Cardiac neurotransmission imaging with 123I-meta-iodobenzylguanidine in postural tachycardia syndrome

Postural orthostatic tachycardia syndrome (POTS) is a disorder of orthostatic intolerance characterised by excessive tachycardia of unknown aetiology. Whether this condition involves abnormal cardiac sympathetic innervation or function remains elusive. Meta-iodobenzylguanidine (MIBG) resembles guanethidine and is a pharmacologically inactive analogue of norepinephrine, which is similarly metabolised in noradrenergic neurons. MIBG myocardial scintigraphy is used clinically to estimate local myocardial sympathetic nerve damage in some forms of heart disease and autonomic neuropathy. The objective of this study was to evaluate cardiac sympathetic innervation in patients with POTS. 20 patients with POTS were studied using (123)I-MIBG-single photon emission computed tomography, standardised autonomic testing, assessment of catecholamine plasma levels and sympathetic skin response. In four POTS patients (20.0%), myocardial MIBG uptake was markedly decreased. The mean heart to mediastinum ratio was reduced to 1.22+/-0.08 compared with the normal range of >1.7. No correlation was found between myocardial MIBG uptake and degree of postural tachycardia, baroreflex sensitivity, catecholamine plasma levels or other autonomic parameters. Sympathetic skin responses were normal in all patients. These findings suggest that POTS may be, in part, a manifestation of autonomic cardiac neuropathy. MIBG myocardial scintigraphy may be helpful to distinguish patients with neuropathic POTS from patients with orthostatic intolerance of other origin.