Sympathetic neural activity, sex dimorphism, and postural tachycardia syndrome.

OBJECTIVE: To elucidate the mechanisms whereby postural tachycardia syndrome (POTS) patients maintain orthostatic blood pressure and explain the greater prevalence of female cases in POTS cohorts.
METHODS: We investigated muscle sympathetic nerve (MSN) discharge characteristics in 12 female POTS patients and in 9 male and 12 female control subjects using the burst amplitude distribution and the relative contribution of burst frequency and burst amplitude.
RESULTS: At rest, burst distribution was shifted toward larger amplitudes in POTS patients (p < 0.005), consistent with increased sympathetic activity. During hypotension, the distribution shifted toward larger amplitude bursts in control subjects (p < 0.001), whereas it did not change in POTS patients. Total MSN activity increase to hypotension did not differ between subject groups, but the relative contribution of burst frequency change to the total activity increase was greater in POTS patients than in female (p < 0.05) and male (p < 0.001) control subjects. In contrast, the relative contribution of burst amplitude change to total MSN activity increase was greater in male compared with female control subjects (p < 0.05) and POTS patients (p < 0.001).
INTERPRETATION: At rest, the burst amplitude distribution was consistent with increased sympathetic activity in POTS and did not change in response to hypotension. In response to hypotension, burst frequency makes a proportionally greater contribution to the increase in total MSN activity in POTS patients compared with female control subjects, and female compared with male control subjects. These physiological differences in MSN discharge characteristics, in the setting of sympathetic fiber loss associated with POTS, may contribute to the predisposition to and greater prevalence of POTS in female individuals.