The brainstem in neurodegenerative diseases.

The brainstem, despite its modest size relative to the cerebral cortex, is critically involved in the pathology and clinical manifestations of numerous neurodegenerative diseases (NDDs). Historically, research on NDDs such as Alzheimer disease, Lewy body disease, and frontotemporal lobar degeneration predominantly adopted a cortico-centric perspective. However, emerging neuropathologic evidence underscores the brainstem's essential role, with early pathologic changes often predating cortical involvement. This chapter highlights salient points regarding the pathology and clinicopathologic correlations of brainstem involvement across major NDDs, emphasizing the chronology of disease progression. Key mechanisms, including protein misfolding and aggregation, selective neuronal vulnerability, and neurotransmitter dysfunction, are explored. Clinical correlations illustrate how early brainstem pathology significantly contributes to prodromal symptoms and helps define distinct clinical phenotypes, such as autonomic dysfunction, sleep disturbances, and mood disorders. Recognizing the chronologic order and specific nuclei affected in the brainstem broadens our understanding of disease progression, highlighting opportunities for targeted interventions at earlier disease stages.