During ischemic stroke, blood supply to part of the brain is decreased, leading to dysfunction of the brain tissue in that area. There are four reasons why this might happen:

1. Thrombosis (obstruction of a blood vessel by a blood clot forming locally)
2. Embolism (obstruction due to an embolus from elsewhere in the body),
3. Systemic hypoperfusion (general decrease in blood supply, e.g., in shock)
4. Cerebral venous sinus thrombosis.