A PHOTOTHROMBOTIC RING MODEL OF RAT STROKE-IN-EVOLUTION DISPLAYING PUTATIVE PENUMBRAL INVERSION

Background and Purpose To facilitate reproducible and rigorous study o f a tissue zone at risk of encroaching ischemic damage, we propose a n ew model in which the potentially compromised tissue lies within rathe r than perifocal to an ischemic locus. The perimeter of the ''zone at risk'' is defined by a photothrombotically produced cortical lesion in the shape of a toroid (or ''ring''). Methods The exposed crania of er ythrosin B-injected rats were irradiated with a 514.5-nm laser beam, c onfigured as a 5-mm-diameter ring, to yield a ring-shaped lesion cause d by photochemically induced platelet occlusion of cortical vasculatur e. Developing perfusion deficits in the interior region were revealed by carbon black infusion. Tissue damage and infarct volumes were asses sed by light and electron microscopy, and blood-brain barrier integrit y was assessed with Evans blue dye and horseradish peroxidase as trace rs. Results For rats injected with 17 mg/kg erythrosin B and irradiate d for 2 minutes with a ring beam intensity of 0.92 W/cm(2) (beam power of 65 mW), carbon black infusion at times up to 4 hours demonstrated a shallow cortical ring lesion encircling a fully patent zone at risk, which by 24 hours evinced an essentially complete perfusion deficit. At times up to 24 hours, the ring lesion was penetrated at the pial su rface by distal branches of the middle cerebral and anterior cerebral arteries. Stereotaxically based histopathological assessment showed th at by 24 hours the lesion spanned the cortical thickness. Lesion volum e increased from 14.5+/-8.0 mm (mean+/-SD) (n=8) to 46.2+/-15.6 mm(3) (n=8) between 4 and 24 hours after irradiation (P<.01), but the antero posterior lesion diameter did not change significantly between 4 hours (6.00+/-1.03 mm; n=9) and 24 hours (6.75+/-1.15 mm; n=9). Conclusions The present model of slowly developing but inevitable cortical tissue death in a sequestered area should facilitate more precise observatio ns of the evolution of tissue metabolic responses, from the impending onset of ischemia to the threshold of irreversible damage. This system may prove efficient for evaluating treatments intended to salvage a p enumbral region.