Magnetic resonance diffusivity indices have been repeatedly proposed as biomarkers of neurodegeneration in amyotrophic lateral sclerosis (ALS), but no consensus exists as to which diffusivity parameter is the most sensitive to identify early degenerative changes. Despite numerous studies, surprisingly little is known of the segmental vulnerability of the corticospinal tracts and corpus callosum. Our objective was to characterize the core three-dimensional white matter signature of ALS, to describe phenotype-specific patterns of white matter degeneration and to evaluate the diffusivity profile of individual patients and controls in specific white matter segments.
A large neuroimaging study was undertaken with 62 patients and 55 age-matched healthy controls. White matter alterations were explored based on fractional anisotropy and radial, mean and axial diffusivity indices. Atlas-based region of interest analyses were carried out in the corona radiata, internal capsules, cerebral peduncles, and in the splenium, body and genu of the corpus callosum. Percentage change and receiver operating characteristic (ROC) curves were used to characterize disease-state discriminating diffusivity measures and white matter regions.
Bulbar onset patients exhibit extensive corticobulbar tract involvement in the genu of the internal capsule and in the lateral fibres of the corona radiata subjacent to the bulbar representation of the motor homunculus. Spinal onset patients show predominantly posterior internal capsule involvement and medial corona radiata pathology. ROC curve analyses revealed that diffusivity measures of the cerebral crura best discriminate patients and controls (area under the curve 80.1%).
Amyotrophic lateral sclerosis is associated with a core, disease-specific three-dimensional white matter signature which is best demonstrated by radial diffusivity measurements. The main ALS motor phenotypes are manifestations of the relatively selective involvement of corticospinal and corticobulbar fibres.