Acute Bacterial Meningitis in Adults

The CSF studies described above are the primary tools in diagnosing bacterial meningitis; however, there are other studies that may be helpful in certain clinical settings. Latex agglutination tests for bacterial antigens may be used in cases in which bacterial meningitis remains a possible diagnosis despite negative CSF studies. This test is available for S. pneumoniae, N. meningitidis, H. influenzae type B, group B Streptococcus, and E. coli. The polymerase chain reaction (PCR) test of the CSF has been developed for some bacterial pathogens including S. pneumoniae, N. meningitidis, H. influenzae type B, and Mycobacterium tuberculosis. The limulus amebocyte lysate assay is a very sensitive test for gram-negative endotoxins, which may aid in identifying gram-negative organisms as potential pathogens in the CSF. While these alternative CSF diagnostic tests are available, many laboratories do not perform the tests on site and require send-out to a specialty laboratory. The time required for this may negate the clinical utility of these tests.

Role of Brain Imaging

The decision to obtain a brain imaging study prior to performing an LP has been a controversial issue for both patient safety and medical-legal reasons. Two large studies have been published in an attempt to derive a clinically useful decision analysis tool (7,8). In summary, the studies found that 5 clinical features were associated with an abnormal head cranial tomography (CT) scan. These were:

1. Age >60 years
2. Immunocompromised state
3. Any history of central nervous system (CNS) disease
4. A history of seizure within 1 week prior to presentation
5. Presence of a focal neurologic abnormality, including altered level of consciousness, inability to answer or follow 2 consecutive requests, gaze palsy, abnormal visual fields, facial palsy, arm or leg drift, and abnormal language.

In patients with none of these findings, there was a 97% negative predictive value of having an abnormal CT scan, with the few patients with positive scans nonetheless tolerating LP without adverse effects. Thus, in patients with none of these findings, it appears that an LP can safely be performed without obtaining a CT scan. One study also demonstrated that patients who underwent a CT scan prior to their LP waited, on average, 2 hours longer to get an LP; with antibiotic administration delayed by an average of 1 hour (8). Antibiotic administration should not be delayed in any patient suspected of having bacterial meningitis, whether brain imaging is performed or not.

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Differential Diagnosis

Given the severe nature of this disease, the diagnosis of bacterial meningitis must be differentiated from other conditions that may present in similar ways. Infectious causes that may present similarly to bacterial meningitis include other types of meningitis (viral, tuberculous, Lyme disease, syphilitic), viral encephalitis, Rocky Mountain spotted fever, fungal meningitis, parasitic causes, brain abscess, and epidural and subdural empyema. Other infectious etiologies not originating from the CNS may be mistaken for bacterial meningitis when these patients present with concomitant mental-status changes. This is especially common in elderly patients with pneumonia and urinary tract infections. Other noninfectious considerations include a CNS bleed such as a subarachnoid hemorrhage, drug-induced aseptic meningitis, and CNS vasculitis.

Treatment

When the patient’s presentation is suggestive of bacterial meningitis, empiric antibiotics should be administered without delay, while awaiting diagnostic evaluation. The initial dose of antibiotics should not alter the results of the diagnostic studies significantly. The choice of antibiotics is based upon the most likely offending organism from epidemiologic data and underlying predisposing conditions. S. pneumoniae and N. meningitidis are the 2 most common causes of bacterial meningitis in adults.