Viral encephalitis is a serious inflammatory condition, commonly due to HSV1 typically affecting the temporal lobes, characterised by necrotising haemorrhagic inflammation, Cowdry A bodies and diffuse oedema of the limbic system.
Description
Viral encephalitis is an inflammation of the brain parenchyma caused by viral infection, often resulting in neurological dysfunction. The condition can affect individuals of all ages and genders, but certain viruses may have a predilection for specific age groups. While many viruses can cause encephalitis, herpes simplex virus (HSV) is the most common cause of sporadic fatal encephalitis worldwide.
Pathogenesis
The pathogenesis of viral encephalitis involves the invasion of the brain parenchyma by a virus, which can occur via haematogenous spread or neuronal transmission. The virus replication leads to cell death and the release of inflammatory mediators, causing oedema, necrosis, and the clinical symptoms of encephalitis.
Subtypes
Viral encephalitis can be caused by numerous types of viruses. Notable ones include:
- Herpes simplex encephalitis (HSE): Due to HSV 1. The most common and severe form of acute encephalitis.
- Arbovirus encephalitis: Caused by arthropod-borne viruses, including West Nile virus, Eastern Equine Encephalitis virus, and others.
- Enterovirus encephalitis: Often affects children and infants.
Epidemiology, Risk Factors & Associations
- HSV encephalitis is the most common cause of fatal sporadic encephalitis worldwide.
- Risk factors include immunocompromised states (such as HIV infection, organ transplant, or chemotherapy), age extremes, and residing in or travelling to areas endemic for certain arboviruses.
Clinical Features
- Fever, headache, altered mental status, and seizures are common.
- Focal neurological deficits may be present, particularly in HSV encephalitis.
Complications
- Long-term neurological complications are common in survivors and may include cognitive impairment, epilepsy, and motor deficits.
- Mortality rate for untreated HSE can exceed 70%.
Pathological Features
Histopathology
- Macroscopic: Brain swelling and petechial haemorrhages may be observed.
- Microscopic: Perivascular and parenchymal inflammation, neuronophagia, and necrosis are common findings. Typically eosinophilic Cowdry A bodies are seen in HSV1.
Serology
- Presence of specific IgM antibodies in serum or CSF can suggest specific viral infections.
Biochemistry
- CSF analysis typically reveals elevated protein and pleocytosis.
Radiological Features
General Features
- Characteristically demonstrates brain swelling, areas of necrosis or haemorrhage, and diffuse or focal areas of increased T2/FLAIR signal indicating oedema or inflammation
- Typically seen in the medial temporal and inferior frontal cortex
CT
- Non-contrast: May show low attenuation areas representing oedema or necrosis. Haemorrhage may appear as hyperdense.
MRI
- T1: Lesions are usually hypointense or isointense.
- T2: Hyperintense signal changes in affected areas due to oedema or inflammation.
- FLAIR: Hyperintense signal changes in affected regions.
- DWI/ADC: Restricted diffusion may be seen in areas of cytotoxic oedema.
- T1 Gad+: Variable enhancement, may be more pronounced in later stages.
Diagnosis
Diagnosis is typically based on clinical presentation, CSF analysis (pleocytosis and elevated protein), and imaging findings. PCR of the CSF for specific viruses can confirm the diagnosis.
Differential Diagnosis
- Acute disseminated encephalomyelitis (ADEM): More likely to have a history of recent vaccination or infection, and typically shows more extensive white matter involvement on imaging.
- Vasculitis/CNS lupus: May have additional systemic symptoms or signs of other organ involvement.
Prognosis
Prognosis varies depending on the causing virus and the patient’s immune status. Despite antiviral treatment, HSV encephalitis still carries a high risk of death or long-term neurological impairment.
Management
Management typically involves supportive care and antiviral therapy. In the case of HSV encephalitis, acyclovir is the treatment of choice. Urgent neurology or infectious disease consultation is usually required.