Autonomic hyperreflexia is an imbalanced reflex sympathetic discharge that occurs in patients with spinal cord injury (SCI) higher than the level of splanchnic sympathetic outflow. Autonomic hyperreflexia is also known as autonomic dysreflexia (AD).

Approximately 65% to 85% of patients with SCI above the T ₇ level experience autonomic hyperreflexia. Prevalence rates vary, with the incidence occurring in 48% to 90% of the patients with an injury above T ₆. Because the male to female ratio of SCI is 4:1, that same ratio applies to the incidence of autonomic hyperreflexia.

Autonomic hyperreflexia is typically seen after the post-injury period of spinal shock has “run its course” and some reflexes have returned. These intact peripheral sensory nerves transmit to the spinothalamic and posterior columns, which then return impulses via the sympathetic neurons located in the intermediolateral gray matter. The inhibitory mechanisms above the SCI are not transmittable past the level of the SCI; thus the sympathetic stimulation is unopposed and can produce serious vasoconstriction, bradycardia, and other symptoms. The box on p. 37 lists stimuli that produce or trigger an episode of autonomic hyperreflexia.