eMedicine Specialties > Pediatrics: General Medicine > Infectious Disease
Meningitis, Bacterial: Differential Diagnoses & Workup
Updated: Nov 23, 2009
- Overview
- Differential Diagnoses & Workup
- Treatment & Medication
- Follow-up
- Multimedia
Differential Diagnoses
Other Problems to Be Considered
Viral meningitis/encephalitis
Brain abscess
Subdural/epidural abscess
Brain tumors
CNS leukemia
Lead encephalopathy
Meningitis, fungal
CNS tuberculosis
Hypersensitivity to drugs (trimethoprim-sulfamethoxazole, intravenous immune globulin, antithymocyte globulin)
Disorders associated with vasculitis such as Kawasaki disease and collagen vascular disease
Workup
Laboratory Studies
- General guidelines
- Bacterial meningitis is a medical emergency. A firm diagnosis usually is made when bacteria are isolated from the cerebrospinal fluid (CSF) and evidence of meningeal inflammation is demonstrated by increased pleocytosis, elevated protein level, and low glucose level in the CSF. Timely collection and processing of CSF and isolation of an organism allows optimization of choice of antimicrobial agent and duration of therapy.
- A lumbar puncture (LP) may be contraindicated in some of the following conditions: unstable patients with hypotension or respiratory distress who may not be able to tolerate the procedure, brain abscess, brain tumors or other cause of raised intracranial pressure, and occasionally infection at the lumbar puncture site. Specific hematologic, radiographic (eg, CT scan, MRI of the head), and other studies assist in diagnosis.
- Measurement of serum glucose level close to the time of CSF collection is useful for interpreting CSF glucose levels and the likelihood of meningitis.
- Group B streptococcal antigen test in urine is unreliable and should not be used to make a diagnosis of sepsis or meningitis.
- CSF chemistries and cytology vary depending upon the maturity and age of the newborn.
- The bacterial meningitis score is under continual evaluation as is its effectiveness as an aid to identify those children with CSF pleocytosis at low risk of having bacterial meningitis. The components of the score include the following:
- Positive cerebrospinal fluid Gram stain
- CSF absolute neutrophil count greater than or equal to 1000 cells/mcL
- CSF protein greater than or equal to 80 mg/dL
- Peripheral blood absolute neutrophil count greater than or equal to 10000 cells/mcL
- History of seizure before or at the time of presentation
- Infants and children
- Definitive diagnosis is based on CSF findings. The opening pressure of CSF should be measured in older children. Similarly, the color of the CSF (eg, turbid, clear, bloody) should be recorded.
- If the spinal fluid is not crystal clear, administer treatment immediately without waiting for the results of CSF tests.
- If the patient shows signs of pending herniation, consider treatment without performing a lumbar puncture. A lumbar puncture can be performed later, when intracranial pressure is controlled and the patient is clinically stable. A CT scan or MRI is helpful in managing patients who require control of intracranial pressure and herniation.
- Perform chemistries (ie, glucose, protein), total and differential cell count, Gram stain, and cultures on all CSF specimens. In a setting of antibiotic pretreatment, rapid bacterial antigen testing may be considered. Generally, CSF glucose is less than 50% of simultaneously obtained blood glucose value, and CSF proteins are greater than 50 mg/dL. However, these values may be within the reference range in patients with very early disease. Patients with both fulminant disease and poor immune response may not show cytological or chemical changes in CSF. Approximately 2-3% of bacterial meningitis cases have a negative Gram stain result and normal cell count, glucose level, and protein level yet positive bacterial cultures.
- Most untreated patients have an increased WBC count with a predominance of polymorphonuclear leukocytes at the time of diagnosis, although bacterial meningitis may present with a lymphocytic predominance. A Gram stain of cytocentrifuged CSF may reveal bacterial morphology. The CSF should be plated immediately onto a chocolate and blood agar media. Blood cultures also should be obtained. Smears of petechial lesions may reveal microorganisms on Gram stain. Similarly, examination of a buffy coat smear also may reveal intracellular microorganisms.
- Several tests based upon the principle of agglutination for the detection of bacterial antigens in body fluids are available. Bacterial antigen detection can be carried out in samples of CSF, blood, and urine. Antigen detection tests are most helpful in patients with partially treated meningitis where bacteria may not grow from the CSF but antigens persist in body fluids. Antigen detection in the urine is particularly helpful in such circumstances because urine can be concentrated several fold in the laboratory. Several gram-negative bacteria and higher serotypes of S pneumoniae have capsular antigens, which cross-react with H influenzae type b polyribophosphate. Capsular antigens of group B meningococcus cross-react with K1-containing Escherichia coli. Gram stains of CSF are more sensitive than these rapid diagnostic tests for the detection of N meningitidis.
- Partially treated meningitis
- Many children receive antibiotics before definitive diagnosis is made. As a rule, a few doses of oral antimicrobial agents, or even a single injection of an antibiotic, do not significantly alter CSF findings, including bacterial cultures, especially in patients with H influenzae type b disease. Oral antibiotics have never convincingly been shown to render patients with bacterial meningitis CSF culture negative.
- CSF cultures may become sterile rapidly if the pathogen was pneumococcus or meningococcus, although cellular changes, an increase in protein, and low glucose levels persist. In such cases, CSF, blood, and urine should be tested for bacterial antigens; however, the presence of a negative antigen result does not entirely rule out a bacterial source.
- More sensitive tests, such as amplification of 16S rRNA gene by polymerase chain reaction (PCR), may become readily available in the future to diagnose bacterial meningitis in antibiotic pretreated patients.
Imaging Studies
- CT scanning and MRI may reveal ventriculomegaly and sulcal effacement, as in the images below.
Acute bacterial meningitis (same patient as in Media files 2-3). This axial nonenhanced CT scan shows mild ventriculomegaly and sulcal effacement.
Acute bacterial meningitis (same patient as in Media files 1 and 3). This axial T2-weighted MRI shows only mild ventriculomegaly.
Acute bacterial meningitis (same patient as in Media files 1-2). This contrast-enhanced, axial T1-weighted MRI shows leptomeningeal enhancement (arrows).
More on Meningitis, Bacterial |
| Overview: Meningitis, Bacterial |
 Differential Diagnoses & Workup: Meningitis, Bacterial |
| Treatment & Medication: Meningitis, Bacterial |
| Follow-up: Meningitis, Bacterial |
| Multimedia: Meningitis, Bacterial |
| References |
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Further Reading
Keywords
pyogenic meningitis, bacterial meningitis, bacterial infection of the meninges, acute bacterial meningitis, treatment, diagnosis, symptoms
