A physician will compile a thorough medical history of the child and perform a complete physical examination. The physician is also likely to ask whether the child has recently had a cold or other respiratory illness, gastrointestinal illness or tick bite. In addition, parents will be asked whether the child has been around pets or other animals, or has traveled to certain areas of the country that might be high-risk areas for contracting viruses that can cause encephalitis.

There are several ways in which physicians may test for the presence of encephalitis (inflammation of the brain). Diagnosis methods may include:

• Blood testing. Removal of a sample of blood (usually from a vein) for laboratory analysis. Certain encephalitis types, such as West Nile encephalitis virus, are detectable through blood testing. Analysis of blood may show a rise in antibodies to a virus or other indicators necessary to make a diagnosis.
  
  
• Brain imaging. A CAT scan (computed axial tomography) or MRI (magnetic resonance imaging) that produces images of the brain may reveal swelling consistent with encephalitis. Pressure within the brain can often be observed through brain imaging, providing valuable insight as to the likelihood of encephalitis infection prior to performing a spinal tap.

• Spinal tap (lumbar puncture). Involves inserting a needle directly into the lower spine area to extract cerebrospinal fluid (CSF) for laboratory testing. A spinal tap is a common method used to diagnose encephalitis. Advances in the testing of DNA (the molecule that encodes genetic information) have made it possible to diagnose encephalitis through more detailed examination of CSF. This may reveal the presence of the virus directly or indirectly (e.g., a heightened white blood cell count which indicates the body is fighting infection). Sometimes a spinal tap may be nondiagnostic.
  
  
• Electroencephalography (EEG). A measure of the brain’s electrical activity. Electrodes that monitor electrical impulses of the brain are pasted or otherwise attached to the scalp. The electrodes send the information of the brain’s activity to an EEG machine for recording on a moving sheet of paper. Abnormal readings may indicate the presence of encephalitis, however normal results do not categorically rule out the presence of encephalitis.
  
  
• Brain biopsy. Removal of a sample of brain tissue for laboratory analysis. The patient is anesthetized and a small hole is drilled into the skull, followed by the insertion of a needle into the brain tissue. Computer-imaging techniques such as x-rays or MRI aid in this technique. A brain biopsy may be necessary in rare instances when diagnosis of encephalitis through other methods is not possible. This test is not without risks and is not commonly performed.