Spinal Cord Injury Medicine Physician-Scientist Mayo Clinic Rochester, Minnesota, United States
Objectives: 1. Quantify symptoms of autonomic dysreflexia (AD) 2. Compare physiologic vessel responsiveness and heart rate sensitivity between individuals with spinal cord injury (SCI) and uninjured controls. 3. Examine whether these physiologic parameters explain who develops symptoms of AD
Design: Individuals with SCI and controls had symptoms of AD quantified using the Autonomic Dysfunction Following SCI (ADFSCI)-AD survey. To asses physiologic responses, participants received three intravenous phenylephrine boluses, reproducibly increasing systolic blood pressure 15-40 mmHg. Continuous heart rate (R-R interval, ECG), beat-to-beat blood pressure (finapres), and popliteal artery flow velocity were recorded. Vascular responsiveness (alpha-1 adrenoreceptor sensitivity) and heart rate responsiveness to increased blood pressure (baroreflex sensitivity) were calculated.
Results: SCI (n=14) and control (n=17) cohorts were well-matched with mean age of 31.9 and 29.6 years (p=0.41), 21.4% and 17.6% female respectively. Baseline mean arterial pressure (MAP) (p=0.83) and R-R interval (p=0.39) were similar. ADFSCI-AD scores were higher following SCI (27.9+/-22.9 vs 4.2+/-2.9 in controls, p=0.002).
To quantify blood pressure response, MAP area under the curve was normalized to dose/bodyweight. Individuals with SCI had a significantly larger responses (0.26+/-0.19 mmHg*s/kg*ug) compared to controls (0.06+/-0.06 mmHg*s/kg*ug, p=0.002). Similarly, leg vascular resistance increased to a greater degree after SCI (24% vs 6% to a normalized dose, p=0.007). Baroreflex sensitivity was significantly lower after SCI compared to controls (15.0+/-8.3 vs 23.7+/-9.3 ms/mmHg, p=0.01). While a discrete range of variables was present after SCI, ADFSCI-AD subscore had no meaningful correlation with vascular responsiveness (R2=0.008) or baroreflex sensitivity (R2=0.092) in individuals with SCI.
Conclusions: While this research confirms smaller previous studies suggesting increased alpha-1 adrenoreceptor sensitivity and lower baroreflex sensitivity in individuals with SCI, these differences had no meaningful correlation to who had more symptoms of AD. Further research into physiologic mechanisms to explain why some individuals with SCI develop symptoms is needed.
