To test the hypothesis that increased venous compliance manifested as inferior vena cava (IVC) dilation is an important substrate for syncope in children.

IVC diameters were measured in 191 children and adolescents with syncope and in 95 controls. Subjects were divided based on age <12 years (younger group) and ≥12 years (older group). IVC measurements at the right atrial junction (IVC-RA), 10 mm below the IVC-RA junction (IVC-RA10), and at the point of maximal diameter (IVCmax) were made. The linear relation to body surface area (BSA) was confirmed, as were dimensions indexed to BSA (iIVC). Relationships between iIVC and the time of day were evaluated.

In the syncope group, the mean age was 12.9 ± 3.6 years, mean weight was 54.7 ± 23 kg, and mean BSA was 1.5 ± 0.4 m². Among controls, all IVC dimensions varied linearly with BSA (P < .001). In the older group (140 patients with syncope and 60 controls), all iIVC dimensions were larger in the syncope cohort: iIVC-RA, 9 vs 7.7 mm/m² (P < .0001); iIVC-RA10, 9.4 vs 8.1 mm/m² (P < .0001); iIVCmax, 11.7 vs 10.6 mm/m² (P = .002). In the younger group (51 patients with syncope and 35 controls), there were no differences in iIVC measurements between the syncope cohort and controls: iIVC-RA, 10.2 vs 11.3 mm/m²; iIVC-RA10, 11.7 vs 12.0 mm/m²; iIVCmax, 14.2 vs 14.7 mm/m² (P > .05 for all).

The IVC is enlarged in teenagers with syncope compared with controls, suggesting that venous capacitance and resultant pooling play roles in the pathogenesis of syncope. In contrast, younger children with syncope do not demonstrate IVC dilation, suggesting that their syncope arises from a different mechanism.