Magnetic resonance markers of ischaemia: their correlation with vasodilatory reserve in patients with carotid artery stenosis and occlusion

Objectives-Fetter methods of identifying patients with asymptomatic carotid artery stenosis who are at high risk of stroke are required. It has been s uggested that proton magnetic resonance spectroscopy (MRS) may allow the id entification of ongoing ischaemia in this patient group by the detection of a potentially reversible reduction of N-acetyl aspartate (NAA), a presumed marker of neuronal integrity, and the presence of lactate, a marker of ana erobic metabolism. Previous studies have reported metabolite ratios rather than absolute concentrations. This study was performed to determine if NAA was reduced ipsilateral to carotid stenosis or occlusion, and if its concen tration was related to carbon dioxide reactivity, a marker of cerebrovascul ar reserve. Methods - Twenty one patients with unilateral carotid stenosis (>70%) or oc clusion were studied. Single voxel proton MRS was performed in the ipsilate ral and contralateral hemispheres, with the voxel positioned in the arteria l borderzone region between the middle and anterior cerebral artery territo ries. Absolute quantification of metabolite concentrations was performed. C erebrovascular reactivity to 6% carbon dioxide was determined in both middl e cerebral artery territories using transcranial Doppler ultrasonography. Results-Mean (SD) cerebrovascular reactivity was significantly lower in the stenosed compared with the contralateral hemisphere (13.3 (7.7)v 19.2 (8.2 )%/kPa, p=0.002). There were no significant differences in the absolute con centrations of NAA, choline, or creatine between the ipsilateral and contra lateral hemispheres (for example, NAA 10.1 (1.1) v 10.5 (1.1) mmol/l, p=0.1 ). No lactate peak was seen in any spectra. For each metabolite measured, t here was no correlation between the absolute concentration and cerebrovascu lar reactivity for either hemisphere. Conclusions-In patients with carotid stenosis and occlusion we found no evi dence that chronic hypoperfusion is associated with a reduction in NAA or t he presence of lactate. Magnetic resonance spectroscopy is unlikely to help in the selection of patients with asymptomatic carotid stenosis for endart erectomy.