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Central nervous system (CNS) infections are often life-threatening and can have severe sequelae. These infections cause inflammation and edema within the unyielding cranium, resulting in damage to brain tissue and loss of function. The most common causes of CNS infections are bacteria and viruses, but fungi, protozoa, and helminths also cause these infections.
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In addition to the history and physical examination, clinical diagnosis of CNS infections requires a spinal fluid analysis combined with neuroimaging using either magnetic resonance imaging (MRI) or computed tomography (CT) scan. Microbiologic diagnosis of bacterial infections frequently is made using Gram stain and culture of spinal fluid and blood. Polymerase chain reaction (PCR) assays and serologic tests are also useful. Antimicrobial therapy requires that the antibiotics be bactericidal and that they penetrate the blood–brain barrier. Some CNS infections, such as a brain abscess, often require surgical drainage.
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CEREBROSPINAL FLUID ANALYSIS
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Examination of cerebrospinal fluid (CSF) is critical in making the diagnosis of CNS infections. CSF is obtained by performing a lumber puncture at the L3–L4 interspace. During the process, the CSF pressure is measured and fluid obtained for analysis of cells (both number and cell type, i.e., neutrophils or lymphocytes), protein, and glucose. The results of CSF analysis in acute bacterial meningitis, acute viral meningitis, and subacute meningitis are described in Table 72–1.
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Although CSF analysis is a very important step in the diagnosis of many CNS infections, a lumbar puncture should not be performed if there are signs of increased intracranial pressure, such as papilledema or focal neurologic signs, because herniation of the brainstem and death may occur. A CT scan should be performed prior to the lumbar puncture to determine whether a mass lesion, such as a brain abscess or cancer, is present. If a mass lesion is seen, a lumbar puncture should not be performed.
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Meningitis is an infection of the meninges, the membranes that line the brain and spinal cord (Figure 72–1). Meningitis can be categorized as acute, subacute, or chronic depending on the speed of onset of the initial presentation and the rate of progression of the illness. Acute meningitis is caused by either pyogenic bacteria, such as Streptococcus pneumoniae and Neisseria meningitidis, or viruses, ...