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Encephalitis

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Overview

Encephalitis is the inflammation of the brain tissue associated with neurological dysfunction. Unlike meningitis, it is the result of direct inflammation of brain tissue, not the meninges. It is most commonly caused by a viral infection, particularly herpes simplex virus (HSV) type 1 which classically affects the temporal and inferior frontal lobes.

Epidemiology

  • HSV-1 is the most common cause (95% of cases)
  • Most frequent and severe in children and the elderly

Causes

Viral causes

  • HSV-1 – responsible for 95% of cases in adults
  • HSV-2
  • Varicella zoster
  • Cytomegalovirus
  • Epstein-Barr virus
  • HIV

Bacterial causes

  • Neisseria meningitidis
  • Tuberculosis
  • Syphilis
  • Listeria species
  • Borrelia burgdorferi – Lyme disease
  • Bartonella – cat-scratch disease
  • Streptococcus pneumoniae
  • Mycoplasma species

Fungal infections

  • Cryptococcus species
  • Candida species

Parasitic causes

  • Toxoplasma gondii
  • Entamoeba histolytica
  • Plasmodium falciparum

Risk Factors

  • Neonates
  • Aged >65 years
  • Immunodeficiency e.g. due to immunosuppressants/HIV
  • Tick bites – Lyme disease
  • Cat scratches/bites – Bartonella 

Presentation

A fever accompanied by changes in consciousness, neurological deficits, or seizures should raise suspicion for encephalitis. Features are:

  • Fevers
  • Altered mental state:
    • Can range from mild sleepiness to coma
    • May include confusion, memory problems, behavioural changes, or psychosis
  • Rashes:
    • Dependent on the infective pathogen e.g. vesicular rashes in HSV
  • Meningism:
  • Focal neurological deficits:
    • E.g. hemiparesis, ataxia, visual field changes, speech difficulties, sensory changes
  • Upper respiratory tract features may be present
  • Seizures – usually generalised tonic-clonic seizures

Differential Diagnoses

Bacterial meningitis

  • Fever and headaches, neck stiffness, and photophobia are usually present
  • Altered consciousness is generally not as significant (except for drowsiness)
  • CSF shows elevated polymorphs, elevated protein, and low glucose
  • Gram staining and polymerase chain reactions can identify the causative organism

Intracranial bleed

  • It may be difficult to differentiate clinically, but subarachnoid haemorrhages (SAH) can have meningism
  • CT/MRI shows intracranial bleeds
  • In SAH, a lumbar puncture at least 12 hours after onset may show xanthochromia

Investigations

  • Full blood count (FBC):
    • May show elevated white cell count in infection
  • Blood cultures:
    • Can show causative pathogen
  • CSF analysis, culture, serology, and PCR:
    • Findings depend on cause
  • Neuroimaging:
    • CT brain – can be normal but may show hypodense lesions
    • HSV-1 classically affects the temporal and inferior frontal lobes
    • MRI brain – preferable to CT
  • Electroencephalogram (EEG):
    • May show background slowing

Management

  • 1st-line: IV aciclovir if suspected/confirmed viral encephalitis
  • Notify Public Health England – encephalitis is a notifiable disease

Complications

Prognosis

  • Many patients are left with residual physical or neuropsychiatric complications that require multidisciplinary management
  • Untreated herpes simplex encephalitis is fatal within 7-14 days
  • Factors associated with a worse prognosis are:
    • Increasing age
    • Immunodeficiency
    • Mechanical ventilation being needed
    • Coma

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