2‐Deoxy‐2‐[18F]fluoro‐d‐glucose positron emission tomography, cortical thickness and white matter graph network abnormalities in brains of patients with amyotrophic lateral sclerosis and frontotemporal dementia suggest early neuronopathy rather than axonopathy

Background and purpose Amyotrophic lateral sclerosis (ALS) is a motor neuron disorder, although extra‐motor degeneration is well recognized, especially in frontotemporal regions manifested as ALS with frontotemporal dementia (ALS‐FTD). Previous neuroimaging studies of the brains of ALS‐FTD patients have measured abnormalities of either grey matter (GM) or white matter (WM) structures but not of both together. Therefore, the aim was to evaluate both GM and WM in the same ALS‐FTD patient using functional and structural neuroimaging. By doing so, insights could be gained into whether neurodegeneration in ALS‐FTD is primarily a neuronopathy or axonopathy. Methods After high‐resolution brain 2‐deoxy‐2‐[18F]fluoro‐D‐glucose (18F‐FDG) positron emission tomography (PET) and magnetic resonance imaging (MRI) scans were obtained in ALS‐FTD patients and in age‐ and sex‐matched neurological controls, changes in metabolic rate, cortical thickness (CT) and WM network analysis using graph theory were analyzed. Results Significant reductions in 18F‐FDG PET metabolism, CT and WM connections were observed in motor and extra‐motor brain regions of ALS‐FTD patients compared to controls. Both CT and underlying WM networks were abnormal in frontal, temporal, parietal and occipital lobes of ALS‐FTD patients with 86 of 90 brain regions showing reductions of CT. Conclusion Abnormalities in significantly fewer WM networks underlying the affected cortical regions suggest that neurodegeneration in brains of ALS‐FTD patients is primarily a ‘neuronopathy’ rather than an ‘axonopathy.’