Meningitis

Definition

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 (bacteria, viruses, or fungi ), although it can also be caused by bleeding into the meninges, cancer, diseases of the immune system, and an inflammatory response to certain types of chemotherapy or other chemical agents. The most serious and difficult-to-treat types of meningitis tend to be those caused by bacteria.

Description

Meningitis is a particularly dangerous infection because of the very delicate nature of the brain. Brain cells are among a very small number of cells in the body that, once killed, will not regenerate. Therefore, if enough brain tissue is damaged, serious, life-long handicaps may remain.

To understand meningitis, it is important to have a basic understanding of the anatomy of the brain. The meninges are three separate membranes layered together that encase the brain and spinal cord:

• The dura is the toughest, outermost layer, and is closely attached to the inside of the skull.
• The arachnoid, the middle layer, is important because of its involvement in the normal flow of the cerebrospinal fluid (CSF), a lubricating and nutritive fluid that bathes both the brain and the spinal cord.
• The pia, the innermost layer, helps direct blood vessels into the brain.

The space between the arachnoid and the pia contains CSF, which helps insulate the brain from trauma. Many blood vessels course through this space.

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 to supply important nutrients for brain cells. CSF is reabsorbed by blood vessels located within the meninges. A careful balance between CSF production and reabsorption is important to avoid the accumulation of too much CSF.

Because the brain is enclosed in the hard, bony case of the skull, any disease that produces swelling of the brain will be damaging. The skull cannot expand at all; so, when any swollen brain tissue pushes up against the skull's hard bone, brain tissue may become damaged and ultimately die. Swelling on one side of the brain will not only cause pressure and damage to that side of the brain, but, because it takes up precious space within the tight confines of the skull, the opposite side of the brain will also be pushed against the skull, causing damage to that side also.

Another way that infections injure the brain involves the way the chemical environment of the brain changes in response to that infection. Brain cells require a very well-regulated environment. Careful balance of oxygen, carbon dioxide, sugar (glucose), sodium, calcium, potassium, and other substances must be maintained to avoid damage to brain tissue. An infection upsets this balance. Brain damage can occur when cells are either deprived of important nutrients or exposed to toxic levels of particular substances.

The cells lining the brain's tiny blood vessels (capillaries) 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 substances that could be harmful to brain tissue (toxins), as well as many agents of infection, from crossing from the blood stream into the brain tissue. While this barrier is obviously an important protective feature for the brain, it also serves to complicate treatment in the case of an infection by making it difficult for medications to pass out of the blood and into the brain tissue where an infection is located.

Causes and symptoms

The most common infectious causes of meningitis vary according to an individual's age, habits, living environment, and health status. While nonbacterial types of meningitis are most common, bacterial meningitis is the more potentially life threatening. Three bacterial agents are responsible for about 80% of all bacterial meningitis cases. These bacteria are Haemophilus influenzae type b, Neisseria meningitidis (causing meningococcal meningitis), and Streptococcus pneumoniae (causing pneumococcal meningitis).

In newborns, the most common agents of meningitis are those contracted from the newborn's mother, including Group B streptococci (which is becoming an increasingly common infecting organism in the new-born period), Escherichia coli, and Listeria monocytogenes. The highest incidence of meningitis occurs in babies younger than a month old, with an increased risk of meningitis continuing through about two years of age.

Older children are more frequently infected by the bacteria Haemophilus influenzae, Neisseria meningitidis, and Streptococci pneumoniae.

Adults are most commonly infected by either S. pneumoniae or N. meningitidis, with pneumococcal meningitis the most common. Certain conditions predispose an individual to this type of meningitis, including alcoholism and chronic upper respiratory tract infections (especially of the middle ear, sinuses, and mastoids).

N. meningitidis is the only organism that can cause epidemics of meningitis. In particular, these appear to occur when a child in a crowded day-care situation, or a military recruit in a crowded training camp, has fallen ill with meningococcal meningitis.

Viral causes of meningitis include the herpes simplex virus, the 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 bites of infected mosquitoes.

A number of medical conditions predispose individuals to meningitis caused by specific organisms. People with AIDS (acquired immunodeficiency syndrome) are more prone to getting meningitis from fungi, as well as from the agent that causes tuberculosis. Persons who have had their spleens removed, or whose spleens are no longer functional (as in the case of individuals with sickle cell disease ) are more susceptible to meningococcal and pneumococcal meningitis.

The majority of meningitis infections are acquired by blood-borne spread. A person may have another type of infection (of the lungs, throat, or tissues of the heart ) caused by an organism that can also cause meningitis. If this initial infection is not properly treated, the organism will continue to multiply, find its way into the blood stream, and be delivered in sufficient quantities to pass the blood brain barrier. Direct spread occurs when an organism spreads to the meninges from infected tissue next to or very near the meninges. This can occur, for example, with a severe, poorly treated ear or sinus infection.

Persons who suffer from skull fractures possess abnormal openings to the sinuses, nasal passages, and middle ears. Organisms that usually live in the human respiratory system without causing disease can pass through such openings, reach the meninges, and cause infection. Similarly, people 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 meningitis.

Organisms can also reach the meninges via an uncommon but interesting method called intraneural spread. This involves an organism invading the body at a considerable distance away from the head, spreading along a nerve, and using that nerve as a ladder into the skull where the organism can multiply and cause meningitis. Herpes simplex virus is known to use this type of spread, as is the rabies virus.

The most classic symptoms of meningitis (particularly of bacterial meningitis) include fever, headache, vomiting, sensitivity to light (photophobia), irritability, severe fatigue (lethargy), stiff neck, and a reddish purple rash on the skin. Untreated, the disease progresses with seizures, confusion, and eventually coma.

A very young infant may not show the classic signs of meningitis. Early in infancy, a baby's immune system is not yet developed enough to mount a fever in response to infection, so fever may be absent. In some infants with meningitis, seizures are the only identifiable symptom. Similarly, debilitated elderly people may not have fever or other identifiable symptoms of meningitis.

Brain 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 the mechanical effects of swelling and compression of brain tissue against the skull. Swelling of the meninges may interfere with the normal absorption of CSF by blood vessels, causing accumulation of CSF and damage from the resulting pressure on the brain. Interference with the brain's carefully regulated chemical environment may cause abnormal and 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.

Diagnosis

A number of techniques are used when examining a person suspected of having meningitis to verify the diagnosis. Certain manipulations of the head (lowering the head, chin towards chest, for example) are difficult to perform and painful for a person with meningitis.

The most important test used to diagnose meningitis is the lumbar puncture (commonly called a spinal tap). Lumbar puncture (LP) involves the insertion of a thin needle into a space between the vertebrae in the lower back and the withdrawal of a small amount of CSF. The CSF is then examined under a microscope to look for bacteria or fungi. Normal CSF contains set percentages of glucose and protein. These percentages will vary with bacterial, viral, or other causes of meningitis. For example, bacterial meningitis causes a greatly decreased percentage of glucose in the CSF because the bacteria are essentially consuming or "eating" that glucose and using it for their own nutrition and energy production. Normal CSF should contain no infection-fighting cells (white blood cells), so the presence of white blood cells in CSF is another indication of meningitis. Some of the withdrawn CSF is put into special lab dishes to allow growth of the infecting organism, which can then be identified more easily. Special immunologic and serologic tests may also be used to help identify the infectious agent.

In rare instances a lumbar puncture cannot be performed because of the amount of swelling and pressure within the skull (intracranial pressure). This pressure is measured immediately upon insertion of an LP needle. If it is found to be high, no fluid is withdrawn because doing so could cause herniation of the brain stem. Herniation of the brain stem occurs when the part of the brain connecting to the spinal cord is thrust through the opening at the base of the skull into the spinal canal. Such herniation will cause compression of those structures within the brain stem that control the most vital functions of the body (breathing, heart beat, consciousness). Death or permanent debilitation follows herniation of the brain stem.

Treatment

Antibiotic medications (forms of penicillin and cephalosporins, for example) are the most important elements of treatment against bacterial meningitis. Because of the effectiveness of the blood-brain barrier in preventing passage of substances into the brain, medications must be delivered directly into an affected person's veins (intravenously, or IV), at very high doses. Antiviral drugs (acyclovir) may be helpful in shortening the duration of viral meningitis, and antifungal medications are also available.

Other treatments involve decreasing inflammation (with steroid preparations) and paying careful attention to the balance of fluids, glucose, sodium, potassium, oxygen, and carbon dioxide in a person's system. People who develop seizures will require medications to halt the seizures and prevent their return.

Prognosis

Viral meningitis is the least severe type, and persons usually recover with no long-term effects from the infection. Bacterial infections, however, are much more severe, and rapidly progress. Without very rapid treatment with the appropriate antibiotic, an infection can swiftly lead to coma and death, often in less than 24 hours. While death rates from meningitis vary depending upon the specific infecting organism, the overall death rate is just under 20%.

The most frequent long-term effects of meningitis include deafness and blindness, which may be caused by the compression of specific nerves and brain areas responsible for the senses of hearing and sight. Some people develop permanent seizure disorders, requiring life-long treatment with antiseizure medications. Scarring of the meninges may result in obstruction of the normal flow of CSF, causing abnormal accumulation of CSF. This may be a chronic problem for some people, requiring the installation of shunt tubes to drain the accumulation on a regular basis.

Health care team roles

Family physicians, pediatricians, emergency physicians, or internists usually make the initial diagnosis of meningitis. Laboratory technicians identify organisms that cause meningitis. Nurses and other hospital staff provide supportive care, and patient and family education. Occasionally, physical therapists are needed to help the patient recover lost muscle functioning.

KEY TERMS

Arachnoid mater— The middle layer of the meninges.

Blood-brain barrier— An arrangement of cells within the blood vessels of the brain that prevents toxic substances, infectious agents, and many medications, from passing from the blood into the brain.

Cerebrospinal fluid (CSF)— Fluid made in chambers within the brain which then flows over the surface of the brain and spinal cord. CSF provides nutrition to cells of the nervous system, as well as providing a cushion for the nervous system structures.

Dura mater— The outermost layer of the meninges.

Lumbar puncture (LP)— A medical test in which a very narrow needle is inserted into a specific space between the vertebrae of the lower back in order to obtain a sample of CSF for examination.

Meninges— The three-layer membranous covering of the brain and spinal cord, composed of the dura, arachnoid, and pia. It provides protection for the brain and spinal cord, as well as housing many blood vessels and participating in the appropriate flow of CSF.

Photophobia— Abnormal sensitivity to light.

Pia mater— The innermost layer of the meninges.

Prevention

Prevention of meningitis primarily involves the appropriate treatment of other infections an individual may acquire, particularly those known to seed to the meninges (such as ear and sinus infections). Preventive treatment with antibiotics is sometimes recommended for those in close contacts with an individual who is ill with meningococcal or H. influenzaetype b meningitis. A meningococcal vaccine is sometimes recommended to individuals traveling to very high risk areas. A vaccine for H. influenzae type b is now given to babies as part of the standard array of childhood immunizations.

Resources

BOOKS

Adams, Raymond D, Maurice Victor, and Allan H. Ropper. Adam's & Victor's Principles of Neurology, 6th ed. New York: McGraw Hill, 1997.

Koroshetz, Walter J, and Morton N. Swartz. "Chronic and Rrecurrent Meniingitis." In Harrison's Principles of Internal Medicine, 14th ed. Ed. Anthony S. Fauci, et al. New York: McGraw-Hill, 1998, 2434-2439.

Nath, Avindra, and Joseph R. Berger. "Acute Viral Meningitis and Encephalitis." In Cecil Textbook of Medicine, 21st ed. Ed. Lee Goldman and J. Claude Bennett. Philadelphia: W.B. Saunders, 2000, 2123-2126.

Pollard, Andrew J., and Martin C. J. Maiden. Meningococcal Disease: Methods and Protocols. Totowa: Humana Press, 2001.

Prober, Charles G. "Acute Bacterial Meningitis Beyond the Neonatal Period." In Nelson Textbook of Pediatrics 16th ed. Ed. Richard E. Behrman et al., Philadelphia: Saunders, 2000, 751-757.

??????. "Eosinophilic Meningitis." In Nelson Textbook of Pediatrics 16th ed. Ed. Richard E. Behrman et al., Philadelphia: Saunders, 2000, 760-761.

??????. "Viral Meningoencephalitis." In Nelson Textbook of Pediatrics 16th ed. Ed. Richard E. Behrman et al., Philadelphia: Saunders, 2000, 757-760.

Scheld, W. Michael. "Bacterial Meningitis, Brain Abscess, and Other Suppurative Intracranial Infections." In Harrison's Principles of Internal Medicine 14th ed. Ed. Anthony S. Fauci, et al. New York: McGraw-Hill, 1998, 2419-2434.

Tyler, Kenneth L. "Aseptic Meningitis, Viral Encephalitis, and Prion Diseases." In Harrison's Principles of Internal Medicine 14th ed. Ed. Anthony S. Fauci, et al. New York: McGraw-Hill, 1998, 2451-2457.

PERIODICALS

Agarwal, R., and A. J. Emmerson. "Should Repeat Lumbar Punctures be Routinely Done in Neonates with Bacterial Meningitis? Results of a Survey into Clinical Practice." Archives of Diseases of Children 84, no. 5 (2001): 451-452.

Colby, C. E., W. J. Steinbach, and A. K. Haiman. "A 10-week Old Infant with Meningitis." Clinics in Pediatrics 40, no. 3 (2001): 155-158.

Glennie, L. "Meningitis: A Rash Diagnosis." Nursing Times 96, no. 12 (2000): 44-45.

Kerr, C. "Meningitis Hits West Africa." Trends in Microbiology 9, no. 5 (2001): 203-205.

van den Berg, H., M. van der Flier, and M. D. van de Wetering. "Cytarabine-Induced Aseptic Meningitis." Leukemia 15, no. 4 (2001): 697-699.

von Vigier, R. O., S. M. Colombo, P. B. Stoffel, P. Meregalli, A. C. Truttmann, and M. G. Bianchetti. "Circulating Sodium in Acute Meningitis." American Journal of Nephrology 21. no. 2 (2001): 87-90.

ORGANIZATIONS

American Academy of Emergency Medicine. 611 East Wells Street, Milwaukee, WI 53202. (800) 884-2236. ?http://www.aaem.org?.

Meningitis Foundation of America Inc. 7155 Shadeland Station, Suite 190, Indianapolis, Indiana 46256-3922. (800) 668-1129 or (317) 595-6383. ?http://www.musa.org/default.htm?.

OTHER

American Academy of Family Practice. ?http://www.aafp.org/afp/990515ap/2761.html.,? and ?http://www.aafp.org/afp/971001ap/tunkel.html?.

Association of State and Territorial Directors of Health Promotion and Public Health Education. ?http://www.astdhpphe.org/infect/bacmeningitis.html?.

Centers for Disease Control and Prevention. ?http://www.cdc.gov/od/oc/media/pressrel/r991021.htm.,? and ?http://www.cdc.gov/ncidod/dbmd/diseaseinfo/meningococcal_g.htm?.

Meningitis Research Foundation of UK. ?http://www.meningitis.org.uk?.

National Institute for Medical Research (UK). ?http://www.nimr.mrc.ac.uk/MillHillEssays/1999/meningitis.htm?.

National Institute of Neurological Disorders and Stroke. ?http://www.ninds.nih.gov/health_and_medical/disorders/encmenin_doc.htm?.

National Library of Medicine. ?http://www.nlm.nih.gov/medlineplus/meningitis.html?.

National Meningitis Trust of UK. ?http://www.meningitis-trust.org.uk/frame.htm?.

University of Illinois School of Medicine. ?http://www.mckinley.uiuc.edu/health-info/dis-cond/commdis/meningit.html?.

University of Maryland College of Medicine. ?http://umm.drkoop.com/conditions/ency/article/000680.htm?.