Encephalitis - Exams and Tests

Exams and Tests

Your doctor will ask questions about your medical history and your symptoms. It is important to tell your doctor if you have taken any recent trips or have been sick lately. Your doctor may also ask about your sexual history to see if it's possible that you have had herpes simplex virus.

If your doctor thinks that you may have encephalitis, lab tests usually will be done to confirm the diagnosis.

Spinal fluid analysis

An analysis of the fluid in the spine (cerebrospinal fluid) is one of the most important tests in diagnosing encephalitis. Samples are taken during a lumbar puncture (spinal tap), in which a needle is inserted in the lower back between the bones of the spine. The spinal fluid is examined for evidence of infection, such as increases in white blood cells and protein. In the case of herpes simplex encephalitis, health professionals can test for traces of the virus's genetic material. Also, your doctor may do a viral culture to identify the virus causing encephalitis. For a viral culture, a small amount of the fluid is placed in a container with other cells that grow a virus. It may take several weeks before the results of a viral culture are known.

Imaging tests

Magnetic resonance imaging (MRI) of the head may be used to detect specific areas of inflammation or bleeding in the brain caused by encephalitis. However, most people with encephalitis have normal MRIs.

Another imaging test, computed tomography (CT) scan of the head and face, also may be used to see these changes in the brain. A CT scan uses X-rays to take pictures of the brain. See an illustration of a CT scan of encephalitis.

Blood tests

Testing for antibodies in the blood can identify some causes of encephalitis, including mosquito-borne viruses and the viruses that cause mononucleosis (Epstein-Barr virus), cytomegalovirus, and toxoplasmosis.

Electroencephalogram (EEG)

An electroencephalogram (EEG) can help confirm a diagnosis of encephalitis. An EEG records electrical activity in the brain through wires (electrodes) taped to your head and hooked up to a computer. If you have encephalitis, the EEG may show an abnormal increase or decrease in electrical activity; however, an EEG does not indicate whether the abnormal electrical activity is caused by encephalitis.

Brain biopsy

Brain biopsy sometimes may be used to find the cause of encephalitis, especially if herpes simplex encephalitis is suspected and you do not appear to be improving after treatment with acyclovir (an antiviral medication used to treat the herpes simplex virus). An MRI may also help guide the doctor in determining which tissue to biopsy should a biopsy be needed. Using MRI to guide the biopsy needle, the doctor removes a small sample of brain tissue and examines it for viral infection. Brain biopsy is seldom used because tests of blood and spinal fluid usually can accurately diagnose encephalitis caused by the herpes simplex virus.