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Novel MRI and MRS Findings of Brain Damage After Bypass Surgery

Magnetic resonance imaging of the brain before and after coronary artery bypass grafting suggests new hypotheses on the cause of brain damage during bypass.

In this unique study, researchers reported neurologic, neuropsychological, and magnetic resonance imaging (MRI) and spectroscopy (MRS) findings before and after coronary artery bypass grafting (CABG). Two novel findings emerged.

First, 17 new lesions were found on diffusion weighted (DW)-MRI images in 9 (26%) of the 35 patients. These lesions were cortical or subcortical and appeared as infarcts on later T2-weighted MRI scans. These embolic infarcts were truly silent: Patients showed no new, related neurologic or neuropsychological impairments. In all previous studies of so-called silent infarcts, there had been no prior examinations or imaging. Thus, those lesions were unexpected by clinicians who ordered the scans, but there may have been symptoms and signs that had not been appreciated or considered important or that were not recalled by patients or families; or the histories and examinations may not have been thorough.

The second novel finding was that MRS performed at specific sites 3 to 5 days after CABG showed significantly decreased N-acetylaspartate (NAA)/creatine ratios. This metabolic abnormality correlated with decreased neuropsychological performance. In 12 of the patients who had follow-up MRS 10 to 14 days after surgery, recovery from the metabolic abnormality occurred, accompanied by a return to baseline on neuropsychological testing.

Comment: This study confirms that bypass surgery very often produces brain infarction, most likely due to emboli arising from the aorta or the heart. Unfortunately, thorough echocardiography (to show potential embolic cardiac and aortic disease) is seldom performed before CABG.

More mysterious are the MRS results. A unifying hypothesis would be that the decreased NAA/creatine ratio might also be related to microembolism-associated, transient ischemia (reversible because the emboli are multiple but quite small). Transcranial-Doppler (TCD) monitoring studies (Stroke 1996; 27:871, and Curr Probl Cardiol 1997; 22:447) and pathology (Ann Neurol 1990; 28:477) show that microembolism is extensive and universal after CABG when the aorta is clamped and bypass pumps are used. These findings suggest that microembolism during the bypass might cause the temporary metabolic effects seen with MRS. However, might there instead be a metabolic or toxic explanation for these effects? The role of microembolism in brain damage after CABG may be clarified by further research that compares the findings from this study with those in patients who undergo "off-pump" CABG without aortic clamps and that compares the findings of TCD microemboli monitoring during surgery with MRS findings.

— Louis R. Caplan, MD

Dr. Caplan is Professor of Neurology, Harvard Medical School, Boston.

Published in Journal Watch Neurology September 26, 2002

Citation(s):

Bendszus M et al. Brain damage after coronary artery bypass grafting. Arch Neurol 2002 Jul; 59:1090-5.

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