Ischemic penumbra: the therapeutic window.

The concept of ischemic penumbra has extended our understanding of the focal ischemic process and in many ways has revitalized our research. Simply stated, the penumbra is the part of the brain that is sandwiched between brain regions committed to die and those that receive enough blood to communicate. Therefore, it is ischemic brain tissue that has just enough energy to survive for a short time but not enough to communicate and function. The life expectancy of the penumbra is short. Although the penumbra is an elegant concept, in practice it has been a difficult one to exploit. For one thing, it is unstable in both time and space. Depending on the severity and the duration of the focal ischemia, it may be anywhere in the ischemic brain. We believe that nimodipine binding experiments have taught us a great deal about the ischemic penumbra. Second, cells in the penumbra may die not by necrosis but by apoptosis. If that is true, then our concepts about the benign transient ischemic attack may need revision. Third, the penumbra may regain its ability to survive not only through reperfusion but also by interrupting the process of commitment to apoptosis.