Autonomic hyperreflexia is a reaction of the autonomic (involuntary) nervous system to overstimulation. This reaction may include high blood pressure, change in heart rate, skin color changes (paleness, redness, blue-grey skin color), and excessive sweating.

The most common cause of autonomic hyperreflexia is spinal cord injury. In this condition, types of stimulation that are tolerated by healthy people create an excessive response from the person's nervous system.

Other causes include medication side effects, use of illegal stimulants such as cocaine and amphetamines, Guillain-Barre syndrome (a severe form of paralysis that can lead to respiratory failure), subarachnoid hemorrhage (a form of brain bleeding), severe head trauma, and other brain injuries.

The following conditions share many similar symptoms with autonomic hyperreflexia, but have a different cause:

• Carcinoid syndrome -- a disease caused by abnormalities of hormone-producing cells in the lungs and the gut
• Neuroleptic malignant syndrome -- a condition characterized by muscle stiffness, high fever, and drowsiness, which can be caused by some antipsychotic and anesthesia medications
• Serotonin syndrome -- an abnormal release of serotonin, a brain chemical
• Thyroid storm -- a condition caused by too much production of thyroid hormone