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:
- Headache
- Photophobia
- Neck stiffness
- 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
- Syndrome of inappropriate antidiuretic hormone secretion (SIADH)
- Disseminated intravascular coagulopathy
- Epilepsy
- Changes in personality
- Residual cognitive, behavioural, or emotional impairments
- Residual amnesia
- Death
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
