Multidimensional apathy: evidence from neurodegenerative disease

Apathy is a demotivation syndrome common in neurodegenerative diseases and is fundamentally multidimensional in nature. Different methodologies have been used to identify and quantify these dimensions, which has resulted in multifarious concepts, ranging in the number and characteristics of apathy subtypes. This has created an ambiguity over the fundamental substructure of apathy. Here we review the multidimensional concepts of apathy and demonstrate that overlapping elements exist, pointing towards commonalities in apathy subtypes. These can be subsumed under a unified Dimensional Apathy Framework: a triadic structure of Initiation, Executive and Emotional apathy. Distinct cognitive processes may underlie these domains, while self-awareness interplays with all subtypes. Evidence from neurodegenerative diseases supports this distinction with differing apathy profiles in amyotrophic lateral sclerosis, Parkinson's disease and Alzheimer's disease.