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Bacterial meningitis

Bacterial meningitis is a condition in which the layers lining the brain (the meninges) have become inflamed as a result of infection with bacteria. more...

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Symptoms and signs

The classic symptoms of bacterial meningitis are similar to those of other forms of meningitis, including fever, headache, light sensitivity (photophobia), and confusion. Because of the continuity of the meninges and the linings of the spinal cord, movements that place strain on the spinal cord can cause worsening pain. In particular, movement of the neck and stretching of the legs cause a worsening of symptoms.

Clinicians will often attempt to elicit objective evidence of these latter symptoms. Brudzinski's sign (a correlate of nuchal rugidity), is detected when the knees passively flex when the neck is brought forward in a patient lying supine. A similar phenomenon, Kernig's sign, refers to an inability to stretch the hamstrings of an affected patient lying supine. Both are likely reflex responses to attempts to place traction on inflamed dura.

Bacterial meningitis can seldom be distinguished from other forms of meningitis based on symptoms alone, and usually requires lumbar puncture for definitive diagnosis.

Diagnosis

Meningitis is diagnosed when the cerebrospinal fluid, obtained via lumbar puncture, reveals the presence of an increased number of leukocytes. The condition is attributed to bacteria when their presence is detected via Gram stain or bacterial culture.

Etiology

In most cases, the reason that the meninges have become infected is never determined. Occasionally, the infection is the result of direct bacterial invasion from infections of adjacent structures, such as the paranasal sinuses or the inner ear. The latter is seen more commonly in children with untreated otitis media, although it should be noted that the true incidence of this complication and the ability of antibiotics to prevent it is a matter of controversy. Finally, bacteria can reach the meninges via the bloodstream, in a phenomenon known as hematogenous spread. In this situation, the most common predisposing infection is bacterial endocarditis, an infection of the structures of the heart.

Microbiology

The most common organisms involved in bacterial meningitis include Neisseria meningitidis (or meningococcus), Streptococcus pneumoniae (G001), Haemophilus influenzae (G000), and Staphylococcus aureus (G003). Less common bacterial causes include Listeria monocytogenes, Staphylococcus and Escherichia coli. In developing countries, Mycobacterium tuberculosis is a common cause of bacterial meningitis. The less common organisms are particularly found in elderly or immunocompromised individuals.

Treatment

The mainstay of treatment for bacterial meningitis is antibiotic therapy. Empiric therapy, directed at the most common organisms, is provided until a microbiologic diagnosis is made. The initiation of antibiotics in a patient suspected to have bacterial meningitis should not be delayed while a diagnosis is made, due to the high incidence of complications in untreated patients—including brain damage, hearing loss, and death.

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Review of Bacterial Meningitis in the Older Patient
From American Family Physician, 3/15/02 by Bill Zepf

The remarkable success of universal childhood immunization for Haemophilus influenzae type B and the recent introduction of an infant regimen for Streptococcus pneumoniae vaccination are shifting the epidemiology of meningitis from a childhood disease to one that occurs mainly in adults. Older adults are not the most common age group affected by meningitis, but their higher rate of morbidity and atypical presentation make for unique challenges. Choi presents an updated review of bacterial meningitis in older adults.

While viral etiologies for meningitis are common in younger adults, these relatively benign agents are much less frequent among older persons. The most common bacterial causes of meningitis in the elderly are S. pneumoniae, Listeria monocytogenes, gram-negative bacilli (especially Escherichia coli and Klebsiella pneumoniae), and Streptococcus agalactiae (group B Streptococcus). Neisseria meningitidis and H. influenzae are not common.

The less common occurrence of the typical presenting symptoms of meningitis among older adults requires a higher index of suspicion to successfully detect infection. The author notes, however, that at least one classic finding is present in more than 99 percent of cases. Meningitis is very unlikely in an older patient without fever, neck stiffness, or confusion. The author advocates obtaining a lumbar puncture in most cases, unless there is concern for possible cerebral herniation from known or suspected brain tumor, abscess, or brain edema from a recent stroke. In such cases, a computed tomographic scan of the brain may be warranted before lumbar puncture is considered.

The cerebrospinal fluid (CSF) findings that strongly predict a bacterial cause include a white blood cell count greater than 500 per mm3 (500 3 106 per L), a differential with greater than 85 percent polymorphonuclear leukocytes, and a CSF glucose level that is less than one third of the serum value. Gram stain can often identify the causative organism and thus guide initial therapy, but it is less reliable for detection of Listeria and gram-negative bacilli, which are important causes of bacterial meningitis in older adults.

Drug-resistant S. pneumoniae and the possibility of Listeria infection complicate the choice of initial therapy (see the accompanying table). The author suggests using ceftriaxone or cefotaxime, plus ampicillin (for Listeria infection), if the Gram stain does not show streptococcal infection. Vancomycin is substituted for ampicillin if streptococcal organisms are visible on the stain. Use of steroids as adjunctive therapy is not routinely recommended, unless there is evidence of increased intracerebral pressure.

Given the higher morbidity in older patients with meningitis, even with prompt treatment, the author notes that preventive measures are obviously important. Previous immunization with the adult pneumococcal vaccine is not completely effective in preventing meningitis but appears to cut the risk by at least one half. Listeriosis is usually caused by contaminated food. A vaccine for group B streptococcal infection is in research but is intended for prevention of neonatal disease, and trials in older adults are not ongoing.

COPYRIGHT 2002 American Academy of Family Physicians
COPYRIGHT 2002 Gale Group

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