Absence of selective deep white matter ischemia in chronic carotid disease.

BACKGROUND AND PURPOSE Deep white matter may be the location of an internal 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. Measurements of oxygen extraction fraction were made in superficial (cortical and subcortical) regions in the middle cerebral artery territory and in deep white matter (internal border zone) regions. The presence of selective ischemia of the deep white matter was assessed by the ratio of deep white matter:superficial oxygen extraction fraction. Ipsilateral hemispheric ratios among patients were assessed as a group as compared with contralateral hemispheric ratios and as compared with normal hemispheric ratios from 15 control volunteers. RESULTS Mean deep white matter to superficial oxygen extraction fraction ratios (+/-95% confidence limits) were 0.99 (+/-0.07), 1.01 (+/-0.06), and 1.02 (+/-0.08) for ipsilateral, contralateral, and normal hemispheres, respectively. No statistically significant difference was found between ipsilateral and contralateral (P = .691) or normal hemispheres (P = .68), nor was any 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 range 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 compromise occurring at an internal arterial border zone.