Unravelling the nexus of stroke and dementia: Deciphering the role of secondary neurodegeneration in orchestrating cognitive decline.

Stroke is the leading cause of acquired disability. The development of acute ischemic stroke treatments, such as mechanical thrombectomy and tissue plasminogen activator, has resulted in more patients surviving the initial insult. However, long-term complications, such as post-stroke cognitive impairment (PSCI) and dementia (PSD), are at an all-time high. Notably, 80% of stroke survivors suffer from cognitive impairment, and a history of stroke doubles a patient's lifetime risk of developing dementia. A combination of greater life expectancy, an increase in the number of strokes in young individuals, and improved survival have inherently increased the number of years patients are living post-stroke, highlighting the critical need to understand the long-term effects of stroke, including how pathological changes in the brain might give rise to functional and behavioral changes in stroke survivors. Even with this increased risk of PSCI and PSD in stroke survivors, understanding of how the stroke itself develops into these conditions remains incomplete. Recently, secondary neurodegeneration (SND) following stroke has been linked with PSCI and PSD. SND is the degeneration of brain regions outside the original stroke site. Degeneration in these sites is thought to arise due to functional diaschisis with the infarct core; however, observation of SND pathology in multiple regions without direct connectivity to the stroke infarct suggests that the degeneration in these regions is likely more complex. Moreover, pathological hallmarks of dementia, such as a deposition of neurodegenerative proteins and iron, cell death, inflammation and blood-brain barrier alterations, have all been found in regions such as the thalamus, hippocampus, basal ganglia, amygdala and prefrontal cortex following stroke. Hence, in this review, we present the current understanding of PSCI and PSD in the context of SND and outline how remote anatomical and molecular changes may drive the development of these conditions.