Acute community-acquired bacterial meningitis in adults: An evidence-based review
ABSTRACTCommunity-acquired bacterial meningitis is still a significant cause of morbidity and mortality. Clinicians should know how to quickly diagnose it, perform a lumbar puncture, order the necessary tests, and start appropriate empiric therapy promptly.
KEY POINTS
- The most common organisms that cause community-acquired bacterial meningitis are Streptococcus pneumoniae and Neisseria meningitidis. The incidence of Listeria infection increases in patients over age 50 and in those with compromised cell-mediated immunity.
- Symptoms and signs are not sensitive or specific enough to diagnose community-acquired bacterial meningitis. A lumbar puncture for cerebrospinal fluid studies is needed to reach the diagnosis, to identify the organism, and to determine antimicrobial susceptibilities.
- Gram stain of cerebrospinal fluid may quickly identify the causative organism. It is not very sensitive, but it is specific.
- Lumbar puncture should be performed as soon as possible. Computed tomography of the head is not necessary in all patients, only in immunocompromised patients and those who have features suggestive of or who are at risk of increased intracranial pressure.
- Try to obtain blood and cerebrospinal fluid cultures before staring antimicrobial therapy, but do not delay therapy if obtaining them is not feasible.
WORKUP AND DIAGNOSTIC TESTS
Which tests are needed?
Blood cultures should be drawn before antimicrobial treatment is started.12–14 Although positive only 19% to 70% of the time, they can help identify the pathogen.15–17
Lumbar puncture with CSF study is essential to make the diagnosis and to identify the organism and its susceptibility to various antibiotics. If lumbar puncture can be performed immediately, it should be done before starting antibiotics, to maximize the yield of cultures. Pediatric studies show that after starting antibiotics, complete sterilization of the cerebrospinal fluid can occur within 2 hours for N meningitides and within 4 hours for S pneumoniae.14 However, starting antimicrobials should not be delayed if a lumbar puncture cannot be done expeditiously.
Is computed tomography of the brain necessary before a lumbar puncture?
The rationale behind performing CT before lumbar puncture is to determine if the patient has elevated intracranial pressure, which would increase the risk of brain herniation due to lowering of the lumbar CSF pressure during lumbar puncture. For ethical and practical reasons, it would be difficult to evaluate this in a randomized clinical trial.
Hasbun et al18 performed a study to evaluate if any features on clinical presentation can predict abnormal findings on CT of the head suggestive of elevated intracranial pressure and thus the risk of herniation. The study included 301 adults with suspected meningitis. It found that abnormal findings on CT were unlikely if all of the following features were absent at baseline:
- Immunocompromised state
- History of central nervous system disease (mass lesion, stroke, or a focal infection)
- New onset of seizure (≤ 1 week from presentation)
- Specific abnormal neurologic findings (eg, an abnormal level of consciousness, inability to answer two consecutive questions correctly or to follow two consecutive commands, gaze palsy, abnormal visual fields, facial palsy, arm drift, leg drift, abnormal language).
Absence of these baseline features made it unlikely that CT would be abnormal (negative likelihood ratio 0.1, 95% CI 0.03–0.31).
According to the guidelines from the Infectious Diseases Society of America (IDSA),19 if none of those features is present, blood cultures and a lumbar puncture should be done immediately, followed by empiric antimicrobial therapy. If any of the features is present, blood cultures should be obtained first, then empiric antimicrobial therapy started, followed by CT of the brain to look for contraindications to a lumbar puncture (Figure 2).
What can lumbar puncture tell us?
Results of lumbar puncture studies can help determine whether meningitis is present and, if so, whether the cause is likely bacterial or viral.20
The opening pressure is elevated (usually > 180 mm H2O) in acute bacterial meningitis. The CSF white blood cell count is usually more than 1.0 × 109/L, consisting predominantly of neutrophils, in acute bacterial meningitis. In viral meningitis, it is usually less than 0.1 × 109/L, mostly lymphocytes.
Protein shows a mild to marked elevation in bacterial meningitis but is normal to elevated in viral meningitis.
The CSF glucose level is lower in bacterial meningitis than in viral meningitis.
The ratio of CSF glucose to blood glucose. Because the glucose levels in the CSF and the blood equilibrate, the ratio of CSF glucose to serum glucose has better diagnostic accuracy than the CSF glucose level alone. The equilibration takes place within a few hours, so the serum glucose level should be ordered at the same time lumbar puncture is done. The CSF glucose-blood glucose ratio is a better predictor of bacterial meningitis than the CSF white blood cell count. Bacterial meningitis is likely if the ratio is lower than 0.4.
Lactate levels are not usually measured, but a lactate level greater than 31.5 mg/dL (3.5 mmol/L) is predictive of meningitis, and a lower level makes the diagnosis unlikely.
The diagnostic accuracies (likelihood ratios) of the CSF tests were analyzed by Straus et al.21 The positive likelihood ratios for the CSF white blood cell count and for the CSF glucose-blood glucose ratio are greater than 10, but these tests have negative likelihood ratios of more than 0.1. (It is generally thought that a test with a positive likelihood ratio of more than 10 is considered good for ruling in a diagnosis, whereas one with a negative likelihood ratio of less than 0.1 is good for ruling out a diagnosis.) Thus, these tests are good to rule in bacterial meningitis, but not as good to rule it out. There are some data to show that CSF lactate and procalcitonin might be more sensitive in ruling out bacterial meningitis, but more studies are needed.22
Gram stain of the cerebrospinal fluid can be done quickly. If no bacteria are seen, the information is not helpful in ruling out bacterial meningitis (negative likelihood ratio 0.14, 95% CI 0.08–0.27). If it is positive, it is almost 100% specific for meningitis due to the organism seen (positive likelihood ratio 735, 95% CI 230–2,295).21
