Branch atheromatous disease (BAD) is a distinct subtype of ischemic stroke that is particularly prevalent among Chinese and other Asian populations. Patients with BAD are prone to early neurological deterioration and poor functional outcomes, and BAD is increasingly recognized as an independent disease entity. However, the association between BAD and cognitive impairment remains unclear. This nationwide, multicenter, prospective observational study will prospectively collect epidemiological, clinical, neuroimaging, functional, and cognitive outcome data from patients with BAD. The study aims to investigate the occurrence of cognitive impairment after BAD and identify the clinical and neuroimaging characteristics associated with cognitive impairment. In particular, the relationship between early neurological deterioration (END) and cognitive impairment will be investigated. As an exploratory objective, this study will further explore the potential brain network mechanisms underlying cognitive impairment associated with END events.