Abnormal Sympathetic Responses To Orthostasis After Concussion Explored Through Blood Pressure Variability

Abstract

PURPOSE: Orthostasis causes a gravity-dependent redistribution of blood volume to the lower extremities. The baroreflex coordinates an effector response through the sympathetic nervous system (SNS) to the peripheral vasculature to buffer changes in systolic blood pressure (SBP). In autonomic dysfunction (AD), impaired or exaggerated responses may lead to orthostatic hypotension (OHypo) or hypertension (OHTN). AD is becoming an increasingly recognized consequence of an acute concussion injury. The purpose of this study was to evaluate the SNS responses to orthostasis in recently concussed athletes. METHODS: A prospective study was performed in 32 concussed college athletes (gender: 22 female, 10 male; age: 20 ± 1 yrs; height: 1.74 ± 0.11 m; weight: 74 ± 13 kg) and 14 athlete controls (gender: 8 female, 6 male; age: 20 ± 2 yrs; height: 1.70 ± 0.10 m; weight: 69 ± 13 kg). Beat-to-beat SBP was assessed digitally at rest for 10 minutes in the supine (SUP) position and then for 3 minutes in the standing (STND) position within 48 hours (48H) of concussion. The change in SBP (ΔSBP) was computed from the difference between SUP and the final 15 seconds of STND. ΔSBP was then categorized as Normal (NR: ΔSBP -19 to +19 mmHg); OHypo (SBP fall ≥20 mmHg); and, OHTN (SBP rise ≥20 mmHg) for each group. Fast-Fourier transform was performed and power calculated for the total, very low, low and high frequency power (TP, vLF, LF and HF, respectively) spectra in each position; the variables were log10 transformed for parametric analyses. RESULTS: Fourteen control and 14 concussion participants had a NR ΔSBP, whereas 11 concussion participants had OHypo and 7 had OHTN. There were no differences across these four groups for the TP, vLF, LF, and HF in the SUP position. In the STND position, TP (p < 0.0001), vLF (p < 0.0001), and HF (p < 0.05) were significant for the omnibus model. Bonferroni post hoc analyses revealed that the OHypo concussion group had a significantly lower TP and vLF than each of the 3 other groups, and the OHTN concussion group had a significant elevation and trend toward a significant elevation of HF compared to the OHypo and NR concussion groups, respectively. CONCLUSIONS: Different mechanisms appear to contribute to and influence AD during orthostasis within 48H of concussion injury.