Encephalitis: An Overview
Encephalitis is defined as inflammation of the brain parenchyma associated with neurologic dysfunction, characterized by altered mental status lasting ≥24 hours with no alternative cause identified. 1 This serious neurological condition can lead to significant morbidity and mortality if not promptly diagnosed and treated.
Definition and Distinction from Encephalopathy
Encephalitis and encephalopathy are often used interchangeably but represent different pathophysiologic processes:
Encephalitis: Characterized by brain inflammation due to:
- Direct infection of brain parenchyma
- Post-infectious processes (e.g., acute disseminated encephalomyelitis)
- Non-infectious conditions (e.g., autoimmune encephalitis) 1
Encephalopathy: A clinical state of altered mental status without necessarily having inflammation, which can be caused by:
- Metabolic derangements
- Toxins
- Systemic infections
- Certain infectious agents without direct inflammation 1
Diagnostic Criteria
According to the International Encephalitis Consortium, diagnosis requires:
Major Criterion (required):
- Altered mental status (decreased consciousness, lethargy, or personality change) lasting ≥24 hours with no alternative cause 1
Minor Criteria (2 required for possible encephalitis; ≥3 for probable or confirmed encephalitis):
- Documented fever ≥38°C within 72 hours before/after presentation
- Seizures (generalized or partial) not attributable to pre-existing seizure disorder
- New focal neurological findings
- CSF WBC count ≥5/cubic mm
- Brain imaging abnormalities suggestive of encephalitis
- EEG abnormalities consistent with encephalitis 1
Epidemiology
- Incidence in children: approximately 10.5-13.8 per 100,000 1
- In industrialized nations, herpes simplex virus (HSV) is the most common identified cause 2
- Annual incidence of HSV encephalitis: 1 in 250,000 to 500,000 people 1, 2
- Age-specific incidence is bimodal, with peaks in childhood and the elderly 1, 2
- Most HSV encephalitis (90%) is caused by HSV type 1, with HSV type 2 accounting for about 10% of cases 2
Etiology
Encephalitis can be broadly categorized into:
Infectious causes:
Autoimmune/post-infectious causes:
Clinical Presentation
Patients with encephalitis typically present with:
- Fever
- Headache
- Altered level of consciousness
- Behavioral or personality changes
- Cognitive impairment
- Focal neurological deficits
- Seizures (focal or generalized)
- Movement disorders
- Autonomic instability 4, 5
Diagnostic Approach
Diagnosis involves:
- Clinical assessment for altered mental status and other neurological symptoms
- Cerebrospinal fluid analysis (looking for pleocytosis, protein elevation)
- Neuroimaging (MRI preferred over CT)
- Electroencephalography
- Microbiological and serological testing 1, 4
Management Implications
- Early treatment is critical: Delays in treatment beyond 48 hours after hospital admission are associated with worse outcomes 1, 2
- For HSV encephalitis, prompt administration of acyclovir is essential
- VZV encephalitis also responds to acyclovir therapy 6
- Specific treatment depends on identified etiology
- Supportive care is crucial for all patients 4
Long-term Consequences
Even after successful treatment, encephalitis may lead to:
- Neurocognitive impairment
- Neuropsychiatric disorders
- Potential contribution to neurodegenerative diseases
- Ongoing disability requiring rehabilitation 3
Special Considerations
Immunocompromised patients are at higher risk for:
- VZV (multifocal leukoencephalopathy)
- Cytomegalovirus
- JC/BK virus (progressive multifocal leukoencephalopathy)
- Fungal and parasitic causes 1
Presentation can be acute, subacute, or chronic depending on the causative agent 1
Early recognition, prompt diagnosis, and appropriate treatment are essential to improve outcomes in patients with encephalitis. The diagnostic approach should be systematic, and treatment should be initiated promptly, particularly for treatable causes like HSV encephalitis.