Bacterial meningitis

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

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 rigidity), 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. When due to bacteria the CSF is cloudy or frankly purulent, it is under increased pressure with as many as 90,000 neutrophils/mm3 and it has an increased protein level with markedly decreased glucose content.

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 (G00.1), Haemophilus influenzae (G00.0), and Staphylococcus aureus (G00.3). 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.

Cephalosporin antibiotics constitute the current standard treatment, although they must be administered in high doses due to their relative inability to cross the blood-brain barrier. In addition, vancomycin is frequently added due to the high incidence of resistance of organisms to cephalosporins, particularly among Streptococcus pneumoniae. When Listeria is suspected, ampicillin may be added. If microbiologic testing eventually reveals an etiologic organism, the antibiotic regimen may then be altered to more specifically cover the appropriate bacteria.

The role of corticosteroids in reducing symptoms and preventing complications from antibiotics is controversial in adults, and less so in children.

External links

• eMedicine ped/198
• Merck