What causes secondary injury?

Author: SCIRE Community Team | Reviewer: Chris S. Bailey | Published: 22 January 2018 | Updated: ~

During inflammation, special immune cells help get rid of foreign substances. This can lead to localized swelling.²

Inflammation is a natural bodily process where cells from the immune system (such as white blood cells) are brought to the site of an injury or illness. These cells remove harmful bacteria and dead cells from the area to help with healing.

Some inflammation is needed for healing. However, when it is excessive or continues for a long time, it can be damaging. Inflammation can end up breaking down healthy nerve cells (neurons), which makes the injury worse. Inflammation and other factors can also contribute to swelling of the spinal cord near the injury. Swelling can compress the spinal cord even more, damaging nerve cells and impairing blood flow.

When the spinal cord is injured, the small blood vessels that provide oxygen and nutrients to the cord are also injured. If the blood vessels are torn, they may bleed within the spinal cord causing a bruise (called haemorrhage), which can increase the pressure on the spinal tissues and cause damage.

If the blood vessels are compressed or if there is not enough whole body blood pressure (which happens as a part of neurogenic shock), they are also unable to maintain adequate blood flow to the cord. This is called ischemia (pronounced ‘iss-kee-mee-ah’). A lack of blood flow means that oxygen or nutrients cannot reach the tissues, which can damage or kill healthy cells. Ischemia can become severe within hours after the injury and can damage nerve cells and surrounding tissues, causing the injury to get bigger.

Damaged nerve cells can cause the release of chemical compounds like glutamate. Glutamate is a neurotransmitter that stimulates nerve cells to fire. However, too much glutamate can overstimulate the cells, causing calcium to build up inside them, which damages and kills healthy cells. This is called excitotoxicity. Excitotoxicity is a major cause of secondary damage after SCI.

Inflammation, excitotoxicity and cell damage can release by-products called free radicals. Free radicals are molecules that are unstable and highly reactive. Free radicals can damage DNA, the fragile outer membranes, and other parts of cells.

Cells can die because of damage or injury (called necrosis) or because the body triggers the cells to self-destruct through a process called apoptosis.

Apoptosis is sometimes called ‘programmed cell death’ because it is a natural part of how the body gets rid of cells it doesn’t need.

Under normal circumstances, apoptosis is carefully controlled by the body. However, certain conditions can trigger apoptosis when it is not needed. For example, damage to parts of the cell or chemical imbalances around or within the cell can trigger apoptosis.

After an SCI, secondary injury processes like excitotoxicity, inflammation, and the release of free radicals can trigger apoptosis. This can affect both nerve cells (neurons) and supporting cells like the cells that maintain myelin. This can even happen far away from the injury. Cell apoptosis after SCI expands the area of damage by killing previously healthy neurons and supporting cells.