Why does autonomic dysreflexia happen?

Author: SCIRE Community Team | Reviewer: Andrei Krassioukov and Janice Eng | Published: 12 October 2017 | Updated: ~

Autonomic dysreflexia is caused by dysfunction of the autonomic nervous system after SCI that leads to poorly controlled blood pressure responses.

The autonomic nervous system controls largely unconscious bodily processes such as blood pressure, heart rate, breathing rate, body temperature, digestion, bladder, bowel, and sexual function. It has two divisions:

The sympathetic nervous system prepares the body for stressful or emergency situations. It is often called the ‘fight or flight’ system, because it prepares the body for action. For example, it increases heart rate and constricts blood vessels.

The parasympathetic nervous system prepares the body for normal, non-emergency situations. It is often called the ‘rest and digest’ system, because it allows the body to restore itself. For example, it slows heart rate and relaxes blood vessels.

The sympathetic and parasympathetic systems have different (and often opposite) effects on the organs and work together to control bodily functions according to the situation.

Blood pressure is carefully controlled by the autonomic nervous system to ensure that circulation works properly. The body monitors blood pressure and makes adjustments to maintain blood pressure within an optimal range. This is done in part by tightening (constricting) or relaxing (dilating) the blood vessels and changing heart rate.

When the body below the SCI detects strong stimulation, it activates the sympathetic nervous system, causing the constriction of blood vessels in the lower body. This causes blood pressure to rise.

Pressure sensors in the arteries then detect the elevated blood pressure and relay this message to the brain. Under normal circumstances, the brain sends signals through the spinal cord and cranial nerves to relax the blood vessels and slow heart rate. This restores blood pressure to normal.

However, when there is an SCI, the signal to relax the blood vessels is blocked from travelling to the lower body. Because of this, the blood vessels remain constricted and the body cannot restore blood pressure to normal. This causes the uncontrolled elevated blood pressure that happens during autonomic dysreflexia.

The level of T6 is important because nerves from this part of the spinal cord constrict a large group of blood vessels in the abdomen called the splanchnic vascular bed. These blood vessels contain a large volume of blood, so when the blood vessels are constricted, it causes the blood pressure to rise significantly. Injuries below T6 do not usually cause enough of a change in blood pressure to cause autonomic dysreflexia.