Viral
encephalitis
Epidemiology
- most cases are sporadic accompaniments of common infections eg mumps,
measles, HSV. (UK & Europe)
- most epidemics are due to Japanese B encephalitis except in USA where most
cases are due to arboviruses (arthropod-bourne)
- viruses are by far the commonest causes of encephalitis globally but in
certain locations and seasons other organisms eg malaria, other protozoa,
rickettsiae and fungi may cause an encephalitic syndrome
Pathogenesis
- haematogenous infection in most cases although in the case of rabies and
HSV virus ascends neurons centripetally
- certain viruses exhibit tropism towards specific cell types eg limbic system
in rabies, temporal lobes in HSV encephalitis
Clinical features
- prodrome of several days, may include:
- malaise
- myalgia
- mild URTI
- fever
- rash
- parotitis
- headache
- mental change
- drowsiness
- meningism
- may progress to: disorientation, behavioural disturbance, speech disturbance
and coma
- fits common
- focal features appropriate to the part of the brain taking the brunt of the
infection may appear. eg hallucinations and memory loss from temporal lobes,
hemiparesis, cerebellar deficits
- +/- signs of raised ICP
- often more severe in very young, very old, and immunocompromised
Differential diagnosis
- all forms of bacterial meningitis
- malaria
- other protozoal and fungal infestations
- intracranial suppuration
- septicaemia & endocarditis
- metastatic disease
- connective tissue disease
- drug abuse
Investigations
- FBC
- U&E
- travel history
- blood films for malaria
- blood cultures
- CT. NB During first 2-3 days no abnormality may be evident and changes may not
show for 5-6 days
- EEG: usually shows non-specific diffuse slow-wave activity +/- seizure
activity. Temporal lobe focal abnormality with high-voltage spike and slow-wave
complexes is highly suggestive of HSV encephalitis
- LP as soon as is deemed safe. Unfortunately this is very difficult to
determine as CT scanning does not convey an accurate picture of ICP.
- raised pressure
- WCC: 10/ml - several thousand/ml. Usually lymphocytes but polymorphs may
predominate in early stages
- RBCs may be found if there is a necrotising element eg HSV encephalitis
- glucose normal
- protein raised
- CSF and serum for viral studies
- CSF for HSV Ag PCR
Treatment
- supportive
- acyclovir 10 mg/kg tds unless HSV has been excluded
- theoretical reasons for not using steroids (may inhibit interferon synthesis)
but many continue to use dexamethasone in severely ill patients
Specific encephalitides
HSV
- most common cause of sporadic encephalitis in Europe and N America.
Accounts for 10-20% of cases of viral encephalitis
- 0.5/100 000/yr. Probably underestimate
- any age
- not more common in immunocompromised
- clinical picture very variable
- onset usually with a 4-10 day prodrome: malaise, pyrexia, irritability
- followed by frontal and temporal lobe disturbance:
- personality change
- hallucinations
- psychiatric disturbance
- memory loss
- increasing focal signs, seizures, deteriorating conscious level
- focal signs in as many as 87%
- in a few patients onset much more acute with evolution over only a few days
- differential diagnosis includes:
- abscess
- granuloma
- vascular disease
- other forms of viral encephalitis
- treatment: acyclovir IV for 10 days. This decreases mortality to 30%. Has
been suggested that multiple courses may be necessary and that a longer initial
course (ie 21 days) should be the standard
VZV
- affects approx 0.09% of cases of chickenpox. May rarely follow shingles
- in 50% cerebellar signs are prominent with ataxia, dysarthria, headache,
drowsiness coming on about a week after onset of rash
- neurological signs may precede rash
- convulsions common
- +/- progression to hemiplegia, cranial nerve palsies, aphasia and coma
- patients with cerebellar form usually recover completely
- 10% of those with general form die
- management supportive
CMV
- only important as a cause of encephalitis in neonates and the
immunocompromised
- no specific treatment of proven value
Human herpes virus 6
- recently identified as a possible cause of human CNS infection although
aetiological link between virus and disease is not clearly established
- ubiquitous in the community and appears to be acquired early in life
- rarely associated with encephalitis
- capable of latent infection with reactivation in response to a variety of
stimuli
- usual presentation is with decreased consciousness and fits
- detection of HHV-6 DNA in CSF is not specific for HHV-6 encephalitis
Measles
- acute encephalitis associated with measles usually caused by postinfectious,
perivenous, demyelinating, allergic phenomenon +/- direct viral damage
- up to 1/1000 cases above age of 2 yrs. Frequency increases with age
- typical presentation: recurrence of fever on 7th or 8th day as rash is fading,
with rapid development of encephalitis: convulsions, focal signs, myoclonus and
coma
- no specific features on investigation
- treatment supportive
- mortality 15% and some disability in most survivors
Mumps
- rare. Occurs in < 1% of cases of mumps
- no specific features
- occurs up to 2 weeks after development of parotitis. Latter may be absent in
50%
- prognosis excellent
Epidemic encephalitis
- caused by arboviruses. All zoonoses with birds and smaller vertebrates as
primary hosts. Transmission is by mosquito or tick
- clinical features are common to all with some variation in incubation,
progression, and severity
- specific diagnosis can only be made by rise in antibody titre
- treatment is symptomatic
Eastern equine encephalitis
- rare
- Atlantic and Gulf coasts of America
- summer and autumn
- children most affected
- 70% mortality
Western equine encephalitis
- less severe
- eastern, central and western USA & Canada and eastern S. America
St Louis encephalitis
- most cases are benign and of short duration
California and La Crosse viruses
- similar
- central and midwest USA
- children
- peak in August and September
- sudden onset
- recovery within 10 days
Central European encephalitis
- woodlands in Scandinavia through to northern Greece and former
Yugoslavia
- tick-borne
- biphasic course: flu-like illnesss followed within 2 weeks by mild meningitis
or encephalitis +/- muscle weakness
- management symptomatic
- recovery usually complete
Russian spring-summer encephalitis
- similar to Central European encephalitis but move severe
Japanese encephalitis
- most common arbovirus infection worldwide
- endemic in SE Asia, Japan, China, Philippines, Borneo, and large parts of
Indian sub-continent
- age: < 15 yrs and elderly predominantly
- mosquito-borne
- incubation 1-2 weeks followed by abrupt onset of encephalitis often with
myalgia
- +/- extra-pyramidal features
- convalescence may be prolonged and sequelae are common
- treatment symptomatic
Further reading
Caserta MT. Human herpesvirus 6 infection of the central nervous system.
Current Infectious Diseases Reports, 2004, 6:316-321
© Charles Gomersall December 1999, February 2005
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