Diagnosis

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 Ma gnetic 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).

Last modified: 18 March 2008