MAPT and Menkes disease: In the context of neurodegenerative diseases, increases in grey matter MD at symptomatic stages are thought to reflect loss of neurons and dendrites, allowing water molecules to diffuse more freely.42 When detected at presymptomatic stages, grey matter MD increases have been suggested to indicate early neuronal loss,43 while MD decreases could signify the preceding inflammation and cellular swelling restricting water diffusion.44,45 Unlike volumetric measurements, however, we did not detect significant presymptomatic MD differences in GRN and MAPT groups compared with non-carriers.