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Because diagnostic and therapeutic decisions in infectious diseases are heavily informed by a patient’s clinical syndrome, they are included with each specific disease.



  • Note: Meningitis and encephalitis are two distinct clinical entities, each with their own list of causative organisms. However, in practice the distinction may be blurred as these syndromes can share clinical features

  • Clinical features:

    • - Classic triad: 1) Fever, 2) Nuchal rigidity, 3) Altered mental status, although less than 50% of patients have all three symptoms

    • - Physical exam: The diagnostic utility of these tests are quite poor and they have limited negative predictive values. In other words, the absence of positive findings for any of the following tests does not meaningfully decrease the probability of meningitis and encephalitis

      • Kernig’s sign: Inability to fully extend knees when patient is supine with hips flexed

      • Brudzinski’s sign: Flexion of legs/thighs that is brought on by passive flexion of neck

      • Jolt test: Painful to turn head side-to-side

      • Rashes:

        • - N. meningitides: Maculopapular rash with petechiae/purpura (~50% of patients will have a rash on presentation)

        • - HSV: Vesicular lesions (may be present, but their absence in a patient with encephalitis does not decrease the likelihood of HSV encephalitis)

        • - VZV: Vesicular lesions (encephalitis may develop weeks before or weeks after the onset of rash)

  • Pathogens:

    • - Bacteria:

      • Neonates: Group B strep, E.coli, L. monocytogenes

      • Children >3 months: N. meningitidis, S. pneumoniae, H. influenzae

      • Adults: S. pneumoniae (>70% cases), N. meningitis (12%), Group B strep (7%), H. influenzae (6%)

      • Elderly/immunocompromised: L. monocytogenes (<5%)

    • - Viruses: Enterovirus, HSV-2, VZV, HIV, mumps, arbovirus (West Nile virus, St. Louis encephalitis virus)

    • - Other pathogens: Mycobacterium tuberculosis, syphilis, Cryptococcus spp., Coccidioides spp.

  • Diagnosis: Lumbar puncture (LP)

    • - Ensure there are no other contraindications to LP (e.g., a deteriorating level of consciousness, anticoagulation, epidural abscess)

    • - Perform non-contrast head CT (NCHCT) prior to LP if new focal neurologic deficits, altered mental status, age >60 yr, immunocompromised, concern for increased intracranial pressure (papilledema, vomiting), seizures, known brain metastases

    • - Obtain blood cultures prior to antibiotics, but do not delay empiric antibiotics while waiting for head imaging or LP

    • - The following studies should be sent from the CSF:

      • Cell count and differential

      • Gram stain and bacterial culture

      • Glucose concentration (check simultaneous serum glucose to evaluate the ratio of CSF:serum glucose)

      • Protein concentration

      • More advanced diagnostic tests can be sent depending on the concern for specific pathogens (E.g., viral PCR testing, fungal testing, metagenomic next generation sequencing, universal PCR testing)

    • - Typical CSF profiles for CNS conditions: See Table 8.15

  • Treatment: See Table 8.16

  • Prevention:

    • - Vaccinate all individuals 65+ yr and immunocompromised patients for S. pneumoniae

    • - Vaccinate all asplenic patient for S. pneumoniae, N. meningitidis, H. influenzae

TABLE 8.15Typical CSF Profiles of CNS conditions

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