum is a unique characteristic of human brain aging.

Proprioceptive recalibration following implicit visuomotor adaptation is preserved in Parkinson's disease

Individuals with Parkinson's disease (PD) and healthy adults demonstrate similar levels of visuomotor adaptation provided that the distortion is small or introduced gradually, and hence, implicit processes are engaged. Recently, implicit processes underlying visuomotor adaptation in healthy individuals have been proposed to include proprioceptive recalibration (i.e., shifts in one's proprioceptive sense of felt hand position to match the visual estimate of their hand experienced during reaches with altered visual feedback of the hand). In the current study, we asked if proprioceptive recalibration is preserved in PD patients. PD patients tested during their "off" and "on" medication states and age-matched healthy controls reached to visual targets, while visual feedback of their unseen hand was gradually rotated 30° clockwise or translated 4 cm rightwards of their actual hand trajectory. As expected, PD patients and controls produced significant reach aftereffects, indicating visuomotor adaptation after reaching with the gradually introduced visuomotor distortions. More importantly, following visuomotor adaptation, both patients and controls showed recalibration in hand position estimates, and the magnitude of this recalibration was comparable between PD patients and controls. No differences for any measures assessed were observed across medication status (i.e., PD off vs PD on). Results reveal that patients are able to adjust their sensorimotor mappings and recalibrate proprioception following adaptation to a gradually introduced visuomotor distortion, and that dopaminergic intervention does not affect this proprioceptive recalibration. These results suggest that proprioceptive recalibration does not involve striatal dopaminergic pathways and may contribute to the preserved visuomotor adaptation that arises implicitly in PD patients.

Morphometric mapping of the macrostructural abnormalities of midsagittal corpus callosum in Wilson’s disease

Background and Purpose

The corpus callosum (CC) consists of topographically arranged white matter (WM) fibers. Previous studies have indicated the CC to be discretely involved in WD. In this study, we strived to characterize the macrostructural properties of the CC using midsagittal cross-sectional area and thickness profile measurements.

Materials and Methods

This study was performed using archived magnetic resonance imaging (MRI) scans of 14 patients with WD and 14 age- and gender-matched healthy controls. Using an automated software pipeline for morphometric profiling, the midsagittal CC was segmented into five sub-regions (CC_1–5) according to the Hofer–Frahm scheme. The mean thickness and area of different CC segments and their clinical and cognitive correlates were identified.

Results

The mean area was significantly different only in CC₂ segment (94.2 ± 25.5 vs. 118.6 ± 19.7 mm², corrected P < 0.05). The mean thickness was significantly different in CC₁ (5.06 ± 1.15 vs. 6.93 ± 0.89 mm, corrected P < 0.05), CC₂ (3.73 ± 0.96 vs. 4.87 ± 1.01 mm, corrected P < 0.05), and CC₃ segments (3.42 ± 0.84 vs. 3.94 ± 0.72 mm, corrected P < 0.05). The age at onset of neurological symptoms and MMSE score was significantly correlated with the morphometric changes of CC₁ and CC₂ segments.

Conclusion

Morphological changes of the CC are discrete in WD. Morphometric loss of CC was associated with an earlier onset of neurological symptoms and cognitive dysfunction in WD.

Shape Profile of Corpus Callosum As a Signature to Phenotype Different Dementia

Background  Phenotyping dementia is always a complex task for a clinician. There is a need for more practical biomarkers to aid clinicians. Objective  The aim of the study is to investigate the shape profile of corpus callosum (CC) in different phenotypes of dementia. Materials and Methods  Our study included patients who underwent neuroimaging in our facility as a part of clinical evaluation for dementia referred from Geriatric Clinic (2017-2018). We have analyzed the shape of CC and interpreted the finding using a seven-segment division. Results  The sample included MPRAGE images of Alzheimer' dementia (AD) ( n = 24), posterior cortical atrophy- Alzheimer' dementia (PCA-AD) ( n = 7), behavioral variant of frontotemporal dementia (Bv-FTD) ( n = 17), semantic variant frontotemporal dementia (Sv-FTD) ( n = 11), progressive nonfluent aphasia (PNFA) ( n = 4), Parkinson's disease dementia (PDD) ( n = 5), diffuse Lewy body dementia ( n = 7), progressive supranuclear palsy (PSP) ( n = 3), and corticobasal degeneration (CBD) ( n = 3). We found in posterior dementias such as AD and PCA-AD that there was predominant atrophy of splenium of CC. In Bv-FTD, the genu and anterior half of the body of CC was atrophied, whereas in PNFA, PSP, PDD, and CBD there was atrophy of the body of CC giving a dumbbell like profile. Conclusion  Our study findings were in agreement with the anatomical cortical regions involved in different phenotypes of dementia. Our preliminary study highlighted potential usefulness of CC in the clinical setting for phenotyping dementia in addition to clinical history and robust biomarkers.

Multifocal alterations of white matter accompany the transition from normal cognition to dementia in Parkinson's disease patients

The purpose of the present study was to investigate the pattern of white matter (WM) changes associated with Parkinson's disease (PD)-related cognitive impairment by using fractional anisotropy (FA), mean diffusivity (MD), axial diffusivity (AD) and radial diffusivity (RD) measures. Diffusion Tensor Imaging (DTI) was performed in 21 PD-patients with dementia (PDD) and in an age-matched control group including 40 PD-patients without dementia (PD-CTRL). The Parkinson's disease-Cognitive Rating Scale (PD-CRS) was used for patients' neuropsychological assessment. Local microstructural WM differences associated with the presence of cognitive impairment were tested using tract-based spatial statistics analysis. Multiple regression models investigated the association of DTI indices with total PD-CRS score, PD-CRS raw items and other clinical measures across the whole study sample. Significant FA decreases were found in PDD compared to PD-CTRL patients mainly in the body of corpus callosum, corona radiata and cingulum. Lower PD-CRS score was significantly associated with decreased FA, MD and AD values in multiple WM tracts primarily located in prefrontal and limbic areas as well as across the corpus callosum. Lower performance in specific PD-CRS raw items was also associated with FA decreases in major WM tracts. The results suggest that multifocal microstructural changes of WM accompany the transition from normal to demented cognitive state in PD-patients. The corpus callosum, the corona radiata and the cingulum are among the regions mostly affected during this course. A progressive axonal degeneration is proposed as a key underlying mechanism.

Corpus callosal atrophy and associations with cognitive impairment in Parkinson disease

OBJECTIVE

To investigate atrophy of the corpus callosum on MRI in Parkinson disease (PD) and its relationship to cognitive impairment.

METHODS

One hundred patients with PD and 24 healthy control participants underwent clinical and neuropsychological evaluations and structural MRI brain scans. Participants with PD were classified as cognitively normal (PD-NC; n = 28), having mild cognitive impairment (PD-MCI; n = 47), or having dementia (PDD; n = 25) by Movement Disorder Society criteria. Cognitive domain (attention/working memory, executive function, memory, language, visuospatial function) z scores were calculated. With the use of FreeSurfer image processing, volumes for total corpus callosum and its subsections (anterior, midanterior, central, midposterior, posterior) were computed and normalized by total intracranial volume. Callosal volumes were compared between participants with PD and controls and among PD cognitive groups, covarying for age, sex, and PD duration and with multiple comparison corrections. Regression analyses were performed to evaluate relationships between callosal volumes and performance in cognitive domains.

RESULTS

Participants with PD had reduced corpus callosum volumes in midanterior and central regions compared to healthy controls. Participants with PDD demonstrated decreased callosal volumes involving multiple subsections spanning anterior to posterior compared to participants with PD-MCI and PD-NC. Regional callosal atrophy predicted cognitive domain performance such that central volumes were associated with the attention/working memory domain; midposterior volumes with executive function, language, and memory domains; and posterior volumes with memory and visuospatial domains.

CONCLUSIONS

Notable volume loss occurs in the corpus callosum in PD, with specific neuroanatomic distributions in PDD and relationships of regional atrophy to different cognitive domains. Callosal volume loss may contribute to clinical manifestations of PD cognitive impairment.

Role of Corpus Callosum Volumetry in Differentiating the Subtypes of Progressive Supranuclear Palsy and Early Parkinson's Disease

Background and Objective

Progressive supranuclear palsy (PSP) is a progressive neurodegenerative disorder. Classic PSP or Richardson-Steele phenotype (PSP-RS) and parkinsonian phenotype (PSP-P) are the common subtypes of PSP. At the early stage, differentiating the subtypes of PSP as well as differentiating PSP from other parkinsonian disorders, especially Parkinson's disease (PD) is challenging. Microstructural abnormalities of corpus callosum (CC) have been reported both in PSP and PD. The objective of this study was to compare the volumes of various segments of CC between patients with PSP-P, PSP-RS, and early PD.

Methodology

This study included 32 patients with PSP (RS: 18, P: 14), 20 patients with early PD, and 25 controls. All subjects underwent 3-Tesla MRI. An automated surface-based analysis package (FreeSurfer) was used to divide CC into five segments: anterior (CC1), midanterior (CC2), central (CC3), midposterior (CC4), and posterior (CC5). Volumes of these segments were compared among the four groups.

Results

The PSP-RS group had significantly lower CC volume in all segments except in CC1 and CC5, whereas the volumes of the five segments of CC were comparable among PSP-P, PD and controls. The PSP-RS group had lower CC3 volume compared to the PSP-P group, and the PSP-RS group had lower volume of both CC2 and CC3 compared to the PD group.

Conclusions

The lower volume of the central segment of CC (CC3) might help in differentiating PSP-RS from PSP-P. There is no significant difference in the pattern of CC atrophy in PSP-P and early PD. Studies with higher sample sizes are warranted to confirm the results of our study.

In vivo imaging of brain glutamate defects in a knock-in mouse model of Huntington's disease

Huntington's disease (HD) is an inherited neurodegenerative disease characterized by motor, cognitive and psychiatric symptoms. Atrophy of the striatum has been proposed for several years as a biomarker to assess disease progression in HD gene carriers. However, it does not provide any information about the biological mechanisms linked to HD pathogenesis. Changes in brain metabolites have been also consistently seen in HD patients and animal models using Magnetic Resonance Spectroscopy (MRS), but metabolite measurements are generally limited to a single voxel. In this study, we used Chemical Exchange Saturation Transfer imaging of glutamate (gluCEST) in order to map glutamate distribution in the brain of a knock-in mouse model (Ki140CAG) with a precise anatomical resolution. We demonstrated that both heterozygous and homozygous mice with pathological CAG repeat expansion in gene encoding huntingtin exhibited an atrophy of the striatum and a significant alteration of their metabolic profile in the striatum as compared to wild type littermate controls. The striatal decrease was then confirmed by gluCEST imaging. Surprisingly, CEST imaging also revealed that the corpus callosum was the most affected structure in both genotype groups, suggesting that this structure could be highly vulnerable in HD. We evaluated for the first time gluCEST imaging as a potential biomarker of HD and demonstrated its potential for characterizing metabolic defects in neurodegenerative diseases in specific regions.

Corpus callosum atrophy associated with the degree of cognitive decline in patients with Alzheimer's dementia or mild cognitive impairment: a meta-analysis of the region of interest structural imaging studies

Individual structural neuroimaging studies of the corpus callosum (CC) in Alzheimer's disease (AD) and mild cognitive impairment (MCI) with the region of interest (ROI) analysis have yielded inconsistent findings. The aim of this study was to conduct a meta-analysis of structural imaging studies using ROI technique to measure the CC midsagittal area changes in patients with AD or MCI. Databases of PubMed, the Cochrane Library, the ISI Web of Science, and Science Direct from inception to June 2014 were searched with key words "corpus callosum" or "callosal", plus "Alzheimer's disease" or "mild cognitive impairment". Twenty-three studies with 603 patients with AD, 146 with MCI, and 638 healthy controls were included in this meta-analysis. Effect size was used to measure the difference between patients with AD or MCI and healthy controls. Significant callosal atrophy was found in MCI patients with an effect size of -0.36 (95% CI, -0.57 to -0.14; P = 0.001). The degree of the CC atrophy in mild AD was less severe than that in moderate AD with a mean effect size -0.69 (95% CI, -0.89 to -0.49) versus -0.92 (95% CI, -1.16 to -0.69), respectively. Comparing with healthy controls, patients with MCI had atrophy in the anterior portion of the CC (i.e., rostrum and genu). In contrast, patients with AD had atrophy in both anterior and posterior portions (i.e., splenium). These results suggest that callosal atrophy may be related to the degree of cognitive decline in patients with MCI and AD, and it may be used as a biomarker for patients with cognitive deficit even before meeting the criteria for AD.

Shape analysis of the corpus callosum in Alzheimer's disease and frontotemporal lobar degeneration subtypes

Morphology of the corpus callosum is a useful biomarker of neuronal loss, as different patterns of cortical atrophy help to distinguish between dementias such as Alzheimer's disease (AD) and frontotemporal lobar degeneration (FTLD). We used a sophisticated morphometric analysis of the corpus callosum in FTLD subtypes including frontotemporal dementia (FTD), semantic dementia (SD), and progressive non-fluent aphasia (PNFA), and compared them to AD patients and 27 matched controls. FTLD patient subgroups diverged in their callosal morphology profiles, with FTD patients showing marked widespread differences, PNFA patients with differences largely in the anterior half of the callosum, and SD patients differences in a small segment of the genu. AD patients showed differences in predominantly posterior callosal regions. This study is consistent with our previous findings showing significant cortical and subcortical regional atrophy across FTLD subtypes, and suggests that callosal atrophy patterns differentiate AD from FTLD, and FTLD subtypes.

Corpus Callosum and Cingulum Tractography in Parkinson's Disease

Background:

In Parkinson's disease (PD) cell loss in the substantia nigra is known to result in motor symptoms; however widespread pathological changes occur and may be associated with non-motor symptoms such as cognitive impairment. Diffusion tensor imaging is a quantitative imaging method sensitive to the micro-structure of white matter tracts.

Objective:

To measure fractional anisotropy (FA) and mean diffusivity (MD) values in the corpus callosum and cingulum pathways, defined by diffusion tensor tractography, in patients with PD, PD with dementia (PDD) and controls and to determine if these measures correlate with Mini-Mental Status Examination (MMSE) scores in parkinsonian patients.

Methods:

Patients with PD (17 Males [M], 12 Females [F]), mild PDD (5 M, 1F) and controls (8 M, 7F) underwent cognitive testing and MRI scans. The corpus callosum was divided into four regions and the cingulum into two regions bilaterally to define tracts using the program DTIstudio (Johns Hopkins University) using the fiber assignment by continuous tracking algorithm. Volumetric MRI scans were used to measure white and gray matter volumes.

Results:

Groups did not differ in age or education. There were no overall FA or MD differences between groups in either the corpus callosum or cingulum pathways. In PD subjects the MMSE score correlated with MD within the corpus callosum. These findings were independent of age, sex and total white matter volume.

Conclusions:

The data suggest that the corpus callosum or its cortical connections are associated with cognitive impairment in PD patients.

Transcallosal diffusion tensor abnormalities in predominant gait disorder parkinsonism

BACKGROUND

There have been no previous diffusion tensor imaging (DTI) studies comparing Parkinson's disease (PD) with postural instability and gait disorder (PIGD) parkinsonism.

OBJECTIVE

Utilizing DTI with 2-region tractography, we conducted a case control study to determine if different brain regions representing the neural network of the motor system are differentially affected in PIGD compared to PD and controls.

METHODS

On a 3 T MR machine, using manual ROI (regions of interest) we determined the fractional anisotropy (FA) and apparent diffusion coefficient (ADC) values on DTI in anatomical brain regions representing the extrapyramidal, pyramidal, and transcallosal tracts, aided by 2-region tractography. FA and ADC were correlated with the Tinetti score (measure of gait and balance).

RESULTS

Sixty-five subjects (21 PD, 25 PIGD, 19 controls) were included in the analysis. We demonstrated greater ADC abnormalities in the extrapyramidal, pyramidal and transcallosal motor systems in PIGD compared to controls. Multivariate analysis taking into consideration various clinical variables showed that the FA (p = 0.02) and ADC (p = 0.001) values in the corpus callosum body differentiated PIGD from PD. PIGD with low Tinetti score had a lower FA (p = 0.02) and a higher ADC value (corpus callosum body) (p = 0.03) compared to those with a high score.

CONCLUSIONS

We demonstrated for the first time that DTI abnormalities along the transcallosal motor tract in the body of the corpus callosum, but not the substantia nigra, differentiated PIGD from PD, and the degree of corpus callosum body abnormality correlated with the Tinetti score (a measure of risk of falls).

N-methyl-D-aspartate receptor-mediated axonal injury in adult rat corpus callosum

Damage to white matter such as corpus callosum (CC) is a pathological characteristic in many brain disorders. Glutamate (Glut) excitotoxicity through AMPA receptors on oligodendrocyte (OL) was previously considered as a mechanism for white matter damage. Recent studies have shown that N-methyl-D-aspartate receptors (NMDARs) are expressed on myelin sheath of neonatal rat OL processes and that activation of these receptors mediated demyelization. Whether NMDARs are expressed in the adult CC and are involved in excitotoxic axonal injury remains to be determined. In this study, we demonstrate the presence of NMDARs in the adult rat CC and their distributions in myelinated nerve fibers and OL somata by means of immunocytochemical staining and Western blot. Incubation of the CC slices with Glut or NMDA induced axonal injury as revealed by analyzing amplitude of CC fiber compound action potentials (CAPs) and input-output response. Both Glut and NMDA decreased the CAP amplitude and input-output responses, suggesting an involvement of NMDARs in Glut- and NMDA-induced axonal injury. The involvement of NMDAR in Glut-induced axonal injury was further assayed by detection of β-amyloid precursor protein (β-APP) in the CC axonal fibers. Treatment of the CC slices with Glut resulted in β-APP accumulation in the CC fibers as detected by Western blot, reflecting an impairment of axonal transport function. This injurious effect of Glut on CC axonal transport was significantly blocked by MK801. Taken together, these results show that NMDARs are expressed in the adult CC and are involved in excitotoxic activity in adult CC slices in vitro.

Diffusion tensor quantification and cognitive correlates of the macrostructure and microstructure of the corpus callosum in typically developing and dyslexic children

Noninvasive quantitative MRI methods, such as diffusion tensor imaging (DTI), can offer insights into the structure-function relationships in human developmental brain disorders. In this article, we quantified the macrostructural and microstructural attributes of the corpus callosum (CC) in children with dyslexia and in typically developing readers of comparable age and gender. Diffusion anisotropy, and mean, radial and axial diffusivities of cross-sectional CC subregions were computed using a validated DTI methodology. The normalized posterior CC area was enlarged in children with dyslexia relative to that in typically developing children. Moreover, the callosal microstructural attributes, such as the mean diffusivity of the posterior middle sector of the CC, correlated significantly with measures of word reading and reading comprehension. Reading group differences in fractional anisotropy, mean diffusivity and radial diffusivity were observed in the posterior CC (CC5). This study demonstrates the utility of regional DTI measurements of the CC in understanding the neurobiology of reading disorders.

Corpus callosum in aging and neurodegenerative diseases

SUMMARY The corpus callosum (CC) is a major white matter bundle that connects primarily homologous areas of the cortex. The structure may be involved in interhemispheric communication and enable the lateralization of certain cerebral functions. Despite its possible role as the main conduit for interhemispheric communication, interest from researchers has, at times, been sparse. Renewed interest has led to research that has shown that the CC may play a role in both cognitive aging and neurodegenerative diseases including Alzheimer´s disease and frontotemporal dementia. Studies employing structural MRI and diffusion-weighted MRI have found distinct subregional patterns of callosal atrophy in aging, Alzheimer´s disease and frontotemporal dementia. Furthermore, imaging studies may help to elucidate the underlying pathological mechanisms of callosal atrophy. The present review aims to provide an overview of the current knowledge of the structure and function of the CC and its role in aging and neurodegenerative disease.

Callosal tissue loss in multiple system atrophy--a one-year follow-up study

Multiple system atrophy (MSA) is a neurodegenerative disease not only affecting the basal ganglia, brainstem, cerebellum, and intermediolateral cell columns of the spinal cord but also the cerebral cortex. Clinically, cerebellar (MSA-C) and parkinsonian variants of MSA (MSA-P) are distinguished. We investigated 14 MSA patients (10 MSA-C, 4 MSA-P, men: 7, women: 7; age: 61.1 ± 3.3 years) and 14 matched controls (men: 7, women: 7; age: 58.6 ± 5.1 years) with voxel-based morphometry (VBM) to analyze gray and white matter differences both at baseline and at follow-up, 1 year later. Baseline comparisons between patients and controls confirmed significantly less gray matter in MSA in the cerebellum and cerebral cortex, and significantly less white matter in the cerebellar peduncles and brainstem. Comparisons of tissue-loss profiles (i.e., baseline versus follow-up) between patients and controls, revealed white matter reduction in MSA along the middle cerebellar peduncles, reflecting degeneration of the ponto-cerebellar tract as a particularly prominent and progressive morphological alteration in MSA. Comparisons between baseline and follow-up, separately performed in patients and controls, revealed additional white matter reduction in MSA along the corpus callosum at follow-up. This was replicated through additional shape-based analyses indicating a reduced callosal thickness in the anterior and posterior midbody, extending posteriorly into the isthmus. Callosal atrophy may possibly reflect a disease-specific pattern of neurodegeneration and cortical atrophy, fitting well with the predominant impairment of motor functions in the MSA patients.

Diffusion tensor imaging of the corpus callosum differentiates corticobasal syndrome from Parkinson's disease

Differential diagnosis between patients with Corticobasal syndrome (CBS) and Parkinson's disease (PD) may be confusing, particularly in early disease stages. However, in contrast to PD, CBS shows a widespread cortical atrophy that suggests an involvement of the corpus callosum (CC). To test this hypothesis, we used diffusion tensor imaging (DTI) with a 1.5T scanner to compare 14 CBS patients, 14 PD patients, and an age-matched control group. The mean diffusivity (MD) and fractional anisotropy (FA) were determined in the whole CC and in five subdivisions. Group comparisons were performed using the Mann-Whitney U-test. We found a significantly increased MD and decreased FA in CBS patients compared to PD, particularly in the posterior truncus. No differences were found between PD patients and controls. A receiver-operating characteristics (ROC) analysis shows that the MD is particularly useful for discriminating between the two neurodegenerative diseases. Our data suggest that abnormal CC diffusivity in CBS reflects an atrophy and degraded transcallosal connectivity, making the CC a potential target to differentiate CBS from PD patients.

Somatosensory responses in normal aging, mild cognitive impairment, and Alzheimer's disease

As a part of a larger study of normal aging and Alzheimer's disease (AD), which included patients with mild cognitive impairment (MCI), we investigated the response to median nerve stimulation in primary and secondary somatosensory areas. We hypothesized that the somatosensory response would be relatively spared given the reported late involvement of sensory areas in the progression of AD. We applied brief pulses of electric current to left and right median nerves to test the somatosensory response in normal elderly (NE), MCI, and AD. MEG responses were measured and were analyzed with a semi-automated source localization algorithm to characterize source locations and timecourses. We found an overall difference in the amplitude of the response of the primary somatosensory source (SI) based on diagnosis. Across the first three peaks of the SI response, the MCI patients exhibited a larger amplitude response than the NE and AD groups (P < 0.03). Additional relationships between neuropsychological measures and SI amplitude were also determined. There was no significant difference in amplitude for the contralateral secondary somatosensory source across diagnostic category. These results suggest that somatosensory cortex is affected early in the progression of AD and may have some consequence on behavioral and functional measures.

Callosal atrophy in mild cognitive impairment and Alzheimer's disease: different effects in different stages

Alzheimer's Disease (AD) is a neurodegenerative disorder that mainly affects grey matter (GM). Nevertheless, a number of investigations have documented white matter (WM) pathology associated with AD. The corpus callosum (CC) is the largest WM fiber bundle in the human brain. It has been shown to be susceptible to atrophy in AD mainly as a correlate of Wallerian degeneration of commissural nerve fibers of the neocortex. The aim of this study was to investigate which callosal regions are affected and whether callosal degeneration is associated with the stage of the disease. For this purpose, we analyzed high-resolution MRI data of patients with amnesic mild cognitive impairment (MCI) (n=20), mild AD (n=20), severe AD (n=10), and of healthy controls (n=20). Callosal morphology was investigated applying two different structural techniques: mesh-based geometrical modeling methods and whole-brain voxel-based analyses. Our findings indicate significant reductions in severe AD patients compared to healthy controls in anterior (genu and anterior body) and posterior (splenium) sections. In contrast, differences between healthy controls and mild AD patients or amnesic MCI patients were less pronounced and did not survive corrections for multiple comparisons. When correlating anterior and posterior WM density of the CC with GM density of the cortex in the severe AD group, we detected significant positive relationships between posterior sections of the CC and the cortex. We conclude that callosal atrophy is present predominantly in the latest stage of AD, where two mechanisms might contribute to WM alterations in severe AD: the Wallerian degeneration in posterior subregions and the myelin breakdown process in anterior subregions.

Diffusion tensor quantification of the macrostructure and microstructure of human midsagittal corpus callosum across the lifespan

The midsagittal cross-sectional area of the human corpus callosum (CC) has been used by many researchers as a marker of development, natural aging, and neurodegenerative and acquired pathologies. The availability of non-invasive MRI methods for quantifying the macrostructural and microstructural organization of the CC would help to clarify the CC contribution to behavior and cognition in both health and disease. In this report, we extended and validated the ability of a recently described semi-automated diffusion tensor imaging tissue segmentation method to utilize the high orientation contrast of the CC on diffusion tensor imaging. Using a cohort of healthy right-handed children and adults aged 7-59 years, we show gender-independent non-linear (quadratic) and strongly correlated growth trends in the CC area and the corresponding diffusion tensor fractional anisotropy (r = 0.67; P < 1 x 10(-10)). Our results provide preliminary evidence that diffusion tensor anisotropy in the living CC may be related to the number of small myelinated fibers.

Diffusion tensor analysis of corpus callosum in progressive supranuclear palsy

INTRODUCTION

Progressive supranuclear palsy (PSP) is a neurodegenerative disease featuring parkinsonism, supranuclear ophthalmoplegia, dysphagia, and frontal lobe dysfunction. The corpus callosum which consists of many commissure fibers probably reflects cerebral cortical function. Several previous reports showed atrophy or diffusion abnormalities of anterior corpus callosum in PSP patients, but partitioning method used in these studies was based on data obtained in nonhuman primates. In this study, we performed a diffusion tensor analysis using a new partitioning method for the human corpus callosum.

METHODS

Seven consecutive patients with PSP were compared with 29 age-matched patients with Parkinson's Disease (PD) and 19 age-matched healthy control subjects. All subjects underwent diffusion tensor magnetic resonance imaging, and the corpus callosum was partitioned into five areas on the mid-sagittal plane according to a recently established topography of human corpus callosum (CC1-prefrontal area, CC2-premotor and supplementary motor area, CC3-motor area, CC4-sensory area, CC5-parietal, temporal, and occipital area). Fractional anisotropy (FA) and apparent diffusion coefficient (ADC) were measured in each area and differences between groups were analyzed.

RESULTS

In the PSP group, FA values were significantly decreased in CC1 and CC2, and ADC values were significantly increased in CC1 and CC2. Receiver operating characteristic analysis showed excellent reliability of FA and ADC analyses of CC1 for differentiating PSP from PD.

CONCLUSION

The anterior corpus callosum corresponding to the prefrontal, premotor, and supplementary motor cortices is affected in PSP patients. This analysis can be an additional test for further confirmation of the diagnosis of PSP.

New insights into brain BDNF function in normal aging and Alzheimer disease

The decline observed during aging involves multiple factors that influence several systems. It is the case for learning and memory processes which are severely reduced with aging. It is admitted that these cognitive effects result from impaired neuronal plasticity, which is altered in normal aging but mainly in Alzheimer disease. Neurotrophins and their receptors, notably BDNF, are expressed in brain areas exhibiting a high degree of plasticity (i.e. the hippocampus, cerebral cortex) and are considered as genuine molecular mediators of functional and morphological synaptic plasticity. Modification of BDNF and/or the expression of its receptors (TrkB.FL, TrkB.T1 and TrkB.T2) have been described during normal aging and Alzheimer disease. Interestingly, recent findings show that some physiologic or pathologic age-associated changes in the central nervous system could be offset by administration of exogenous BDNF and/or by stimulating its receptor expression. These molecules may thus represent a physiological reserve which could determine physiological or pathological aging. These data suggest that boosting the expression or activity of these endogenous protective systems may be a promising therapeutic alternative to enhance healthy aging.

Protective effect of BDNF against beta-amyloid induced neurotoxicity in vitro and in vivo in rats

We examined the potential protective effect of BDNF against beta-amyloid-induced neurotoxicity in vitro and in vivo in rats. In neuronal cultures, BDNF had specific and dose-response protective effects on neuronal toxicity induced by Abeta(1-42) and Abeta(25-35). It completely reversed the toxic action induced by Abeta(1-42) and partially that induced by Abeta(25-35). These effects involved TrkB receptor activation since they were inhibited by K252a. Catalytic BDNF receptors (TrkB.FL) were localized in vitro in cortical neurons (mRNA and protein). In in vivo experiments, Abeta(25-35) was administered into the indusium griseum or the third ventricle and several parameters were measured 7 days later to evaluate potential Abeta(25-35)/BDNF interactions, i.e. local measurement of BDNF release, number of hippocampal hilar cells expressing SRIH mRNA and assessment of the corpus callosum damage (morphological examination, pyknotic nuclei counting and axon labeling with anti-MBP antibody). We conclude that BDNF possesses neuroprotective properties against toxic effects of Abeta peptides.

A modern hypothesis: The distinct pathologies of dementia associated with Parkinson's disease versus Alzheimer's disease

BACKGROUND/AIMS

Parkinson's disease dementia (PDD) is common, but its neuropathological basis has been controversial. The aim of this review was to survey the recent literature on the pathology of PDD and compare the pathology of PDD to that of Alzheimer's disease (AD).

METHODS

A literature search was performed to identify relevant research published since 2001.

RESULTS

There is widespread Lewy body pathology in the neocortex and subcortical regions in PDD, and Lewy neurites in the CA2 region of the hippocampus that correlate with cognitive decline. Genetic forms of PD, which frequently lead to dementia, are associated with deposition of alpha-synuclein; PDD is not related to the apolipoprotein E epsilon 4 genotype. Compared with AD, central cholinergic deficits occur earlier, are greater and more widespread in PDD, but PDD can occur without the abundant senile plaques and neurofibrillary tangles indicative of AD.

CONCLUSION

Epidemiological investigations, neuroimaging, as well as genetic and neuropathological studies increasingly support PDD as distinct from AD.

Regional atrophy of the corpus callosum in dementia

The regional distribution of degeneration of the corpus callosum (CC) in dementia is not yet clear. This study compared regional CC size in participants (n = 179) from the Cache County Memory and Aging Study. Participants represented a range of cognitive function: Alzheimer's disease (AD), vascular dementia (VaD), mild ambiguous (MA-cognitive problems, but not severe enough for diagnosis of dementia), and healthy older adults. CC outlines obtained from midsagittal magnetic resonance images were divided into 99 equally spaced widths. Factor analysis of these callosal widths identified 10 callosal regions. Multivariate analysis of variance revealed significant group differences for anterior and posterior callosal regions. Post-hoc pairwise comparisons of CC regions in patient groups as compared to the control group (controlling for age) revealed trends toward smaller anterior and posterior regions, but not all were statistically significant. As compared to controls, significantly smaller anterior and posterior CC regions were found in the AD group; significantly smaller anterior CC regions in the VaD group; but no significant CC regional differences in the MA group. Findings suggest that dementia-related CC atrophy occurs primarily in the anterior and posterior portions.

Is Alzheimer's disease a disconnection syndrome?

In Alzheimer's disease (AD), loss of connectivity in the patient's brain has been evidenced by a range of electrophysiological and neuroimaging studies. However, few neuropsychological research projects have sought to interpret the cognitive modifications following the appearance of AD in terms of a disconnection syndrome. In this paper, we sought to investigate brain connectivity in AD via the study of a crossmodal effect. More precisely, we examined the integration of auditory and visual speech information (the McGurk effect) in AD patients and matched control subjects. Our results revealed impaired crossmodal integration during speech perception in AD, which was not associated with disturbances in the separate processing of auditory and visual speech stimuli. In conclusion, our data suggest the occurrence of a specific, audio-visual integration deficit in AD, which might be the consequence of a connectivity breakdown and corroborate the observation from other studies of crossmodal deficits between the auditory and visual modalities in this population.

Sexual dimorphism of the human corpus callosum: Digital morphometric study

Background/Aim. Changes in the morphology and the size of the corpus callosum, are related to various pathological conditions. An analysis of these changes requires data about sexual dimorphism of the corpus callosum, which we tried to obtain in our study. We also investigated the method of digital morphometry and compared the obtained results with the results of other authors obtained by magnetic resonance imaging or by planimetry. Methods. A morphological research included 34 human brains (cadavers of both sexes ? 19 female and 15 male aged 26?72 years). By digital morphometry using an AutoCAD software we performed measurements in the corpus callosum: the length (L), width in the half of its length (WW?), length of its cortical margin (LCM), area and perimeter of the anterior and posterior callosal segments, as well as the area and perimeter of the corpus callosum section area. The investigated parameters were analyzed and compared between the females and males. Results. There was not a statistically significant difference between the males and females in the investigated parameters of the corpus callosum (t test; p > 0.05), including the mean values of the two most important parameters, the surface of its midsagittal section area (males 654.11 mm2; females 677.40 mm2) and of its perimeter (males 19.61 cm; females 19.72 cm). The results obtained by digital morphometry were in the range of the results of other authors obtained by magnetic resonance and by planimetry. However, the value of Pearson coefficient of linear correlation between the section surface area and perimeter of the corpus callosum in the males was highly significant (rxy = 0.6943, p < 0.01), while in the females this value was statistically insignificant. Conclusion. Digital morphometry is accurate method in encephalometric investigations. Our results suggest that the problem of sexual dimorphism of the corpus callosum is very complex, because the identical variables (section surface area or its perimeter) do not exhibit the same behavior in males and in females, implicating that these variables even cannot be simply compared between the sexes.