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Meningoencephalitis: Viral
Axial FLAIR MRI at the level of the medulla. Areas of inflammation in the brainstem and cerebellum adjacent to the 4th ventricle. Patient floridly symptomatic.
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Meningoencephalitis: Viral

Overview
Viral meningoencephalitis divides into:
  • Acute, aseptic (presenting with severe headache but non-lethal symptoms).
  • Encephalitis (with prominent confusion).
  • Arthropod-borne encephalitis (those from insect bites).
Normal CSF
  • < 10 cells, 70% of which are lymphocytes
  • Glucose is 60% of that of the serum glucose
  • Protein is from 15 to 45
  • CSF pressure is usually 70 to 180 mmH2O.
Viral meningitis
  • Cell counts are typically from 50 to 250, the majority of cells are usually lymphocytes
  • Glucose is often normal
  • Protein is elevated
  • Pressure is often normal
viral meningitis
Acute, aseptic
  • Enterovirus is the most common (by far).
  • Influenza was formerly the most common cause but now typically only manifests in those not vaccinated
  • Lymphocytic choriomeningitis (LCM) is a rodent-borne disease.
  • Herpes simplex virus type 2 causes Mollaret meningitis, which manifests with recurrent headaches and is treatable with acyclovir.
  • HIV classically causes an aseptic meningitis as its first symptom, but this is often dismissed by patients.
Viral encephalitis
  • Herpes simplex type 1, which manifests with a limbic encephalitis (medial temporal inflammation) and pathologic findings of Cowdry, Type A intranuclear inclusions in neurons and glia.
  • CMV is an important congenital cause of meningitis (it's one of the TORCH syndromes) and also occurs, notably, in the immunosuppressed. On pathologic exam it demonstrates: owl-eye inclusions.
  • PML occurs from a reactivation of JC virus in the immunosuppressed (notably from iatrogenic causes); it infects oligodendrocytes, so it causes demyelination.
  • HIV – we learned that it causes an aseptic meningitis at presentation, but it also causes a chronic meningitis, which manifest with pathologic findings of microglial nodules and multinucleated giant cells.
  • Measles, notably, can cause a chronic meningitis called: subacute sclerosing panencephalitis, which is a progressive encephalopathy.
We can group CMV, PML, HIV, and SSPE (measles) as leukoencephalopathies in that they typically manifest with white matter invasion (as specifically address with PML).
Arthropod-borne encephalitis
  • Typically manifest with pathologic findings of perivascular cuffs of lymphocytes (meaning aggregation of lymphocytes around a blood vessel. And also microglial nodules.
  • Western hemisphere:
    • West Nile virus (which can manifest with encephalitis and also poliomyelitis (anterior horn cell disease – flaccid paralysis); Eastern Equine virus, Western Equine virus, St. Louis virus, and La Crosse virus.
  • Far East
    • Japanese B virus
  • Eastern Europe and Russia
    • Tick-borne virus.