Meningitis is usually caused by infection by viruses or microorganisms. Most cases are due to infection with viruses, with bacteria, fungi, and parasites being the next most common causes. It may also result from various non-infectious causes.
Bacterial
The types of bacteria that cause bacterial meningitis vary by age group. In premature babies and newborns up to three months old, common causes are group B streptococci (subtypes III which normally inhabit the vagina and are mainly a cause during the first week of life) and those that normally inhabit the digestive tract such as Escherichia coli (carrying K1 antigen). Listeria monocytogenes (serotype IVb) may affect the newborn and occurs in epidemics. Older children are more commonly affected by Neisseria meningitidis (meningococcus), Streptococcus pneumoniae (serotypes 6, 9, 14, 18 and 23) and those under five by Haemophilus influenzae type B (in countries that do not offer vaccination, see below). In adults, N. meningitidis and S. pneumoniae together cause 80% of all cases of meningitis, with increased risk of L. monocytogenes in those over 50 years old.
Recent trauma to the skull gives bacteria in the nasal cavity the potential to enter the meningeal space. Similarly, individuals with a cerebral shunt or related device (such as an extraventricular drain or Ommaya reservoir) are at increased risk of infection through those devices. In these cases, infections with staphylococci are more likely, as well as infections by pseudomonas and other Gram-negative bacilli. The same pathogens are also more common in those with an impaired immune system. In a small proportion of people, an infection in the head and neck area, such as otitis media or mastoiditis, can lead to meningitis. Recipients of cochlear implants for hearing loss are at an increased risk of pneumococcal meningitis.
Tuberculous meningitis, meningitis due to infection with Mycobacterium tuberculosis, is more common in those from countries where tuberculosis is common, but is also encountered in those with immune problems, such as AIDS.
Recurrent bacterial meningitis may be caused by persisting anatomical defects, either congenital or acquired, or by disorders of the immune system. Anatomical defects allow continuity between the external environment and the nervous system. The most common cause of recurrent meningitis is skull fracture, particularly fractures that affect the base of the brain or extend towards the sinuses and petrous pyramids. A literature review of 363 reported cases of recurrent meningitis showed that 59% of cases are due to such anatomical abnormalities, 36% due to immune deficiencies (such as complement deficiency, which predisposes especially to recurrent meningococcal meningitis), and 5% due to ongoing infections in areas adjacent to the meninges.
Aseptic
The term aseptic meningitis refers loosely to all cases of meningitis in which no bacterial infection can be demonstrated. This is usually due to viruses, but it may be due to bacterial infection that has already been partially treated, with disappearance of the bacteria from the meninges, or by infection in a space adjacent to the meninges (e.g. sinusitis). Endocarditis (infection of the heart valves with spread of small clusters of bacteria through the bloodstream) may cause aseptic meningitis. Aseptic meningitis may also result from infection with spirochetes, a type of bacteria that includes Treponema pallidum (the cause of syphilis) and Borrelia burgdorferi (known for causing Lyme disease). Meningitis may be encountered in cerebral malaria (malaria infecting the brain). Fungal meningitis, e.g. due to Cryptococcus neoformans, is typically seen in people with immune deficiency such as AIDS. Amoebic meningitis, meningitis due to infection with amoebae such as Naegleria fowleri, is contracted from freshwater sources.
Viral
Viruses that can cause meningitis include enteroviruses, herpes simplex virus type 2 (and less commonly type 1), varicella zoster virus (known for causing chickenpox and shingles), mumps virus, HIV, and LCMV.
Non-infectious
Meningitis may occur as the result of several non-infectious causes: spread of cancer to the meninges (malignant meningitis) and certain drugs (mainly non-steroidal anti-inflammatory drugs, antibiotics and intravenous immunoglobulins). It may also be caused by several inflammatory conditions such as sarcoidosis (which is then called neurosarcoidosis), connective tissue disorders such as systemic lupus erythematosus, and certain forms of vasculitis (inflammatory conditions of the blood vessel wall) such as Behçet's disease. Epidermoid cysts and dermoid cysts may cause meningitis by releasing irritant matter into the subarachnoid space. Mollaret's meningitis is a syndrome of recurring episodes of aseptic meningitis; it is now thought to be caused by herpes simplex virus type 2. Rarely, migraine may cause meningitis, but this diagnosis is usually only made when other causes have been eliminated.
Further Reading
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