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Encephalitis: Viral


An in-depth report on the causes, diagnosis, treatment, and prevention of encephalitis.


In many cases, the symptoms of encephalitis are too similar to aid the physician 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 viral agents 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 patients lower back. The sample is taken to count white blood cells and identify specific blood cell types, to measure proteins and glucose levels (blood sugar), and to determine spinal fluid pressure. In equine arbovirus 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. Exceptions occur in immunocompromised patients, who may have normal white blood cell counts.

Standard Laboratory Tests

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

Serum Antibody Tests for Acute Viral Encephalitis

An arbovirus can be isolated only within the first two to four 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 microscopic foreign agents. They remain in the system after the organism or agent has gone.

Tests for antibodies are only useful in patients who survive the acute attack. They must be performed at the following times:

  • The first test is taken when the symptoms first occur. At that time, antibody levels are low or absent. Antibodies rise over time.
  • The next test is taken during convalescence (about three to five weeks later). For a diagnosis of the virus, antibody levels at that time should be at least four times the initial measurement.

In the case of West Nile Virus, a 2003 study suggested that antibodies may persist a year or more after an initial infection.

Polymerase Chain Reaction (PCR)

A test called the polymerase chain reaction (PCR) is a rapid diagnostic test that is used to detect some viruses:

  • PCR uses blood, spinal fluid, or any other tissue specimen that may contain the virus.
  • It replicates a piece of the viral DNA millions of times until the virus is detectable, usually within two or three hours.
  • Sensitivity is almost equal to viral culture and results are much quicker.

PCR and Herpes Simplex Encephalitis. PCR analysis of spinal fluid is particularly important in diagnosing herpes simplex virus encephalitis. It has allowed the identification of even mild and unusual forms, allowing rapid, life-saving treatments. PCR may not always be effective in diagnosing suspected herpes simplex virus encephalitis in infants and should not be used as the sole test in this age group. PCR is also proving to useful in monitoring treatment in these patients.

PCR and Varicella-Zoster Virus (VZV). PCR is also useful for identifying the VZV virus (which is responsible for chicken pox and shingles).

PCR and Arboviruses. PCR may be less useful for identifying arboviruses, although methods are being developed for identifying many of the common ones. One, for example, can detect West Nile virus within five hours.

Brain Biopsy

If possible, samples of brain tissue are removed for examination and testing for the presence of the virus. Tissue is examined using staining techniques and then viewed under an electron microscope. In a few cases, brain cells are able to be cultured; that is, the viruses can actually be made to replicate in samples. It is the gold standard for diagnosing rabies.

Virus Culture

A viral culture uses specimens taken from various tissue samples. Samples may be taken of blood, spinal fluid, or from the blisters of infections caused by varicella zoster or herpes, or throat washing for those caused by cytomegalovirus or Epstein-Barr virus. Samples are sent to a laboratory where it takes between one and fourteen days to detect the virus in the preparation made from the specimen. A culture is also sometimes used in vaccinated patients to determine if encephalitis may be caused by a natural virus or by the vaccine.

Ruling Out Other Conditions with Similar Symptoms

It is very important to differentiate viral encephalitis from other disorders that may cause similar neurological symptoms and which may be treatable.

Other Viral Infections of the Central Nervous System. The signs and symptoms for meningitis (infection in the meninges, the membrane around the brain), for meningoencephalitis (viral infection in the brain and meninges), and for encephalomyelitis (infection in the brain and spinal cord) are very similar to encephalitis. Viral meningitis may have the following symptoms:

  • It is self limiting, typically causing only headache, sensitivity to light, and a stiff neck.
  • It also may cause fever, loss of appetite, nausea and vomiting, and fatigue.
  • Viral meningitis does not cause altered consciousness, confusion, or drowsiness, as encephalitis does.

Bacterial Infections in the Brain. Bacterial encephalitis may occur in Legionnaire disease or mycoplasma infection. Bacterial meningitis may cause similar symptoms and is also very serious.

Infections from Other Organisms. The parasitic infection toxoplasmosis and the tick-borne diseases Rocky Mountain spotted fever and ehrlichiosis can cause neurological symptoms. Amebic diseases that cause neurological damage include naegleria (found in soil and warm stagnant water during the summer months) and acanthamoeba (also found in soil and water as well as contaminated contact lens solution).

Stroke. Sometimes encephalitis symptoms can resemble those of stroke. (This is common with older patients with certain cases encephalitis caused by the varicella-zoster virus.) In such cases, diagnostic tests for stroke are needed. [SeeWell-Connected Report #45 Stroke.]


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