In NEJM Knowledge+ Internal Medicine Board Review, we have a number of questions about managing patients with suspected bacterial meningitis, and these tend to generate comments from our learners. How do you balance the need to give antibiotics rapidly with the time needed to obtain required diagnostic studies?
Evaluation and Treatment for Most Patients with Suspected Bacterial Meningitis
In general, most patients who present with features of bacterial meningitis, such as fever, headache, and nuchal rigidity, should have blood tests obtained immediately, including blood cultures. These tests should be followed by a lumbar puncture for cerebrospinal fluid (CSF) analysis. Typically, these studies can be obtained rapidly, and the results will be immensely helpful in tailoring therapy.
Once the samples have been obtained, empiric antibiotic therapy is started. The Infectious Diseases Society of America (IDSA) recommends that empiric treatment be tailored to the patient’s age and comorbidities (especially the presence of immunosuppression). Empiric therapy in otherwise healthy adults should cover Streptococcus pneumoniae, Neisseria meningitidis, group B Streptococcus, and Haemophilus influenzae. Such coverage can be achieved with vancomycin plus a third-generation cephalosporin such as ceftriaxone or cefotaxime, with the addition of ampicillin in adults older than 50 years of age in whom Listeria monocytogenes is an additional pathogen to consider. Once the microscopic examination of the CSF (including a Gram stain) and the culture results from the blood cultures and CSF return, the antibiotic therapy can often be matched to a specific pathogen.
In addition, dexamethasone is recommended in patients in whom S. pneumoniae is suspected as it has been shown to reduce the risk of neurologic sequelae. Dexamethasone should be administered at the same time as the antibiotics, but the causative organism is often not known at that time. However, because S. pneumoniae is the most common cause of meningitis in the United States, dexamethasone is typically given to all patients with suspected meningitis — and then discontinued if S. pneumoniae is not isolated from the blood or CSF.
In short, the typical order is: Blood cultures ➔ lumbar puncture ➔ antibiotics/steroids.
When to First Perform a CT of the Head
Not all patients can proceed directly to a lumbar puncture. This procedure carries a risk of brain stem herniation and death in patients with elevated intracranial pressure; therefore, patients at risk of elevated intracranial pressure require a CT of the head before proceeding.
The IDSA recommends that a CT be obtained before lumbar puncture in the following groups of patients:
- Patients who are immunocompromised (HIV infection, taking immunosuppressants, or after transplantation)
- Patients with a history of central nervous system disease (mass lesion, stroke, or focal infection)
- Patients with new onset of seizure within one week of presentation
- Patients with papilledema on fundoscopy
- Patients with an abnormal level of consciousness
- Patients with a focal neurologic deficit
The following decision tree from the IDSA guideline may be helpful:
Management Algorithm for Adults with Suspected Bacterial Meningitis
“STAT” indicates that the intervention should be done emergently.
- See table 2 of IDSA Guidelines.
- See text of IDSA Guidelines for specific recommendations for use of adjunctive dexamethasone in adults with bacterial meningitis.
- See table 4 of IDSA Guidelines.
- See table 3 of IDSA Guidelines.
- Dexamethasone and antimicrobial therapy should be administered immediately after CSF is obtained.
What if Obtaining a CT Scan Would Take Too Long?
In most care settings, obtaining a CT scan is a time-consuming task. If antibiotic therapy is delayed until after a CT scan and the subsequent lumbar puncture has been obtained, there is a greater likelihood of delays of up to 6 hours before antibiotics are administered, with resulting worsened patient outcomes. Therefore, in patients who require a CT before a lumbar puncture can be performed safely, it is better to give antibiotics before obtaining the CT.
In other words, whereas the typical order is: blood cultures ➔ lumbar puncture ➔ antibiotics/steroids, the order should be altered in patients who need a CT to be: blood cultures ➔ antibiotics/steroids ➔ CT ➔ lumbar puncture.
There are some drawbacks to providing antibiotics before obtaining the lumbar puncture. The primary concern is that the yield of the CSF Gram stain and culture may be reduced — results of CSF white blood cell count, glucose, and protein do not change rapidly. Studies have shown that, even though the yield of CSF Gram stain and culture is reduced in patients who receive antibiotics before lumbar puncture is performed, the Gram stain and culture can remain positive and allow identification of the pathogen for several hours after the administration of antibiotics. The slightly reduced yield is a small price to pay if lives can be saved and disability prevented thanks to earlier antibiotic therapy.
When to Give Antibiotics Early
There are times where antibiotics may be administered early even if a CT is not needed. If there are delays in obtaining blood cultures or lumbar puncture — typically, a delay of more than one hour — then antibiotics should be administered even though the diagnostic evaluation is not yet completed.
Putting Knowledge into Practice
Evaluating patients with suspected bacterial meningitis can be challenging, particularly determining the appropriate sequence of events. We hope that the guideline-based information we have provided is helpful.
What types of challenges have you encountered with suspected bacterial meningitis in your own practice?