In a recent study of stroke patients, those who exhibited continuing deterioration of the nerve cells in the brain after the stroke also exhibited the highest levels of the protein glutamate in their blood  Glutamate, which functions within nerve cells as a neurotransmitter, can kill surrounding nerve cells if it leaks from damaged or oxygen-starved nerve cells
Thus glutamate leaking from damaged or oxygen-starved nerve cells is a cause of longterm brain damage resulting from strokes
Leakage from damaged or oxygen-starved nerve cells is the only possible source of glutamate in the blood.