Meningitis is a potentially fatal inflammation of the meninges, the thin, membranous covering of the brain and the spinal cord. Meningitis is most commonly caused by infection (by bacteria, viruses, or fungi), although it can also be caused by bleeding into the meninges, cancer, or diseases of the immune system.
The meninges are three separate membranes, layered together, which serve to encase the brain and spinal cord. The dura is the toughest, outermost layer, and is closely attached to the inside of the skull. The middle layer, the arachnoid, is important in the normal flow of the cerebrospinal fluid (CSF), a lubricating fluid that bathes both the brain and the spinal cord. The innermost layer, the pia, helps direct brain blood vessels into the brain. The space between the arachnoid and the pia contains CSF, which serves to help insulate the brain from trauma. Through this space course many blood vessels. CSF, produced within specialized chambers deep inside the brain, flows over the surface of the brain and spinal cord. This fluid serves to cushion these relatively delicate structures, as well as supplying important nutrients for brain cells. CSF is reabsorbed by blood vessels that are located within the meninges.
The cells lining the brain's capillaries (tiny blood vessels) are specifically designed to prevent many substances from passing into brain tissue. This is commonly referred to as the blood-brain barrier. The blood-brain barrier prevents various toxins (substances which could be poisonous to brain tissue), as well as many agents of infection, from crossing from the blood stream into the brain tissue. While this barrier obviously is an important protective feature for the brain, it also serves to complicate therapy in the case of an infection, by making it difficult for medications to pass out of the blood and into the brain tissue where the infection resides.
The most common infectious causes of meningitis vary according to an individual host's age, habits and living environment, and health status. In newborns, the most common agents of meningitis are those that are contracted from the newborn's mother, including Group B Streptococci (becoming an increasingly common infecting organism in the newborn period), Escherichia coli, and Listeria monocytogenes. Older children are more frequently infected by Haemophilus influenzae, Neisseria meningitidis, and Streptococcus pneumoniae, while adults are infected by S. pneumoniae and N. meningitidis. N. meningitidis is the only organism that can cause epidemics of meningitis. These have occurred in particular when a child in a crowded day-care situation, a college student in a dormitory, or a military recruit in a crowded training camp has fallen ill with N. meningitidis meningitis.
Viral causes of meningitis include the herpes simplex viruses, mumps and measles viruses (against which most children are protected due to mass immunization programs), the virus that causes chicken pox, the rabies virus, and a number of viruses that are acquired through the bite of infected mosquitoes. Patients with AIDS (Acquired Immune Deficiency Syndrome) are more susceptible to certain infectious causes of meningitis, including by certain fungal agents, as well as by the agent that causes tuberculosis. Patients who have had their spleens removed, or whose spleens are no longer functional (as in the case of patients with sickle cell disease) are more susceptible to certain infections, including those caused by N. meningitidis and S. pneumoniae.
The majority of meningitis infections are acquired by blood-borne spread. An individual may have another type of infection (of the lungs, throat, or tissues of the heart) caused by an organism that can also cause meningitis. The organism multiplies, finds its way into the blood stream, and is delivered in sufficient quantities to invade past the blood-brain barrier.
Direct spread occurs when an already resident infectious agent spreads from infected tissue next to or very near the meninges, for example from an ear or sinus infection. Patients who suffer from skull fractures provide openings to the sinuses, nasal passages, and middle ears. Organisms that frequently live in the human respiratory system can then pass through these openings to reach the meninges and cause infection. Similarly, patients who undergo surgical procedures or who have had foreign bodies surgically placed within their skulls (such as tubes to drain abnormal amounts of accumulated CSF) have an increased risk of the organisms causing meningitis being introduced to the meninges.
The most classic symptoms of meningitis (particularly of bacterial meningitis) include fever, headache, vomiting, photophobia (sensitivity to light), irritability, lethargy (severe fatigue), and stiff neck. The disease progresses with seizures, confusion, and eventually coma.
Damage due to meningitis occurs from a variety of phenomena. The action of infectious agents on the brain tissue is one direct cause of damage. Other types of damage may be due to mechanical effects of swelling of brain tissue, and compression against the bony surface of the skull. Swelling of the meninges may interfere with the normal absorption of CSF by blood vessels, causing accumulation of CSF and damage due to resulting pressure on the brain. Interference with the brain's carefully regulated chemical environment may cause damaging amounts of normally present substances (carbon dioxide, potassium) to accumulate. Inflammation may cause the blood-brain barrier to become less effective at preventing the passage of toxic substances into brain tissue.
Antibiotic medications (forms of penicillins and cephalosporins, for example) are the most important element of treatment against bacterial agents of meningitis. Because of the effectiveness of the blood-brain barrier in preventing passage of substances into the brain, medications must be delivered directly into the patient's veins (intravenous or IV) at very high doses. Antiviral medications (acyclovir) may be helpful in the case of viral meningitis, and antifungal medications are available as well.
Other treatment for meningitis involves decreasing inflammation (with steroid preparations) and paying careful attention to the balance of fluids, glucose, sodium, potassium, oxygen, and carbon dioxide in the patient's system. Patients who develop seizures will require medications to halt the seizures and prevent their return.
A series of immunizations against Haemophilus influenzae, started at two months of age, has greatly reduced the incidence of that form of meningitis. Vaccines also exist against Neisseria meningitidis and Streptococcus pneumoniae bacteria, but these vaccines are only recommended for those people who have particular susceptibility to those organisms, due to certain immune deficiencies, lack of a spleen, or sickle cell anemia.
Because N. meningitidis is known to cause epidemics of disease, close contacts of patients with such meningitis are treated prophylactically, often with the antibiotic Rifampin. This measure generally prevents spread of the disease.
See also Bacteria and bacterial infection; Viruses and responses to viral infection