Encephalitis: Viral - Diagnosis

Diagnosis

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:

• Arboviruses or other viruses that can only be managed by relieving symptoms
• Herpes simplex or other conditions that are potentially treatable

Imaging Techniques

If encephalitis is suspected, a scanning technique is often the first diagnostic step. Computerized tomography (CT) scan or magnetic resonance imaging (MRI) scans may show the extent of the inflammation in the brain and help differentiate encephalitis from other conditions. MRI 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.

Advanced high-tech scanning techniques including single photon emission computed tomography (SPECT) and 18F-fluorodeoxyglucose positron emission tomography (PET) may prove helpful in some cases of encephalitis.

Cerebrospinal Fluid Tests

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. In equine encephalitis, white blood cell count and protein levels are usually elevated (very high in the Eastern variant) and glucose levels are normal. The changes detected in the cerebrospinal fluid of the Western forms are less severe. While cerebrospinal fluid tests may help diagnose encephalitis, they cannot provide information on how severe the disease will be. A 2006 study of patients who had West Nile virus encephalitis and meningitis found that cerebrospinal fluid tests did not help predict disease outcome.

Standard Laboratory Tests

Blood tests may be used to determine blood counts, blood sugar levels, liver function, and blood clotting factors. Urine samples may also be taken.

Serum Antibody Tests for Acute Viral Encephalitis

An arbovirus can be isolated only within the first 2 - 4 days of the attack, so methods for directly detecting it in blood or cerebrospinal fluid samples are rarely successful.

In surviving patients, tests may be conducted using a clear fluid called serum, which can be derived from blood, cerebrospinal fluid, or other fluids. Such tests may identify many of the viruses by detecting specific antibodies against them in the serum. Antibodies are factors produced by the immune system to fight specific organisms and other infections. They remain in the system after the organism or infection has gone.