Heterogeneity of the human myocardial sympathetic innervation: In vivo demonstration by iodine 123-labeled meta-iodobenzylguanidine scintigraphy

Abstract

The normal pattern of the myocardial sympathetic innervation was studied in 15 subjects using single photon emission computed tomography (SPECT) gamma scintigraphy with iodine 123-labeled meta-iodobenzylguanidine (I 123MIBG). Seven young subjects (mean age 29.4 ± 7.5 years SD) with supraventricular tachycardia and eight older patients (mean age 53.0 ± 5.1 years SD) with normal coronary arteries at cardiac catheterization and normal thallium-201 scintigrams were studied. MIBG uptake in the hearts of six patients with complete cardiac denervation after orthotopic cardiac transplantation was also studied. MIBG scintigrams were reconstructed into bull's-eye target plots and divided into eight equal sectors. Within each sector, four areas representing the apical, two midventricular, and basal regions were defined. There was a reduction in counts in the older group of subjects with normal coronary anatomy (1218.2 ± 198.4) as compared with younger subjects with supraventricular tachycardia (1124.4 ± 317.6), (F = 15.0, df = 1, p < 0.001). The difference was lost after adjustment for age ( p = 0.2) by means of analysis of covariance. There was a difference in counts within different sectors of the scan (F = 5.7, df = 7, p < 0.001), with lateral and anterior sectors having higher counts than septal and inferior sectors. There was no difference in the counts within areas of the scan (F = 0.04, df = 3, not significant [NS]) or different areas within the sectors (F = 1.1, df = 21, NS). A small diminution of counts (approximately 10%) in the 10 o'clock region of the bull's-eye target plot was observed in some scans. There was no uptake of MIBG above background levels in the hearts of patients who had undergone cardiac transplantation despite normal perfusion on thallium-201 scintigraphy. These data suggest that there is heterogeneity of the cardiac sympathetic innervation, with fewer catecholaminergic nerve terminals in the inferior and septal walls of the left ventricle. However, cardiac sympathetic nerve terminals do not differ in distribution in the apical to basal regions. Cardiac sympathetic innervation appears to diminish with age. After cardiac transplantation patients do not appear to have evidence of sympathetic nerve terminals within the myocardium. We conclude that there is a degree of heterogeneity of MIBG uptake in normal hearts and that the abnormalities observed in pathologic conditions should be interpreted with caution in light of these findings.