Introduction

Stroke is a leading cause of death in Western countries, with a case mortality rate of 24% within the first month, higher than most forms of cancer. It is the commonest cause of long-term adult disability. Recent important advances in acute stroke therapy include the established benefits of stroke units, and the introduction of the first proven interventional therapies. These include intravenous t-PA within 3 hours, acute use of aspirin and administration of intra-arterial thrombolysis within 6 hours of stroke onset. A range of new strategies are being investigated, which generally involve either rapid reperfusion with thrombolytic agents, or neuroprotection from the damaging neurotoxic cascade that follows ischemia.

Both strategies focus on treatment of the ischemic penumbra. An understanding of the pathophysiologic basis of brain ischemia and the penumbra is important in interpreting the magnetic resonance (MR) changes imaged in acute stroke. The penumbra is a variable region of critically underperfused but still viable tissue surrounding the infarcted core. Despite symptomatic hypoperfusion, essential energy-requiring processes such as the Na-K ATPase system, are still able to maintain ionic gradients, and hence membrane and neuronal integrity. The penumbra represents a therapeutic window for intervention. However, the duration of this interval is uncertain and is likely to vary considerably from person to person. Rapid identification of the penumbra can allow rational therapeutic decisions to be made based on assessment of pathophysiology in the individual patient, rather than relying on the arbitrary time windows used in clinical trials.