Modeling contributions of cognition and apathy to functional impairment in younger-onset dementia.

INTRODUCTION: Overlaps in symptom presentation limits the capacity to predict functional impairment and future care needs in younger-onset dementia syndromes. METHODS: A general additive model (GAM) was applied to cross-sectional retrospective data from 375 participants with younger-onset dementia; 152 behavioral-variant frontotemporal dementia (bvFTD), 118 Alzheimer's disease (AD), 66 semantic dementia, and 39 progressive nonfluent aphasia (PNFA). This GAM aimed to explore the dynamic interrelationships between established measures of global cognition, apathy, and functional impairment. RESULTS: Our GAM significantly predicted functional impairment in all syndromes with a high explained variance (59.5%). Cognition and apathy emerged as significant predictors of functional impairment in each syndrome (p-values < .015). These relationships were consistently linear in AD, non-linear in SD, and mixed in bvFTD and PNFA (i.e., cognition linear and apathy non-linear). DISCUSSION: Our study shows the potential prognostic utility of GAMs for identifying syndrome-specific transition periods across group-level staging's of functional impairment. Highlights: First study to apply a general additive model to functional impairment in younger-onset dementia.Studied 375 individuals with younger-onset Alzheimer's disease or frontotemporal dementia.Apathy and cognition were significant predictors of functional impairment in all syndromes.This modeling has significant implications for syndrome-specific prognosis and management.