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654 PART 5: Infectious Disorders

of meningeal irritation, and no fever. In this subgroup, altered mental amplification test, loop-mediated isothermal amplification, is a promis-
status should not be ascribed to other causes until bacterial meningitis ing tool especially in resource-poor settings. Further refinements in
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has been excluded by CSF examination. A patient after neurosurgery or PCR may render it useful in the diagnosis of bacterial meningitis when
a patient who has undergone head trauma also presents a unique clinical the CSF Gram stain and cultures are negative; PCR may also prove to
situation because these patients already have many of the symptoms and be beneficial to detect in vitro susceptibility of meningeal pathogens to
signs of meningitis from their underlying disease processes ; clinical specific antimicrobial agents.
13
features are variable, but most commonly include fever and an altered Neuroimaging techniques have little role in the diagnosis of acute
level of consciousness. One must have a low threshold for CSF exami- bacterial meningitis, except to rule out the presence of other pathologic
nation in these patients should they develop any clinical deterioration. conditions or to identify a parameningeal source of infection. However,
21
■ DIAGNOSIS computed tomography (CT) or magnetic resonance imaging (MRI)
may be useful in patients who have a persisting fever several days after
The diagnosis of bacterial meningitis rests on the CSF examination. 21,29,30 initiation of antimicrobial therapy, prolonged obtundation or coma,
The opening pressure is elevated in virtually all cases; values above new or recurrent seizure activity, signs of increased intracranial pres-
600 mm H O suggest cerebral edema, the presence of intracranial suppu- sure, or focal neurologic deficits. MRI is better than CT for evaluation
2
rative foci, or communicating hydrocephalus. The fluid may be cloudy or of subdural effusions, cortical infarctions, and cerebritis, although it is
turbid if the white blood cell count is elevated (>200/µL). If the lumbar more difficult to obtain an MRI in a critically ill patient, which limits its
puncture is traumatic, the CSF may appear bloody initially, but it should usefulness in many patients with meningitis.
color of the supernatant of centrifuged CSF, is found in patients with ■ TREATMENT
clear as flow continues. Xanthochromia, a pale-pink to yellow-orange
subarachnoid hemorrhage, usually within 2 hours after hemorrhage. Antimicrobial Therapy: The initial approach to the patient with sus-
The CSF white cell count is usually elevated in untreated bacterial pected bacterial meningitis is to perform a lumbar puncture to deter-
meningitis, ranging from 100 to at least 10,000 per microliter, with a mine whether the CSF findings are consistent with that diagnosis
predominance of neutrophils. About 10% of patients present with (Fig. 71-1). 21,29,30 Patients should receive empirical antimicrobial therapy
a lymphocytic predominance (>50%) in CSF. Some patients (especially based on their age and underlying disease status (Table 71-2), 21,29,32
those with septic shock and systemic complications) have a very low if no etiologic agent is identified by Gram stain; if the Gram stain is
CSF white cell count (0-20/µL) despite high bacterial concentrations in positive, targeted antimicrobial therapy can be initiated (Table 71-3).
CSF; these patients have a poor prognosis. Therefore, a Gram stain and In patients with certain clinical features at presentation (see below), CT
culture should be performed on all CSF specimens, even those with a should be performed immediately to exclude an intracranial mass lesion
normal cell count. A CSF glucose concentration of less than 40 mg/dL because lumbar puncture is relatively contraindicated in that setting.
is found in about 60% of patients with bacterial meningitis, and a However, obtaining a CT scan generally entails some delay, so empiri-
CSF:serum glucose ratio of less than 0.31 is observed in 70% of cases. cal antimicrobial therapy should be started immediately, before the CT
The CSF glucose level must always be compared with a simultaneous scan and lumbar puncture are done and after obtaining blood cultures,
serum glucose concentration. The CSF protein concentration is elevated because of the high mortality rate in patients with bacterial meningitis
in virtually all cases of bacterial meningitis, presumably because of dis-
ruption of the blood-brain barrier. Suspicion for bacterial meningitis
CSF examination by Gram stain permits a rapid, accurate identifica-
tion in 60% to 90% of cases of bacterial meningitis; the likelihood of Yes
detecting the organism by Gram stain correlates with the specific bacte- Immunocompromise, history of CNS disease, new onset seizure,
rial pathogen and the concentration of bacteria in CSF. False-positive papilledema, altered consciousness, or focal neurologic deficit; or
findings may occur as a result of contamination in the collection of tubes delay in performance of diagnostic lumbar puncture
or during staining. Cultures of CSF are positive in 70% to 80% of cases.
These percentages may be lower in patients who have received prior No Yes
antimicrobial therapy. Blood cultures and lumbar
Several rapid diagnostic tests have been developed to aid in the diag- puncture STAT Blood cultures STAT
nosis of bacterial meningitis. 21,29,30 Latex agglutination tests are rapid
and sensitive, although the routine use of CSF bacterial antigen tests
for the etiologic diagnosis of bacterial meningitis has been questioned; Dexamethasone + empiric Dexamethasone + empiric
positive results have not modified therapy and false-positive and false- antimicrobial therapy antimicrobial therapy
negative results may occur. Measurement of serum C-reactive protein
or procalcitonin may also be useful in discriminating between bacterial
and viral meningitis because elevated serum concentrations of these CSF findings c/w Negative CT scan
proteins (≥20 mg/L and ≥0.5 ng/mL, respectively) have been observed bacterial meningitis of the head
in patients with acute meningitis. In patients with acute meningitis Yes
in whom the CSF Gram stain is negative, serum concentrations of
C-reactive protein or procalcitonin that are normal or below the limit Positive CSF Gram stain Perform lumbar puncture
of detection have a high negative predictive value in the diagnosis of
bacterial meningitis. An immunochromatographic test for the detection
of S pneumoniae in CSF was found to have an overall sensitivity of 95% No Yes
to 100% for the diagnosis of pneumococcal meningitis, although more Dexamethasone + empiric Dexamethasone + targeted
31
studies are needed. Nucleic acid amplification tests, such as polymerase antimicrobial therapy antimicrobial therapy
chain reaction (PCR), have been used to amplify DNA from patients with
bacterial meningitis. Several studies have shown that broad-based PCR FIGURE 71-1. Management algorithm for adults with suspected bacterial meningitis.
has excellent sensitivity, specificity, and positive and negative predic- See text and tables for specific recommendations for empirical (Table 71-2) and targeted
tive values in the diagnosis of bacterial meningitis. 21,29,30 The sensitivity therapy (Table 71-3). (Reproduced with permission from Tunkel AR, Hartman BJ, Kaplan SL,
and specificity of PCR for the diagnosis of pneumococcal meningitis are et al. Practice guidelines for the management of bacterial meningitis. Clin Infect Dis. November
92% to 100% and 100%, respectively. A recently developed nucleic acid 1, 2004;39(9):1267-1284.)
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