Widespread microstructural white matter involvement in amyotrophic lateral sclerosis: a whole-brain DTI study

25 years of neuroimaging in amyotrophic lateral sclerosis

Amyotrophic lateral sclerosis (ALS) is a fatal motor neuron disease for which a precise cause has not yet been identified. Standard CT or MRI evaluation does not demonstrate gross structural nervous system changes in ALS, so conventional neuroimaging techniques have provided little insight into the pathophysiology of this disease. Advanced neuroimaging techniques--such as structural MRI, diffusion tensor imaging and proton magnetic resonance spectroscopy--allow evaluation of alterations of the nervous system in ALS. These alterations include focal loss of grey and white matter and reductions in white matter tract integrity, as well as changes in neural networks and in the chemistry, metabolism and receptor distribution in the brain. Given their potential for investigation of both brain structure and function, advanced neuroimaging methods offer important opportunities to improve diagnosis, guide prognosis, and direct future treatment strategies in ALS. In this article, we review the contributions made by various advanced neuroimaging techniques to our understanding of the impact of ALS on different brain regions, and the potential role of such measures in biomarker development.

Altered cortical communication in amyotrophic lateral sclerosis

Amyotrophic lateral sclerosis (ALS) is a disorder associated primarily with the degeneration of the motor system. More recently, functional connectivity studies have demonstrated potentially adaptive changes in ALS brain organization, but disease-related changes in cortical communication remain unknown. We recruited individuals with ALS and age-matched controls to operate a brain-computer interface while electroencephalography was recorded over three sessions. Using normalized symbolic transfer entropy, we measured directed functional connectivity from frontal to parietal (feedback connectivity) and parietal to frontal (feedforward connectivity) regions. Feedback connectivity was not significantly different between groups, but feedforward connectivity was significantly higher in individuals with ALS. This result was consistent across a broad electroencephalographic spectrum (4-35 Hz), and in theta, alpha and beta frequency bands. Feedback connectivity has been associated with conscious state and was found to be independent of ALS symptom severity in this study, which may have significant implications for the detection of consciousness in individuals with advanced ALS. We suggest that increases in feedforward connectivity represent a compensatory response to the ALS-related loss of input such that sensory stimuli have sufficient strength to cross the threshold necessary for conscious processing in the global neuronal workspace.

Quantitative tractography and tract shape modeling in amyotrophic lateral sclerosis

PURPOSE

To investigate brain-wide white matter structural changes associated with amyotrophic lateral sclerosis (ALS) using an automatic single seed point tractography-based segmentation method, probabilistic neighborhood tractography (PNT), which provides quantitative measures of both tract integrity and shape.

MATERIALS AND METHODS

Diffusion MRI data were acquired from 30 patients with ALS (ALS Functional Rating Scale-Revised score > 20) and 30 matched controls. PNT was used to segment 12 major projection, commissural and association fibers, and assess differences in how the shape of an individual subject's tract compares to that of a predefined reference tract, in addition to providing tract-average mean diffusivity (〈D〉) and fractional anisotropy (FA) data.

RESULTS

Across all 12 tracts, group-averaged 〈D〉 was larger, while group-averaged FA was equal to or smaller in value for patients than controls. These differences were significant for right cingulum 〈D〉, and left and right corticospinal tract (CST) 〈D〉 and FA (P-values 6 × 10(-5) to 0.03). Tract shape modeling indicated that there were significantly greater topological differences from the reference tract in left and right CST, and right uncinate fasciculus (P-values 0.02 to 0.04) for patients than controls. The rate of disease progression was significantly negatively correlated with bilateral CST FA (P-values 0.01 to 0.02).

CONCLUSION

ALS, although particularly affecting CST, is associated with subtle changes in white matter tract integrity and shape in several other major fibers within the brain. Correlations between CST integrity and disease progression rate suggest that quantitative tractography may provide useful biomarkers of disease evolution in ALS.

Widespread structural and functional connectivity changes in amyotrophic lateral sclerosis: insights from advanced neuroimaging research

Amyotrophic lateral sclerosis (ALS) is a severe neurodegenerative disease principally affecting motor neurons. Besides motor symptoms, a subset of patients develop cognitive disturbances or even frontotemporal dementia (FTD), indicating that ALS may also involve extramotor brain regions. Both neuropathological and neuroimaging findings have provided further insight on the widespread effect of the neurodegeneration on brain connectivity and the underlying neurobiology of motor neurons degeneration. However, associated effects on motor and extramotor brain networks are largely unknown. Particularly, neuropathological findings suggest that ALS not only affects the frontotemporal network but rather is part of a wide clinicopathological spectrum of brain disorders known as TAR-DNA binding protein 43 (TDP-43) proteinopathies. This paper reviews the current state of knowledge concerning the neuropsychological and neuropathological sequelae of TDP-43 proteinopathies, with special focus on the neuroimaging findings associated with cognitive change in ALS.