West Nile virus
In many cases, the symptoms of encephalitis are too similar to aid the doctor in differentiating among the many causes of brain inflammation. The primary objective in diagnosing viral encephalitis is to determine if it is caused by:
If the doctor suspects encephalitis, a scanning technique is often the first diagnostic step. Computerized tomography (CT) or magnetic resonance imaging (MRI) scans can show the extent of the inflammation in the brain and help differentiate encephalitis from other conditions. MRI are recommended over CT scans because they can detect injuries in parts of the brain that suggest infection with herpes virus at the onset of the disease, while CT scans cannot.
Electroencephalogram (EEG), which records brain waves, may reveal abnormalities in the temporal lobe that are indicative of herpes simplex encephalitis.
When encephalitis is suspected, a sample of cerebrospinal fluid is taken using a lumbar puncture, which involves inserting a needle between two vertebrae in the patient's lower back. The sample is taken to count white blood cells and identify specific blood cell types, to measure proteins and blood sugar levels, and to determine spinal fluid pressure. Doctors use cerebrospinal fluid to test for herpes simplex virus, Epstein-Barr virus, varicella-zoster virus, enteroviruses, and to look for the presence of antibodies to the West Nile virus. While cerebrospinal fluid tests may help diagnose encephalitis, they cannot provide information on how severe the disease will be.
Blood tests are used to test for West Nile virus and other arbovirus infections.
If necessary, tiny samples of brain tissue are surgically removed for examination and testing for the presence of the virus. Tissue is prepared using staining techniques and then viewed under an electron microscope. In a few cases, the viruses in brain cells are able to be cultured; that is, the viruses can actually be made to replicate in samples. A brain biopsy is the gold standard for diagnosing rabies.
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