Central nervous system disorders

III./4.1.: Epidemiology

By definition, meningitis is a viral, bacterial, fungal or parasitic

inflammation of the meninges (leptomeningitis). The inflammation of the dura mater is called pachymeningitis. The central nervous system is rather well protected against infections by its barrier system. The brain tissue does not have its own lymphoid system, thus its cellular immune protection is limited.

Since both viral and bacterial infections result in increased capillary permeability, immunised lymphocytes, macrophages and histiocytes can reach the meninges and the brain tissue. A cellular immune reaction builds up, but since the infection and the immune reaction are separated by days, the infective agents can cross the brain-blood barrier. The majority of purulent causative agents reach the meninges haematogenously. Some viruses enter the liquor via the blood vessel walls of the choroid plexus. Agents originating from the paranasal sinuses reach the subdural space through the diploic veins. From there they can migrate to the brain itself.

Immunosuppression or the immature immune system of newborns make it easier for the infectious agents to reach the nervous system. It is important to note that in case of inflammations of the central nervous system secondary tissue damage is not caused by the infectious agent itself, but by the succeeding cellular reaction.

Purulent meningitis is caused by Pneumococcus (Streptococcus pneumoniae), Haemophilus influenzae, Meningococcus (Neisseria meningitidis) in 3/4th of the cases, while the remaining 1/4th is caused by Staphylococcus aureus and a few Gram-negative bacteria. The prevalence of bacterial meningitises is 5-10/100 000 in Central Europe.