The cerebral ischemic penumbra.

When neuronal activity is suppressed by ischemia of sufficient severity, maintained for a long enough period of time, permanent clinical deficit can be accompanied by radiographic signs of cerebral infarction and histological evidence of ischemic damage. Recent work suggests that this is not an all-ornone event. Between the most densely ischemic region and the more normally perfused brain, an intermediate zone exists where blood flow is reduced to a level that interrupts neuronal function and the consequent electrical activities such as somatosensory evoked potentials (SEP) and electroencephalography (EEG), yet permits maintenance of membrane pumps and preservation of ion gradients. This area is referred to as the ischemic penumbra. This intermediate zone has two characteristics that explain its potential clinical importance: 1) the interruption of clinical and electrical function that characterizes this zone is fundamentally reversible, but 2) the reversibility is time-limited. This description of the penumbra suggests that complete understanding of its properties and its potentially successful management may minimize the clinical deficit resulting from an acute cerebral infarct. Table 1: Some Characteristics of the Ischemic Penumbra*