Meningitis, encephalitis and brain abscesses: Clinical

In order to properly “osmose” new information, your brain is protected from the rest of the body by three meningeal layers.

From outside to inside they’re the dura mater, arachnoid mater, and pia mater, with cerebrospinal fluid in the space between the arachnoid and pia.

If pathogens make their way into these layers, the inflammatory response results in meningitis.

If they find a way to penetrate into the brain parenchyma itself, the inflammatory response results in encephalitis.

And if a pathogen walls itself off - it’s called a brain abscess.

Meningitis can be caused by viruses, bacteria - including mycobacteria like TB, fungi, and parasites.

When it’s caused by viruses, tuberculosis, fungi or parasites, it’s called aseptic meningitis, because routine bacterial cultures of the cerebrospinal fluid are negative.

Sometimes, the infection may spill over from the meninges into the brain parenchyma, and that’s called meningoencephalitis.

Now, the most common causes of acute bacterial meningitis depend on the individual’s age.

For example, in infants less than 3 months, the most common causes in a descending order are group B streptococci, Escherichia coli, and Listeria monocytogenes.

In adolescents 13 to 17 years old, the most common causes are Neisseria meningitidis, followed by Streptococcus pneumoniae, and Haemophilus influenzae.

In non-adolescent children 3 months to 12 years and in adults, Streptococcus pneumoniae is the most common cause, followed by Neisseria meningitidis and Haemophilus influenzae.