Tests and diagnosisBy Mayo Clinic staff
Questions about symptoms, risk factors and medical history are important in making a diagnosis of encephalitis. Diagnostic tests that may be needed include the following:
- Brain imaging. Brain imaging is often the first test if symptoms and patient history suggest the possibility of encephalitis. The images may reveal swelling of the brain or another condition that may be causing the symptoms, such as a tumor. Technologies may include magnetic resonance imaging (MRI), which can produce detailed cross-sectional and 3-D images of the brain, or computerized tomography (CT), which produces cross-sectional images.
- Spinal tap (lumbar puncture). With a spinal tap, the doctor inserts a needle into the lower back to extract cerebrospinal fluid (CSF), the protective fluid that surrounds the brain and spinal column. A particular profile of blood cells and immune system proteins can indicate the presence of infection and inflammation in the brain. In some cases, samples of CSF can be tested in a laboratory to identify the causative virus or other infectious agent.
- Other lab tests. Your doctor or other members of the care team may take samples of blood, urine, or excretions from the back of the throat. These can be tested in the laboratory to identify some of the viruses or other infectious agents that can cause encephalitis.
- Electroencephalogram (EEG). Your doctor may order an electroencephalogram (EEG), a test in which a series of electrodes are affixed to the scalp. The EEG records the electrical activity of the brain. Certain abnormal patterns in this activity may be consistent with a diagnosis of encephalitis.
- Brain biopsy. Rarely, a procedure to remove a small sample of brain tissue (brain biopsy) is used if symptoms are worsening, treatments are having no effect, and there is no working diagnosis.
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