Regarding “Can carotid endarterectomy improve metabolic status in patients with asymptomatic internal carotid artery flow lesion? Studies with localized in vivo proton magnetic resonance spectroscopy”

Noninvasive evaluation of the carotid artery and its branches has progressed from indirect assessment of the hemodynamic consequences of carotid occlusive disease to direct assessment with real-time B-mode imaging and pulsed Doppler techniques. More recently, advances in magnetic resonance and high resolution computed tomographic angiography have permitted noninvasive threedimensional visualization of the extracranial and intracranial circulation. During the same time period, cerebral imaging has progressed from conventional computed tomography and magnetic resonance imaging techniques to the more advanced and constantly unfolding applications of H magnetic resonance spectroscopy (MRS), P MRS, and functional magnetic resonance imaging. H MRS has made it possible to investigate cerebral metabolism in vivo and noninvasively. It can provide significant insight into the neurochemical alterations associated with ischemic, infectious, and neoplastic pathology by measuring markers of neuronal density, regeneration, and maturation; indices of phospholipid metabolism and gliosis; and indirect markers of energy reserve and storage. The molecules primarily visible with H MRS are: N-acetylaspartate, choline, creatine, and myo-inostitol. N-acetylaspartate is widely used as a marker of neuronal density. It has been shown that N-acetylaspartate is synthesized exclusively in the mitochondria of neurons, and decreases in N-acetylaspartate may be due, at least in part, to reduction in mitochondrial metabolism. Thus, reversible N-acetylaspartate losses may be associated with recovery of mitochondrial metabolism. Neuronal cell death will cause an irreversible loss of N-acetylaspartate due to the localization of N-acetylaspartate in the neuron. The choline peak mainly consists of glycerophosphocholine and phosphocholine compounds. These molecules are involved in cellular biosynthesis of phospholipids and the synthesis of acetylcholine in neurons. Total creatine concentration is the amount of creatine plus phosphocreatine in brain tissue. Creatine is part of the creatine kinase energy metabolism buffer system used to maintain adenosine triphoshate (ATP) levels in times of acute mismatch between oxidative ATP supply and ATP demand. Total creatine tends to remain relatively constant in most brain cell types, and when comparing similar cell populations, it can be used as a concentration reference for the other measured molecules. Myo-inostitol appears to act as a marker of glial cell numbers, an osmoregulator, and an intracellular messenger. In animal models, a rapid decline in N-acetylaspartate was shown within the first 8 hours after the onset of focal ischemia. This was followed with a slower rate of reduction over the next 24 hours. Most studies have been able to correlate an increase in cerebral lactate with a decrease in N-acetylaspartate. In an animal model of global ischemia, followed by recirculation, choline levels progressively decreased during reperfusion and were lower than those in control animals after 150 minutes of recirculation. The increased choline levels after ischemia may be associated with cellular and phospholipid breakdown or the degradation of acetylcholine. Although experimental models have shown an increase in choline, clinical models have either shown no change or a reduction. Initial publications using in vivo H MRS to evaluate human cerebral ischemia began to appear around 1990. Studies to date have assessed either the metabolic changes associated with an ischemic event (stroke or transient ischemic attack) or those associated with a carotid stenosis or occlusion. Studies evaluating metabolic changes after an ischemic event have been further divided into those performed acutely after stroke and those evaluating changes occurring from 2 to 6 months after an ischemic event. Measured metabolites include N-acetylaspartate, choline, creatine, and lactate. N-acetylaspartate and choline have been reported as absolute values and relative to creatine; From the Departments of Surgerya and Radiology,b University of Nebraska Medical Center. Competition of interest: nil. Reprint requests: Thomas G. Lynch, MD, Department of Surgery, 983280 Nebraska Medical Center, Omaha, NE 68198-3280 (e-mail: tlynch@unmc.edu). J Vasc Surg 2002;36:646-8. Copyright © 2002 by The Society for Vascular Surgery and The American Association for Vascular Surgery. 0741-5214/2002/$35.00 0 24/9/126554 doi:10.1067/mva.2002.126554