Autonomic responses in children and adolescents with orthostatic syncope and presyncope: children are not small adults

Children and adolescents commonly experience orthostatic intolerance associated with impaired participation and quality of life. We aimed to characterize autonomic responses to provoked presyncope in children with recurrent presyncope/syncope and healthy adolescents. We determined orthostatic tolerance (OT, time to presyncope [mins]) in 36 pediatric patients (age 15 ± 3 yrs., 26 female) with recurrent presyncope/syncope, and 17 asymptomatic controls (age 13 ± 3 yrs., 8 female), using a tilt test with graded lower body negative pressure. Cardiovascular parameters, forearm vascular resistance (FVR), mean middle cerebral artery velocity (MCAv), and breath-by-breath end tidal gases were continuously monitored. Responses to the Valsalva maneuver (VM), cerebral autoregulation, and cerebral reactivity to carbon dioxide were also determined. OT was similar in pediatric patients (21 ± 1.5 min) and controls (20 ± 2.0 min, p = 0.74), but smaller than adult reference values (33.8 ± 0.8 min, p < 0.01). Tilting decreased systolic arterial pressure in pediatric patients (p = 0.009), but not pediatric controls (p = 0.12). Tilting decreased MCAv (p = 0.002) in pediatric patients, with impairments in cerebral autoregulation (p = 0.02) that were negatively correlated with OT (r = -0.322; p = 0.024). Both pediatric patients (+48.9 ± 8.0 %) and controls (+36.7 ± 14.7 %) had small FVR responses compared to adult reference data (+100 ± 12 %, p < 0.01). Blood pressure responses to the VM were abnormal in pediatric patients, with a lower nadir in mean arterial pressure (81.7 ± 2.0 mmHg) compared to pediatric controls (94.0 ± 2.8 mmHg, p = 0.001). Pediatric patients with recurrent presyncope/syncope had impaired orthostatic cardiovascular and autoregulatory responses compared to pediatric controls. Sympathetically-mediated responses were small in children, underscoring the need for pediatric-specific standards for orthostatic cardiovascular control, and treatments targeting enhancement of vascular resistance in children with syncope.