Autonomic Dysreflexia Revisited

Autonomic dysreflexia (AD) is a clinical phenomenon that affects patients with spinal cord injury (SCI) above the major sympathetic outflow tract. The lesion is most often at or above the T-6 level. Any noxious stimuli below this level initiate reflex sympathetic activity resulting in life threatening hypertension uncontrollable by the feedback parasympathetic activity. The episodes of hypertension generally persist until the offending stimulus is removed. Absence of higher control over reflex sympathetic activity due to transection of the cord is an anatomical explanation of the phenomenon. Current evidence suggests additional factors such as supersensitivity and possibly increased numbers of spinal alpha adrenoreceptors and peripheral microvascular adrenoreceptors as well as accumulation of substance P below the lesion. It has been suggested that substance P acts as a modulator, initiating the sympathetic event to produce a strong, slow and prolonged excitatory action. Autonomic dysreflexia is further accentuated by the absence of gamma amino benzoic acid (GABA), norepinephrine (NE) and 5-hydroxytryptamine (5-HT) below the lesion. GABA is an inhibitory neurotransmitter. It has been suggested that either NE or 5-HT may also act as an inhibitory neurotransmitter. Resetting of the baroreceptors at a lower level also plays an important role. The anatomical transection at or above T-6 then helps in maintaining and accentuating the biochemical changes that develop in patients with high spinal cord lesions. The current article reviews the pathophysiology and management of this potentially life threatening, yet easily treatable, phenomenon.