Absence of selective deep white matter ischemia in chronic carotid disease: A positron emission tomographic study of regional oxygen extraction

BACKGROUND AND PURPOSE: Deep white matter may be the location of an interna l arterial border zone. The purpose of this study was to determine whether the deep white matter was subject to a greater degree of ischemia than was the cortex among patients with chronic carotid occlusion, METHODS: Thirty-six patients with carotid occlusion and structurally normal deep white matter were studied with positron emission tomography, Measurem ents of oxygen extraction fraction were made in superficial (cortical and s ubcortical) regions in the middle cerebral artery territory and in deep whi te matter (internal border zone) regions, The presence of selective ischemi a of the deep white matter was assessed by the ratio of deep white matter:s uperficial oxygen extraction fraction, Ipsilateral hemispheric ratios among patients were assessed as a group as compared with contralateral hemispher ic ratios and as compared with normal hemispheric ratios from 15 control vo lunteers, RESULTS: Mean deep white matter to superficial oxygen extraction fraction r atios (+/-95% confidence limits) were 0.99 (+/-0.07), 1.01 (+/-0.06), and 1 .02 (+/-0.08) for ipsilateral, contralateral, and normal hemispheres, respe ctively. No statistically significant difference was found between ipsilate ral and contralateral (P = .691) or normal hemispheres (P = .68), nor was a ny statistically significant difference found when the analysis was limited to patients with increased superficial oxygen extraction fraction (n = 9). Individual deep white matter:superficial ratios were within the normal ran ge for all patients, CONCLUSION: Normal deep white matter among patients with carotid occlusion is not subject to a greater degree of ischemia than is the overlying cortex , It is unlikely that deep white matter infarctions observed among patients with carotid occlusion are owing to chronic selective hemodynamic compromi se occurring at an internal arterial border zone.