Viral encephalitis is the commonest cause of encephalitis. The general features and causes of encephalitis are described in a separate section. Here we discuss the various viral encephalitides.
HSV Encephalitis:
Diagnosis:
Clinical findings combined with MRI features and isolation of virus by PCR or pathology
Findings on investigations:
CT:
- Low density in: Insular regions, hippocampus, cingulated gyrus
MRI:
- T2 high signal intensity, especially in the temporal lobes or orbitofrontal lobe
- May cause Rhombencephalitis
Electroencephalography, EEG:
- Temporal lobe disease
- Periodic lateralised epileptiform discharges (PLEDs),
- Temporal spikes at 2-3Hz
CSF PCR: false negatives can occur
Lumbar puncture & CSF analysis:
- White cell counts: high,
- Lymphocytes or monocytes, can be very high
- RCC: raised in HSV encephalitis
- Opening pressure: Normal or mildly elevated 100-350 mmH2O
- Glucose normal or slightly decreased
- Protein: normal or slightly increased 0.2-0.8 g/L (20-80 mg/dL)
- Oligoclonal bands & electrophoresis abnormalities may occur
Pathology, Biopsy:
- Perivascular lymphocytes
- Lymphocytic infiltrate
- Microglial proliferation
- Intranuclear & cytoplasmic inclusions, Cowdry A inclusions (large with halo)
Treatment:
- Acyclovir I.V. for at least 3 weeks
West Nile Virus encephalitis, WNV meningoencephalitis & WMV neuropathy WNV:
Clinical features:
- – Meningeal symptoms: fever, headache, neckstiffness, rash
- Encephalopathy, seizures may occur
- Myoclonus, tremour, rigidity, postural instability, bradykinesia, Cranial nerve palsies, vertigo, cerebellar ataxia
- Flaccid paralysis syndrome may occur (with or without encephalitis)
- Optic neuritis & chorioretinitis
Diagnosis:
- Clinical features plus CSF antibody tests or isolation of the virus
Findings on Investigations:
+CSF IgG & IgM for West nile virus:
- Microspheric immunoassay (MIA): if serology is positive to exclude a heterologous response to other flaviviruses
+CSF PCR for WNV:
- False negatives occur
+Serology: IgG & IgM for West nile virus: not 100% specific, negative test doesn’t rule out infection
CSF analysis:
- CSF: lymphocytic pleocytosis (rarely normal), raised protein, normal glucose,
CT: is usually normal
MRI:
- leptomeningeal or pervascular enchancement. T2: normal initially, high signal in caudate nuclei, thalami, brainstem, spinal cord,
- MRI brachial plexus: Thickened brachial plexus on T2 in WNV neuropathy
EEG: non-specific features of encephalopathy
NCS/EMG:
- normal SNAPs, reduced CMAPs
Rabies encephalitis:
Clinical features:
- Prodrome: headache, fever, malaise, anxiety
- Dysphagia,
- Hydrophobia: laryngeal spasm on swallowing liquids
- Seizures
Pathology, biopsy:
- Negri bodies: eosinophilic intracytoplasmic inclusions in pyramidal cells & Purkinje cells
- Babes nodules: microglial nodules
Treatment:
- Clean the wound
- Human rabies Ig
- Human diploid cell vaccine