Medicine – Encephalitis

Encephalitis is inflammation of the brain parenchyma, most commonly caused by viral infection. Because the brain tissue itself is affected, patients characteristically develop evidence of cerebral dysfunction, such as confusion, altered consciousness, seizures, behavioural changes, or focal neurological deficits.

This differs from meningitis, in which inflammation primarily involves the meninges surrounding the brain and spinal cord.

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1. Encephalitis versus Meningitis

The major distinction is the anatomical site of inflammation.

Encephalitis → brain parenchyma.

Meningitis → meninges.

Because encephalitis affects functioning brain tissue, abnormalities of mental state and neurological function are particularly prominent.

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2. Clinical Presentation

The typical presentation of encephalitis includes:

Headache.

Fever.

Confusion.

Altered level of consciousness.

Seizures.

Focal neurological abnormalities.

The presence of encephalopathy or altered mental status is especially important in distinguishing encephalitis from uncomplicated meningitis.

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3. Headache

Headache is common and may occur because of:

Inflammation of intracranial structures.

Cerebral oedema.

Associated meningeal irritation.

Patients may therefore have overlapping features of both encephalitis and meningitis.

When both the meninges and brain parenchyma are involved, the term:

Meningoencephalitis

may be used.

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4. Fever

Fever is common in infectious encephalitis and reflects the underlying inflammatory or infectious process.

Therefore, the combination:

Fever + altered mental state

should raise suspicion for a CNS infection, particularly when accompanied by seizures or focal neurological signs.

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5. Confusion

Confusion is one of the characteristic features of encephalitis.

Patients may become:

Disorientated.

Agitated.

Behaviourally abnormal.

Unable to concentrate.

Unable to form new memories.

Behavioural or personality abnormalities may be particularly prominent when the temporal or frontal lobes are involved.

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6. Altered Level of Consciousness

More severe encephalitis can cause progressive impairment of consciousness.

The patient may progress from:

Confusion → drowsiness → stupor → coma.

This may result from widespread cerebral inflammation, cerebral oedema, seizures, or raised intracranial pressure.

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7. Seizures

Seizures are an important feature of encephalitis because inflamed cerebral tissue becomes electrically unstable.

Patients may develop:

Focal seizures.

Focal to bilateral tonic-clonic seizures.

Generalised convulsive seizures.

Status epilepticus.

Non-convulsive seizures should also be considered in a patient with persistent unexplained impaired consciousness.

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8. Focal Neurology

Encephalitis can produce focal neurological abnormalities depending on the cerebral regions affected.

Examples include:

Hemiparesis.

Dysphasia.

Cranial nerve abnormalities.

Visual-field defects.

Focal sensory abnormalities.

Focal seizures.

Focal neurological abnormalities are particularly important because they indicate involvement of the brain parenchyma rather than isolated meningeal inflammation.

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9. Viral Encephalitis

Viruses are among the most important infectious causes of encephalitis.

Important viral causes include:

Herpes simplex virus.

Enteroviruses.

Arboviruses.

Varicella-zoster virus.

HIV-associated infections or neurological disease.

Other viruses can also cause encephalitis depending on geography, age, immune status, vaccination history, and exposure.

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10. Herpes Simplex Virus

HSV-1 is a particularly important cause of sporadic encephalitis in adults.

HSV encephalitis is clinically important because it is:

Potentially fatal.

but also

Specifically treatable with intravenous aciclovir.

Treatment should therefore begin promptly when HSV encephalitis is suspected.

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11. HSV and the Temporal Lobes

HSV encephalitis characteristically affects the:

Medial and inferior temporal lobes.

It may also involve the:

Insular cortex.

and sometimes the:

Frontal lobes.

This produces the classic association between HSV encephalitis and temporal-lobe abnormalities.

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12. Clinical Features of HSV Encephalitis

Patients may develop:

Fever.

Headache.

Confusion.

Personality or behavioural change.

Memory impairment.

Seizures.

Focal neurological deficits.

The combination of fever + altered behaviour/confusion + seizures + temporal-lobe abnormalities is highly suggestive.

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13. Enteroviruses

Enteroviruses can cause CNS infection.

Examples include:

Coxsackieviruses.

Echoviruses.

Other enteroviruses.

They are particularly well known as causes of viral meningitis, although encephalitis and meningoencephalitis can also occur.

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14. Arboviruses

Arboviruses are viruses transmitted by arthropods such as:

Mosquitoes.

or

Ticks.

The specific viruses encountered depend strongly on geographical location and travel history.

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15. Japanese Encephalitis

Japanese encephalitis virus is an important mosquito-borne flavivirus causing encephalitis, particularly in parts of Asia and the Western Pacific.

It can cause severe neurological disease with:

Fever.

Altered consciousness.

Seizures.

Movement abnormalities.

Focal neurological deficits.

Vaccination is available for people at appropriate exposure risk.

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16. Varicella-Zoster Virus

Varicella-zoster virus (VZV) can cause encephalitis, particularly in older or immunocompromised patients, although it can occur in other groups.

VZV can also produce:

Meningitis.

Myelitis.

Cerebellitis.

Cerebral vasculopathy and stroke.

Importantly, neurological VZV disease does not always require a prominent shingles rash.

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17. HIV

HIV can affect the CNS through several mechanisms.

Neurological disease may result from:

Direct effects of HIV.

or, particularly with advanced immunosuppression:

Opportunistic CNS infections.

Therefore, an immunocompromised patient with encephalopathy requires consideration of a broader differential diagnosis.

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18. Other Important Viral Causes

Other viruses capable of producing encephalitis include:

West Nile virus.

Tick-borne encephalitis virus.

Measles virus.

Mumps virus.

Influenza-associated neurological disease.

EBV and CMV in selected patients, particularly immunocompromised individuals.

The likely cause depends heavily on epidemiological and clinical context.

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19. History

The history should assess:

Time course of illness.

Fever and systemic symptoms.

Seizures.

Behavioural or personality change.

Travel history.

Mosquito or tick exposure.

Animal exposure.

Immunosuppression.

Vaccination history.

Recent infections.

Medication or toxin exposure.

These clues may substantially narrow the differential diagnosis.

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20. CSF Findings

Lumbar puncture is usually an important investigation when safe.

Typical viral encephalitis CSF findings include:

Lymphocytic pleocytosis.

Mild to moderately elevated protein.

Usually normal glucose.

However, the exact pattern varies according to the pathogen and timing of the lumbar puncture.

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21. HSV PCR

When HSV encephalitis is suspected, CSF should be tested for:

HSV PCR.

PCR detects HSV DNA and is the key specific diagnostic test.

Other viral PCR or antibody tests may be performed depending on the suspected pathogen.

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22. MRI Brain

MRI brain is generally the preferred imaging modality for encephalitis because it is more sensitive than CT for many forms of cerebral inflammation.

The distribution of abnormalities can sometimes provide important diagnostic clues.

For example:

Temporal/insular involvement → strongly suggests HSV in the appropriate clinical context.

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23. EEG

EEG may demonstrate:

Diffuse cerebral slowing.

Focal abnormalities.

Epileptiform discharges.

It is particularly useful when seizures or non-convulsive status epilepticus are suspected.

In HSV encephalitis, temporal-lobe EEG abnormalities may occur.

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24. Treatment

Management depends on the underlying cause.

However, because HSV encephalitis is both dangerous and treatable, suspected encephalitis commonly requires urgent consideration of:

Intravenous aciclovir.

If HSV encephalitis is clinically suspected, treatment should not be delayed while waiting for PCR confirmation.

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25. Seizure Management

Associated seizures require appropriate antiseizure treatment.

A prolonged convulsive seizure is initially treated according to status epilepticus protocols, generally beginning with a:

Benzodiazepine.

Further antiseizure medication may be required if seizures continue.

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26. Supportive Treatment

Severe encephalitis may require:

Airway and respiratory support.

Fluid and electrolyte management.

Treatment of seizures.

Management of cerebral oedema and raised intracranial pressure.

Critical-care monitoring.

Nutritional support.

Early recognition of neurological deterioration is essential.

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27. Encephalitis – Note Form

Definition: inflammation of the brain parenchyma, commonly infectious and often viral.

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Main features: headache + fever + confusion + altered consciousness + seizures ± focal neurology.

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HSV: particularly important treatable cause; HSV-1 predominates in typical adult sporadic HSV encephalitis.

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Enteroviruses: including coxsackieviruses; meningitis is particularly common, but encephalitis can occur.

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Arboviruses: mosquito- or tick-borne viruses causing encephalitis.

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Japanese encephalitis: important mosquito-borne cause, especially in endemic parts of Asia and the Western Pacific.

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VZV: can cause encephalitis, meningitis, vasculopathy and other neurological disease.

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HIV: CNS disease may occur directly or through opportunistic infections in immunosuppression.

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28. Encephalitis versus Meningitis – Note Form

Encephalitis:

Brain parenchyma affected.

Confusion/behavioural change prominent.

Altered consciousness common.

Seizures common.

Focal neurological deficits may occur.

MRI brain may be abnormal.

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Meningitis:

Meninges affected.

Headache prominent.

Neck stiffness/meningism prominent.

Photophobia common.

Mental status may initially remain relatively preserved in uncomplicated disease.

Seizures and focal neurological deficits are less characteristic than in encephalitis.

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Key Clinical Pattern

Think:

ENCEPHALITIS = BRAIN DYSFUNCTION.

The classic clinical pattern is:

FEVER + HEADACHE + ALTERED MENTAL STATE ± SEIZURES ± FOCAL NEUROLOGY.

For the major treatable viral cause:

HSV-1 → temporal-lobe encephalitis → behavioural/memory changes + seizures → CSF HSV PCR → IV aciclovir.

The most important distinction is:

Meningitis → predominantly meningeal irritation.

Encephalitis → cerebral dysfunction with confusion, altered consciousness, seizures or focal neurological abnormalities.