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

The most common cause of autonomic hyperreflexia is spinal cord injury. Stimuli which are otherwise tolerated in healthy people (such as filling of the urinary bladder) create an excessive response from the patient's nervous system.

Other causes include medication side effects, illicit use of stimulants such as cocaine and amphetamine (which enhance the effects of adrenaline in the body), Guillain-Barre syndrome (a severe form of paralysis which 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, but have a different underlying cause:

• Carcinoid syndrome -- a disease caused by abnormal cells of the lungs and gut that produce hormones
• Thyroid storm -- a condition caused by excessive production of thyroid hormone
• Neuroleptic malignant syndrome -- a condition characterized by muscle stiffness, high fever, and drowsiness, which can be caused by some antipsychotic medications and anesthetic agents
• Serotonin syndrome -- an abnormal release of serotonin, a brain chemical

Symptoms include:

• High blood pressure
• Slow pulse or fast pulse
• Fever
• Flushing
• Bladder or bowel dysfunction
• Headache
• Profuse sweating
• Anxiety
• Nasal congestion
• Muscle spasm

A complete neurological and medical examination must be performed. Patients must provide an accurate medication and drug history in order to help determine which tests are necessary. These may include:

• Brain pictures including head CT or MRI
• Spine pictures, particularly MRI
• Lumbar puncture
• Blood and urine tests
• Toxicology screening
• X-rays
• EKG
• Tilt-table testing

Treatment depends on the cause. Offending medications or drugs must be discontinued. Any underlying illness that is causing the symptoms needs to be treated. If a substantial slowing of the heart rate is shown to produce symptoms, some drugs called anticholinergics (such as atropine) may be tried.

Very high blood pressure needs to be treated carefully, as it may be quite unpredictable, falling abruptly. If symptoms do not improve with medications, a pacemaker may be required.

In patients with spinal cord injury, autonomic hyperreflexia may be caused by pain, fecal impaction, bladder distension, pressure sores, or suctioning. Careful attention to the patient's needs can prevent these problems.