Association between vasovagal hypotension and low sympathetic neural activity during presyncope.

Recent studies suggest that an underlying mechanism for susceptibility of patients and astronauts to presyncope includes hypoadrenergic responses to orthostatic stress. However, data used to reach this conclusion are open to various interpretations. In this report, maintenance of sympathetic neural activity (MSNA; peroneal nerve microneurography) during -60 mmHg lower body negative pressure (LBNP) was associated with maintenance of orthostatic tolerance, and disappearance of MSNA was associated with hypotension and pre-syncope. However, MSNA was substantially higher during progressive increases of negative pressure in the presyncopal subject, compared to the non-presyncopal subjects. The data from this case report question the notion that orthostatic hypotension occurs due to inadequate sympathetic neural activation during orthostatic stress in apparently normal, healthy subjects.