The meninges is a membrane that covers and protects your brain. It is composed of three layers called the pia, arachnoid and dura. You also have cerebrospinal fluid (CSF), which cushions and protects your brain and spinal cord as it circulates around both in the space between the pia and the arachnoid layers. If any microorganism gets into that same space, this causes inflammation in both layers. This is meningitis and it can be due to bacterial, viral or fungal agents.
Bacterial Meningitis
According to Martin Samuels, M.D., Chairman of the Department of Neurology at Brigham and Women's Hospital, any bacteria can theoretically cause meningitis, but 75% is caused by H. influenzae, N. meningitidis and S. pneumoniae. These bacteria ultimately reach your meninges by traveling through your bloodstream. Listeria is the next most common cause; it can cause meningitis if your immune system is not fully functional. This is the case if you are elderly, taking steroids or undergoing chemotherapy. Poor immune function also affects newborn babies and people suffering from leukemia, lymphoma or HIV. S. aureus can cause meningitis as a result of surgery, abscess in your brain or due to head trauma. The most common cause of meningitis in newborns, however, is Group B Streptococcus and E. coli.
Viral Meningitis
Several viruses can cause meningitis, including cytomegalovirus, mumps, adenovirus, herpes simplex virus (HSV) and the enteroviruses. Seventy-five percent of viral meningitis is caused by enteroviruses. But, as Karen Roos, M.D., writes in "Harrison's Principles of Internal Medicine," the number of cases of viral meningitis is not reported on a nationwide basis. Thus, it is only an estimate that there are perhaps 75,000 cases of viral meningitis every year. And since enteroviruses are more prevalent in the summer, the number of viral meningitis cases is also thought to increase during this time, particularly among children who are younger than 15 years old. For adults, HSV is now seen as an important cause of viral meningitis.
Fungal Meningitis
C. neoformans is a fungus, found worldwide in soil and bird droppings, which can cause an infection in your lungs. It is also the most common fungus to cause meningitis. In fact, the first sign of a cryptococcal infection is usually that of meningitis. And it is also a common meningeal infection in those who are suffering with AIDS. H. capsulatum is another fungus that is found in the soil in areas with bird and bat droppings, but this fungus is found in the central part of the U.S. It, too, causes lung infections and can cause meningitis. You can get an infection from either fungus by inhaling their spores. The lung infection will usually go away on its own. But the fungi can stay in your lungs. And if your immune system does not function properly, the fungi can spread to your brain.
References
- "Adams & Victor's Principles of Neurology"; Allan Ropper, M.D., Martin Samuels, M.D.; 2009
- "Harrison's Principles of Internal Medicine"; Anthony Fauci, M.D., Dennis Kasper, M.D., Dan Longo, M.D. et al.; 2008
- "Robbins Pathologic Basis of Disease"; Ramzi Cotran, M.D., Vinay Kumar, M.D., Stanley Robbins, M.D.; 1994
