Clinical meaning
Autonomic dysreflexia (AD) is a potentially life-threatening medical emergency occurring in patients with spinal cord injury (SCI) at or above T6. It is triggered by a noxious stimulus below the level of injury (most commonly bladder distension from urinary retention or blocked catheter, followed by fecal impaction). The pathophysiology: (1) A noxious stimulus below the injury activates sensory neurons, sending pain/pressure signals up the spinothalamic tract. (2) These signals cannot pass through the spinal cord lesion to reach the brain for processing but DO activate the sympathetic nervous system in the intact thoracolumbar sympathetic chain (T5-L2). (3) Massive uninhibited sympathetic discharge occurs below the lesion: release of norepinephrine causes severe vasoconstriction → profound hypertension (systolic BP can rise to 250-300 mmHg). (4) Baroreceptors in the carotid sinus and aortic arch detect the hypertension and signal the brainstem to activate the parasympathetic response via the vagus nerve: bradycardia, vasodilation ABOVE the lesion (flushing, sweating above injury level). (5) However, the descending inhibitory signals from the brainstem CANNOT pass through the spinal cord lesion to reach the sympathetic neurons below — so...