Autonomic dysreflexia is a hypertensive episode in spinal cord-injured individuals induced by exaggerated sympathetic activity and thought to be α-adrenergic mediated. α-Adrenoceptor antagonists have been a rational first choice; nevertheless, calcium channel blockers are primarily used in autonomic dysreflexia management. However, α-adrenoceptor blockade may leave a residual vasoconstrictor response to sympathetic nonadrenergic transmission unaffected. The aim was to assess the α-adrenergic contribution and, in addition, the role of supraspinal control to leg vasoconstriction during exaggerated sympathetic activity provoked by autonomic dysreflexia in spinal cord-injured individuals and by a cold pressure test in control individuals. Upper leg blood flow was measured using venous occlusion plethysmography during supine rest and during exaggerated sympathetic activity in 6 spinal cord-injured individuals and 7 able-bodied control individuals, without and with phentolamine (α-adrenoceptor antagonist) and nicardipine (calcium channel blocker) infusion into the right femoral artery. Leg vascular resistance was calculated. In spinal cord-injured individuals, phentolamine significantly reduced the leg vascular resistance increase during autonomic dysreflexia (8±5 versus 24±13 arbitrary units; P=0.04) in contrast to nicardipine (15±10 versus 24±13 arbitrary units; P=0.12). In controls, phentolamine completely abolished the leg vascular resistance increase during a cold pressure test (1±2 versus 18±14 arbitrary units; P=0.02). The norepinephrine increase during phentolamine infusion was larger (P=0.04) in control than in spinal cord-injured individuals. These results indicate that the leg vascular resistance increase during autonomic dysreflexia in spinal cord-injured individuals is not entirely α-adrenergic mediated and is partly explained by nonadrenergic transmission, which may, in healthy subjects, be suppressed by supraspinal control.

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Erasmus MC: University Medical Center Rotterdam

Groothuis, J.T, Rongen, G.A, Deinum, J, Pickkers, P, Danser, A.H.J, Geurts, A.C.H, … Hopman, M.T.E. (2010). Sympathetic nonadrenergic transmission contributes to autonomic dysreflexia in spinal cord-injured individuals. Hypertension, 55(3), 636–643. doi:10.1161/HYPERTENSIONAHA.109.147330