What is meningitis and how does it cause brain injury?
Meningitis is an infection of the meninges, the three layers of membrane that envelop the brain and the spinal cord. The outermost layer lies under the skull, it is a thick membrane known as the Dura Mater, which literally means ‘tough mother’. The next layer is the Arachnoid Mater which looks a bit like a spider’s web, hence the reference to ‘arachnoid’. The final layer, which lies over the brain itself is the Pia Mater, which can be translated as ‘gentle mother’. Between the arachnoid mater and the pia mater is the arachnoid space which is full of Cerebrospinal Fluid (CSF), a watery substance that circulates around the brain.
The lining of blood vessels within the brain are much denser than those found elsewhere in the body. The purpose of this is to provide a protective barrier (the blood-brain-barrier) between the circulating blood and the brain itself, so that essential nutrients can pass through but harmful organisms cannot.
Various different types of viruses and bacteria can infect the meninges. In newborn babies, meningitis is most commonly caused by Streptococcus Group B and E coli bacteria. The bacteria may spread to the meninges from an adjacent infected area, or via the blood stream, penetrating the brain where the blood-brain-barrier is weak. In some situations, for example where there is a congenital abnormality or where there has been a surgical procedure to the brain, the bacteria can reach the meninges by direct contact between the meninges and the outside world.
Once the bacteria has got into the CSF, the bacteria can spread and multiply quickly.
How does brain damage occur?
Brain damage in bacterial meningitis is caused both by the direct action of toxins released from the bacteria and by the body’s own response to infection.
Toxins released from the bacteria damage brain cells directly but also cause damage to blood vessels, leading to a lack of blood (ischaemia) and subsequent energy deprivation and further brain cell death.
When the bacteria are detected by the immune system, the immune system releases chemical messengers called Cytokines, which in turn attract white blood cells. The white blood cells essentially ingest the bacteria which is obviously good news. The bad news is that the process of ingesting the bacteria leads to the release of toxins which can damage both brain cells and blood vessels too.
Therefore although the immune system serves to protect the brain and the rest of the body, the system starts a destructive cascade that can cause as much damage to the brain as the bacteria it aims to eliminate.
People who have suffered from meningitis may experience a wide range of different types of brain damage, depending on which areas of the brain were affected, including cerebral palsy, epilepsy, hearing loss and cognitive difficulties.