Ischaemic Penumbra

The penumbra is defined by blood flow thresholds rather than by anatomy. Below roughly eight to ten millilitres per hundred grams of tissue per minute, ion pumps fail and the tissue infarcts within minutes; between that and about twenty, neurons stop firing and fall electrically silent while still maintaining membrane integrity. That intermediate band is the penumbra: function lost, viability retained, fate dependent on how quickly flow returns. Left alone it is progressively recruited into the core, and it shrinks at a rate that varies enormously between patients with collateral circulation.

In practice it is estimated by imaging rather than by flow measurement. Diffusion-weighted magnetic resonance imaging marks the core while perfusion imaging marks the whole hypoperfused territory, and the mismatch between them approximates salvageable tissue; computed tomography perfusion with automated software does the same in most centres. Two 2018 trials, DAWN and DEFUSE 3, used exactly this selection to extend mechanical thrombectomy to patients presenting up to twenty-four and sixteen hours after last known well, with substantial benefit in the selected groups.

That is the concept's real consequence: it replaced the clock on the wall with a tissue clock. Eligibility for reperfusion became a question of what imaging shows is still alive, and the penumbra is the only tissue a neuroprotective agent could plausibly rescue.

Where it is misapplied is preclinical reporting. A reduction in rodent infarct volume is routinely presented as penumbral salvage when the same measurement can shrink because oedema was reduced, because staining was done too early, or because cell death was delayed past the assessment point rather than prevented. Treating the penumbra as a fixed compartment also misleads, since its size at three hours says little about its size at nine.