Diagnostic and Treatment of Meningitis and Encephalitis

Cerebrospinal Fluid (CSF) Analysis

The presence of pleocytosis with a predominance of polymorphonuclear neutrophils in the CSF is indicative of bacterial meningitis, according to the Clinical Microbiology and Infection guidelines 1
A low glucose level in the CSF, with a low CSF-to-blood glucose ratio, is characteristic of bacterial meningitis, as recommended by the Clinical Microbiology and Infection guidelines 1
Elevated protein levels in the CSF are also typical of bacterial meningitis, as stated by the Clinical Microbiology and Infection guidelines 1
The presence of pleocytosis with a predominance of lymphocytes in the CSF is indicative of viral encephalitis, such as herpes simplex virus (HSV) encephalitis, according to the Journal of Infection guidelines 2
A normal or slightly decreased glucose level in the CSF is characteristic of viral encephalitis, as recommended by the Journal of Infection guidelines 2
Moderately elevated protein levels in the CSF are also typical of viral encephalitis, as stated by the Journal of Infection guidelines 2
The presence of red blood cells in the CSF may be seen in HSV encephalitis, according to the Journal of Infection guidelines 2
A positive PCR for HSV or other neurotropic viruses in the CSF is diagnostic of viral encephalitis, as recommended by the Clinical Infectious Diseases guidelines 3

Bacterial meningitis typically presents with a predominant meningitic syndrome (headache, neck stiffness, photophobia), fever, and normal consciousness at the outset, according to the Clinical Microbiology and Infection guidelines 4
Viral encephalitis typically presents with altered consciousness, cognitive disturbances, behavioral changes, focal neurological signs, and seizures, according to the Clinical Infectious Diseases guidelines 5

In suspected bacterial meningitis, immediate lumbar puncture is recommended if there are no contraindications, along with standard CSF analysis, Gram stain, and culture, as recommended by the Clinical Microbiology and Infection guidelines 6
In suspected viral encephalitis, MRI of the brain, lumbar puncture with PCR for HSV and other neurotropic viruses, and EEG are recommended, according to the Clinical Infectious Diseases guidelines 5

Empiric antibiotic therapy should be started within the first hour of admission for suspected bacterial meningitis, without waiting for CSF results, according to the Clinical Microbiology and Infection guidelines 4
Aciclovir intravenously (10 mg/kg every 8 hours) should be started urgently in suspected viral encephalitis, without waiting for diagnostic confirmation, as recommended by the Journal of Infection guidelines 2
The duration of treatment for viral encephalitis is typically 14-21 days, according to the Journal of Infection guidelines 2

Encéphalite démyélinisante aiguë post-infectieuse (ADEM) should be treated with high-dose corticosteroids (methylprednisolone 1g/day for 3-5 days), according to the Clinical Infectious Diseases guidelines 3
HSV-2 meningitis should be treated with aciclovir IV 10 mg/kg every 8 hours until resolution of symptoms, followed by valaciclovir orally to complete 14 days, as recommended by the Clinical Infectious Diseases guidelines 7
Immunocompromised patients require a lower diagnostic threshold and more aggressive treatment due to often atypical clinical and biological presentation, according to the Clinical Microbiology and Infection guidelines 1

Prior antibiotic administration may reduce the yield of CSF cultures and Gram stain, but CSF findings such as elevated WBC count, decreased glucose, and elevated protein will likely still provide evidence for or against bacterial meningitis, as noted by the Infectious Diseases Society of America 8

The American Academy of Pediatrics and the Infectious Diseases Society of America recommend that empiric antimicrobial therapy should be started immediately after obtaining blood cultures if lumbar puncture is delayed, with the strength of evidence being high 8