Blunted tachycardia and cardiac sympathetic denervation in isolated rapid eye movement sleep behavior disorder.

Background: Isolated rapid eye movement sleep behavior disorder (iRBD) serves as a prodromal phase of Parkinson's disease (PD) and dementia with Lewy bodies (DLB). Blunted tachycardia (BT) during postural changes indicates neurogenic orthostatic hypotension, a marker of autonomic dysfunction. We aimed to investigate whether BT is associated with cardiac sympathetic neurogenic denervation. Additionally, we conducted a preliminary short-term follow-up to examine the potential prognostic significance of BT regarding phenoconversion and mortality.

Methods: Forty-three patients with iRBD at Shiga University of Medical Science Hospital underwent active standing tests to identify BT, defined by a specific ratio of decrease in systolic blood pressure to inadequate increase in heart rate after standing, and orthostatic hypotension. ¹²³I-metaiodobenzylguanidine myocardial scintigraphy (¹²³I-MIBG) and dopamine transporter single-photon emission computed tomography (DAT-SPECT) were performed. Participants were followed up for 3.4 ± 2.4 years for phenoconversion and 4.0 ± 2.3 years for mortality assessment, and the risk of events was analyzed using log-rank tests.

Results: Among the 43 participants (mean age, 72.3 ± 7.9 years; 8 female), 17 met the BT criteria. We found no significant comorbidity-related differences in hypertension or diabetes between the BT(+) and BT(-) groups. Orthostatic hypotension was more prevalent in the BT(+) group than in the BT(-) group (47.1% vs 7.7%, p = 0.003). BT(+) patients were older with a lower early and delayed MIBG uptake; however, no significant differences were observed in DAT accumulation. Phenoconversion was observed in seven (41.2%) BT(+) and seven (26.9%) BT(-) patients. Three deaths were recorded in the BT(+) group (17.6%) and three in the BT(-) group (11.5%). No significant differences were observed in the risk of phenoconversion or mortality between the groups.

Conclusions: We have identified the possibility that BT reflects cardiac sympathetic neurogenic denervation in patients with iRBD. Future research is needed to elucidate the potential prognostic value of BT.