Dr Nicholas Davies, Locum Consultant Neurologist, Imperial College Healthcare NHS
Trust
It is very important to differentiate encephalitis from other disorders that may
cause similar neurological symptoms and which may have very different
treatments. Disorders that mimic those of encephalitis include bacterial meningitis,
stroke, brain tumours, drug reactions and metabolic disturbances. Symptoms
alone often do not allow a doctor to distinguish between the many diseases that
can mimic encephalitis and therefore a variety of hospital tests are required.
Tests which can help confirm the diagnosis and rule out other disorders include:
Brain Scans Computerized Tomography (CT) or Magnetic Resonance Imaging (MRI)
scans may show the extent of the inflammation in the brain and help differentiate
encephalitis from other conditions. Brain scans can exclude stroke, brain
tumours and aneurysms. In the early stages brain scans may show nothing abnormal
with significant inflammatory changes only occurring later in the illness.
Electroencephalogram (EEG), which records brain waves, can detect abnormal
patterns of activity. Abnormal patterns found in encephalitis include
slowing of brain activity as well as epileptic seizures.
Lumbar puncture (LP) allows a doctor to sample the cerebrospinal fluid
(CSF), that surrounds the brain and spinal cord. CSF is produced within
the brain and flows out at the base of the brain to surround and cushion the brain
and spinal cord. It contains substances essential for normal activity of the
brain but also takes waste products back to blood.
A lumbar puncture involves passing a needle, under local anaesthetic, between two
of the back bones at the base of the spine. In an adult, several tablespoons
full of fluid can be collected safely. Usually blood tests are taken at the same
time as the LP in order to compare blood contents with that in CSF.
Approximately 10% of adult patients suffer a mild headache after a LP. This headache
is typically worse upon being upright and better lying flat. It usually improves
with simple pain killers and good hydration.
A variety of laboratory tests are performed on CSF. Some tests give results within
hours whereas others often take days.
Immediate tests performed on CSF include analysis under the microscope to assess
the number and type of white blood cells present. A raised number of white
blood cells in CSF are indicative of inflammation within the brain, spinal cord,
or lining of the brain (meninges). The types of white blood cell found help differentiate
between viral or bacterial infections.
More complicated tests on CSF include culture for bacteria, antibody studies, or
molecular tests to detect the genetic footprint of viruses or bacteria. The
latter molecular tests include the polymerase chain reaction (PCR), which
when applied to CSF is used most frequently to identify genetic material from
herpes simplex virus, varicella zoster virus (chickenpox virus), and enteroviruses.
PCR has significantly advanced the ability to diagnose viral encephalitis.
LP may have to be repeated during an individual's illness.
Laboratory screening of blood, urine, as well as other body fluids can help
detect and identify brain and/or spinal cord infection. Results from these tests
can help exclude other diseases that mimic encephalitis.
The cause of the infection cannot always be determined. Research suggests
that despite extensive testing, the infectious causes of encephalitis cannot be
determined in approximately 60% of cases.
For some individuals, their diagnosis of encephalitis will be diagnosis of “exclusion”
based upon other diseases having been dismissed on the basis of laboratory tests.
In most cases the type, either infectious or autoimmune/post-infectious will have
been determined.
Some people may have a descriptive diagnosis based on:
- The cause of the infection – if known. Examples include herpes simplex
virus, West Nile virus, enterovirus, or Japanese encephalitis virus.
- The area of the brain affected. Examples include brain-stem encephalitis or
limbic encephalitis.
- The type of inflammation, when not caused directly by an infection. Examples
include Rasmussen’s encephalitis Hashimoto’s encephalitis, or Acute
Disseminated Encephalomyelitis.
Additional information about types of encephalitis can be found on the Encephalitis
Society web site or requested in print form.
If a doctor suspects a diagnosis of encephalitis , they are legally obliged to report
this to the local Health Protection Unit of the Health Protection Agency (details
can be found through www.hpa.org.uk/lars_hpus.htm