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Acute bacterial
meningitis is rapidly developing inflammation of the subarachnoid
space (located within the layers of tissue covering the brain and
spinal cord) that is caused by bacteria.
The subarachnoid space is located between the middle layer (arachnoid mater) and the thin inner layer (pia mater) of tissues (called meninges) that cover the brain and spinal cord (see Biology of the Nervous System: Viewing the Brain ). This space contains the cerebrospinal fluid, which flows through the meninges, fills internal spaces within the brain, and helps cushion the brain and spinal cord.
When bacteria invade the subarachnoid space, the immune system eventually reacts to the invaders, and immune cells gather to defend the body against them. The result is inflammation. Severe inflammation can spread to blood vessels within the brain, sometimes causing clots to form. A stroke can result. Inflammation can also cause widespread damage to brain tissue, causing swelling (edema) and small areas of bleeding. If swelling is severe, it can increase pressure within the skull (intracranial pressure), causing parts of the brain to shift. If these parts are pressed through one of the small natural openings in the tissues that separate the brain into compartments, a life-threatening disorder called brain herniation results.
Bacterial meningitis is most common among infants, children, adolescents, and people over 55. Small epidemics of one particularly dangerous type of meningitis, called meningococcal meningitis, may occur among people living in close quarters, as occurs in military barracks and college dormitories.
Meningitis may also be caused by viruses, fungi, protozoa, cancer cells, certain drugs (which trigger an allergic reaction), and irritating substances (including air and chemicals).
Causes
Different species of bacteria cause meningitis in different settings.
If meningitis is acquired outside of a hospital or nursing home (in the community), it is usually caused by Neisseria
meningitides (which causes meningococcal meningitis) or by Streptococcus
pneumoniae. Both species are normally present in the external environment. They also reside in the nose and upper respiratory system of some people without causing harm. Occasionally, these organisms infect the brain without an identifiable reason. In other cases, infection develops because the immune system is weakened by a disorder or by a drug that suppresses it (immunosuppressant). The following increase the risk of developing bacterial infections, including meningitis:
Sometimes meningitis results from a head injury. For example, a skull fracture may create an opening between the nasal sinuses and the subarachnoid space. Bacteria can travel from the sinuses through the opening and infect the meninges.
Meningitis due to Streptococcus
pneumoniae (pneumococcal meningitis) is becoming less common because people are now routinely vaccinated against it.
Listeria
monocytogenes causes bacterial meningitis in newborns, pregnant women, and people over 50. Having kidney or liver failure or taking corticosteroids or immunosuppressants increases the risk of developing meningitis due to these bacteria.
Meningitis due to Escherichia
coli (which normally resides in the colon and in feces) or Klebsiella
bacteria usually develops after a widespread infection of the blood (sepsis), an infection acquired in a hospital, or surgery on the brain or spinal cord. People with a weakened immune system are more likely to develop sepsis and infections in a hospital and thus to develop meningitis due to these bacteria.
Meningitis due to Pseudomonas bacteria is more common among people with a weakened immune system.
Meningitis due to Staphylococcus
aureus can occur after an injury or a surgical procedure that penetrates the skull or after infection of the heart valves (causing endocarditis) by these bacteria.
Newborns, whose immune system is not completely formed, are at increased risk of developing meningitis due to Escherichia coli or group B streptococci.
Symptoms
In older children and adults, the following symptoms may occur early:
Vomiting is also common. These symptoms are sometimes preceded by a sore throat, cough, runny nose, or other symptoms suggesting a respiratory illness. The stiff neck is more than just sore. Trying to lower the chin to the chest causes pain and may be impossible. Moving the head in other directions is not as difficult.
In children up to 2 years old, some combination of the following usually occurs first:
Unlike older children or adults, infants younger than 1 year may not develop a stiff neck (see Bacterial Infections: Meningitis).
Adults may become seriously or desperately ill within 24 hours, and children even sooner.
In meningococcal meningitis, a rash (usually red and purple spots) sometimes develops. The rash is most prominent on the trunk and lower extremities. It may be difficult to see at first if people have dark skin.
The bacterial infection causes swelling of brain tissue. In children up to 2 years old, the swelling may make the soft spots between the skull bones (fontanelles) bulge. (These soft spots enable the skull to pass through the birth canal. They harden by about age 2 years.) The swelling may block the flow of cerebrospinal fluid around the brain, causing the fluid to accumulate and put pressure on the brain (a disorder called hydrocephalus). Sometimes a collection of pus (subdural empyema) forms under the outer layer (dura mater) of the meninges.
Older children and adults can become irritable, confused, then increasingly drowsy. Drowsiness can progress to unresponsiveness that requires vigorous stimulation for arousal (stupor), coma, and death. The swelling increases pressure inside the skull and can hamper blood flow, sometimes causing symptoms of stroke, including paralysis. Some people have seizures.
In most people with meningococcal meningitis, the bloodstream and many organs are also infected―a disorder called meningococcemia. Meningococcemia can become severe within hours. As a result, areas of tissue may die, and bleeding may occur under the skin (causing red spots or purple blotches), in mucous membranes, and within the digestive tract and other organs. Without treatment, blood pressure drops, leading to shock and death. Typically, bleeding occurs within the adrenal glands, which shut down, making shock worse. This disorder, called the Waterhouse-Friderichsen syndrome, is often fatal unless treated promptly.
Sometimes meningitis develops while people are being treated for another infection (such as an ear or throat infection). Or early meningitis may be mistaken for another infection and be treated with antibiotics. In either case, the symptoms of meningitis are much milder than normal, making meningitis more difficult to recognize.
Diagnosis
If a child 2 years old or younger has an unexplained fever and a parent senses that the child is ill, the parent should see or call a doctor immediately, particularly if symptoms do not resolve after an adequate dose of acetaminophen . Children require immediate medical attention if they do any of the following:
Adults require immediate medical attention if they have any of the following:
During the physical examination, doctors look for telltale signs of meningitis, such as a stiff neck and skin rash. When doctors suspect rapidly developing meningitis, they withdraw a sample of the person's blood and send it to a laboratory, where the bacteria can be grown (cultured) overnight. If bacteria are detected, bacterial infection is confirmed. Culture also helps identify which bacteria are causing infection. Culture results can take up to 2 days. At some hospitals, new blood tests can provide the same information within a few hours.
A spinal tap (lumbar puncture—see Diagnosis of Brain, Spinal Cord, and Nerve Disorders: How a Spinal Tap Is Done) is done but usually not until after tests, such as computed tomography (CT) of the brain and sometimes blood clotting tests, are done to determine whether a spinal tap is safe. CT of the head is done to check for a mass (such as a hemorrhage, tumor, or abscess), which may increase pressure within the skull. If pressure is increased, the brain may shift downward, causing brain herniation.
During a spinal tap, a thin needle is inserted between two vertebrae in the lower spine to withdraw cerebrospinal fluid. Doctors look closely at the fluid, which is normally clear but is cloudy in meningitis. The fluid's pressure is measured. Pressure is usually high in meningitis. Sugar and protein levels and the number and type of white blood cells in the fluid are determined. This information helps doctors distinguish between bacterial and viral infections. The fluid is examined under a microscope to check for and identify bacteria. If bacteria are seen, other tests are done to rapidly identify certain bacteria, such as Neisseria meningitidis and Streptococcus
pneumoniae. Some of these tests can detect proteins (antigens) on the surface of bacteria and thus identify them. The polymerase chain reaction (PCR) technique, which produces many copies of a gene, may be used to identify the bacteria's unique DNA sequence.
The cerebrospinal fluid is also cultured, and after 24 hours, the resulting bacteria are tested to determine which antibiotics are effective against them (called susceptibility testing). Then, antibiotic therapy, which was already started, can be adjusted if necessary.
Until the cause of meningitis is confirmed, other tests using cerebrospinal fluid or blood samples may be done to check for viruses, fungi, cancer cells and other substances that routine tests do not identify. Testing for herpes simplex virus, which can infect the brain (causing encephalitis), is particularly important.
Doctors also take samples of blood, urine, and mucus from the nose and throat, and in people who have a rash, they may use a small needle to remove fluid and tissue from under the skin where the rash is. These samples are cultured and examined under the microscope to see whether bacteria are present.
Treatment
Because acute bacterial meningitis can lead to death within hours, treatment is started as soon as possible, without waiting for the results of diagnostic tests and usually before a spinal tap is done. Several antibiotics (see Antibiotics) are given intravenously, often in the emergency department. Doctors choose antibiotics that are effective against the bacteria most likely to be causing the infection. However, because doctors cannot identify the bacteria causing the infection based on symptoms alone, they usually choose several antibiotics that are effective against many organisms. Also, an antiviral drug that is effective against the herpesvirus that causes inflammation of the brain (encephalitis) is given. Once the infecting organism, usually a specific species of bacteria, is identified, the antibiotics are changed to ones that are most effective against that organism, and any unnecessary antibiotics and antiviral drugs are stopped.
A corticosteroid, such as dexamethasone, is given 15 minutes before or at the same time as the first antibiotic dose. The corticosteroid is continued for 2 to 4 days. Corticosteroids are given to suppress inflammation caused by fragments of bacteria, which are produced when antibiotics break bacteria apart. This inflammation causes swelling that can damage the brain. Corticosteroids can also reduce pressure within the skull and, if the adrenal glands are damaged, replace the corticosteroids normally produced by these glands.
Fluids lost because of fever, sweating, vomiting, and poor appetite are replaced. Because bacterial meningitis often affects many organs and causes serious complications, people are usually admitted to the intensive care unit.
Complications may require specific treatment.
Prognosis
If treated early, most people recover well. But when treatment is delayed, permanent brain damage or death is more likely, especially in very young children and older people. In some people, seizures require lifelong treatment. Neurologic problems, such as permanent mental impairment, paralysis, and hearing loss, may also result.
Prevention
People with acute meningitis (particularly meningococcal meningitis) are usually placed in isolation until the infection is controlled and they can no longer spread the infection, usually for a few days or less. Vaccines for several forms of meningitis are available.
Meningococcal
Meningitis:
A vaccine can help prevent this type of meningitis. It is given to children 2 years old or older whose immune system is weakened. It is also recommended for the following people:
The vaccine is also used when an epidemic occurs or when there is a threat of an epidemic in a self-contained group of people (such as those living in military barracks). Family members, medical personnel, and others in close contact with people who have meningococcal meningitis should be given an antibiotic (such as rifampin or ciprofloxacin taken by mouth or ceftriaxone given by injection) as a preventive measure.
Meningitis Due
to Streptococcus pneumoniae:
A vaccine that helps protect against this infection is now routinely given to children.
Meningitis Due
to Haemophilus influenzae:
Children are now routinely immunized with Haemophilus influenzae type b vaccine, which has virtually eliminated what once was the most common cause of meningitis in children.
Last full review/revision May 2008 by Michael Jacewicz, MD
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