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Background and Objectives: 123 I-meta-iodobenzyl-guanidine ( 123 I-MIBG) myocardial scintigraphy is used as a diagnostic imaging test to differentiate Lewy body diseases (LBDs), including Parkinson disease and dementia with Lewy bodies, from other similar diseases. However, this imaging test lacks validation of its diagnostic accuracy against the gold standard. Our aim was to validate the diagnostic accuracy of 123 I-MIBG myocardial scintigraphy for LBD against autopsy, the gold standard., Methods: This retrospective, cross-sectional study included consecutive autopsy patients from the Brain Bank for Aging Research who had undergone 123 I-MIBG myocardial scintigraphy. We compared the 123 I-MIBG myocardial scintigraphy findings with autopsy findings. Furthermore, the proportion of residual tyrosine hydroxylase (TH)-immunoreactive sympathetic fibers in the anterior wall of the left ventricle was investigated to assess the condition of the cardiac sympathetic nerves assumed to cause reduced 123 I-MIBG uptake in LBDs., Results: We analyzed the data of 56 patients (30 with pathologically confirmed LBDs and 26 without LBD pathology). Compared with the neuropathologic diagnosis, the early heart-to-mediastinum (H/M) ratio had a sensitivity and specificity of 70.0% (95% CI 50.6%-85.3%) and 96.2% (95% CI 80.4%-99.9%), respectively. The delayed H/M ratio had a sensitivity and specificity of 80.0% (95% CI 61.4%-92.3%) and 92.3% (95% CI 74.9%-99.1%), respectively. The washout rate had a sensitivity and specificity of 80.0% (95% CI 61.4%-92.3%) and 84.6% (95% CI 65.1%-95.6%), respectively. The proportion of residual TH-immunoreactive cardiac sympathetic fibers strongly correlated with the amount of cardiac 123 I-MIBG uptake when assessed with early and delayed H/M ratio values (correlation coefficient 0.75 and 0.81, respectively; p < 0.001)., Discussion: This clinicopathologic validation study revealed that 123 I-MIBG myocardial scintigraphy could robustly differentiate LBDs from similar diseases. Abnormal 123 I-MIBG myocardial scintigraphy findings strongly support the presence of LBD and cardiac sympathetic denervation. However, LBD pathology should not necessarily be excluded by normal myocardial scintigraphy results, especially when other biomarkers suggest the presence of comorbid Alzheimer disease pathology., Classification of Evidence: This study provides Class II evidence that 123 I-MIBG myocardial scintigraphy accurately identifies patients with LBD., (Copyright © 2022 The Author(s). Published by Wolters Kluwer Health, Inc. on behalf of the American Academy of Neurology.)