Diagnostic Approach to Central Nervous System Infections

Introduction to Diagnostic Evaluation

The American College of Critical Care Medicine suggests that imaging studies and culture of cerebrospinal fluid are the cardinal features of diagnostic evaluation for CNS infection, with MRI serving a complementary but not substitutive role 1, 2, 3
Imaging is performed before LP only to exclude contraindications, such as mass lesions or obstructive hydrocephalus, in patients with focal neurologic findings, not to replace the LP itself 1, 2, 3
A non-contrast CT scan is adequate for this safety screening purpose; MRI is not specifically required 1, 2, 3

If altered consciousness or focal neurologic signs are unexplained, lumbar puncture should be considered in any patient with new fever, unless there is a contraindication 1, 2, 3
CSF analysis provides essential diagnostic information, including cell count with differential, glucose and protein concentrations, Gram stain, bacterial culture, and potentially PCR testing 1, 2, 3, 4
Normal opening pressure, <5 WBC/μL, and normal CSF protein essentially exclude meningitis in immunologically normal hosts 4
CSF PCR has sensitivity of 87-100% and specificity of 98-100%, especially valuable if antibiotics were given before LP 4

Central nervous system infection rarely causes encephalopathy without focal abnormalities on neurologic examination, but in any febrile ICU patient, even without focal findings, infection must be considered because of inherent limitations of the neurologic examination in critically ill patients 1, 2, 3
Approximately 25% of neurocritical patients develop fever, with nearly half being of non-infectious origin, making the differential diagnosis challenging without CSF analysis 3, 5
The yield of lumbar puncture in patients without immune compromise or central nervous system instrumentation who develop mental status changes in the ICU may be low, but this does not eliminate the need for LP when the patient lacks an obvious alternative source of fever or when there are no clear risk stratification criteria suggesting low risk 1, 2, 3

If bacterial meningitis is suspected and the lumbar puncture is delayed for any reason, including an imaging study, then appropriate empirical antibiotic therapy for meningitis should be started after blood cultures are obtained, as recommended by the American College of Critical Care Medicine 1, 2, 3, 4
The Infectious Diseases Society of America recommends collecting at least 22 mL of CSF for comprehensive testing 4