Autonomic dysreflexia in tetraplegic patients: evidence for alpha-adrenoceptor hyper-responsiveness

A controlled study of acute pharmacological intervention was designed to determine whether decreased sympathetic nerve activity in tetraplegic patients results in increased responsiveness of alpha-adrenoceptors which might contribute to vascular hyperreactivity and the clinical scenario of autonomic dysreflexia. The study took place in a university teaching hospital and included six male tetraplegic patients and six age-matched normal male controls. All tetraplegics were 5 months or longer post-traumatic spinal cord injury and all had experienced symptoms of autonomic dysreflexia on at least one occasion. The dorsal foot vein diameter was recorded with a tonometer during local infusions of noradrenaline 0.125-256 ng/min given through a short intravenous needle. In tetraplegic patients, there was a significant shift to the left of the dose-response curve indicating increased venous responsiveness to noradrenaline. The concentration of noradrenaline required to cause a 50% reduction of the resting vein diameter was decreased in tetraplegics (1.6 ng/min, geometric mean) compared to normal controls (10.9 ng/min, p < 0.02). alpha-Adrenoceptor responsiveness in dorsal foot veins is increased in patients with tetraplegia. Hypersensitivity of vascular alpha-adrenoceptors may contribute to autonomic dysreflexia in patients with high spinal cord injury.