Autonomic hyperreflexia is an abnormal triggering of the autonomic nervous system. It occurs after a spinal cord injury. The body is unable to turn off the nerves that cause blood pressure to rise.

What is going on in the body?

Normally, the autonomic nervous system controls blood pressure automatically. It does this by commanding muscles around blood vessels to tighten or relax in order to raise or lower blood pressure. The nervous system can monitor blood pressure and continually adjusts the commands to keep blood pressure normal.

When a person has a spinal cord injury, it can cause damage to nerves in the spinal cord. And this results in a loss of control in the nerves. This can happen if the damage to the spinal cord is in the neck or upper back. The bones of the spine are each labeled and numbered. Damage at the 6th thoracic bone or higher means the person may develop this problem. If the damage is lower, autonomic hyperreflexia will not be a problem.

In autonomic hyperreflexia, the autonomic nervous system can still cause the blood pressure to rise. But it cannot easily cause it to come back down.

What are the causes and risks of the disease?

Below the level of damage in the spinal cord, the person usually is paralyzed and cannot feel normal sensations. Autonomic hyperreflexia occurs when an irritating stimulus occurs that the person cannot feel. The most common cause is a very full bladder. The bladder sends signals through the nervous system to the brain. The signals cannot get through because of the spinal cord damage. The person cannot feel that the bladder is full and needs to be emptied. But the automatic part of the nervous system that controls the blood pressure still gets triggered and causes the blood pressure to go up.

Other common causes are a full bowel, an infection, or a sore on the skin. Women with spinal cord injuries who have menstrual cramps may also have autonomic hyperreflexia. Sometimes the trigger is as simple as accidentally sitting on a hard object or having clothing bunched up.