1.1. Background

• Encephalitis indicates inflammation of the brain. Meningoencephalitis signifies inflammation of both the brain and meninges. Myelitis is inflammation of spinal cord.
• Clinically the terms are useful to differentiate that area which is most affected by the pathologic process.
• Any of the viruses associated with encephalitis can be among those which cause aseptic meningitis, however, there are a group of viruses (arboviruses) which cause epidemic encephalitis. In any epidemic, however, one should recognize that there will be a wide spectrum of disease ranging from asymptomatic infection to aseptic meningitis to frank encephalitis.

1.2. Epidemiology

• Arboviruses tend to predominate and epidemic disease occurs almost yearly in the US. Certain arboviruses have a predilection for different geographic areas of the US depending on the climate, mosquito vector and the virus itself.
• A comparison of the different arboviruses can be found in the accompanying table.
• Herpes simplex virus is the most important cause of sporadic fatal encephalitis. Recognition is critical since this infection is a treatable form of viral encephalitis.
• Many other viruses are capable of causing acute encephalitis including other members of the Herpes virus family, enteroviruses, mumps, LCM, adenovirus, and postvaccinal.

1.3. Clinical Manifestations

• Meningoencephalitis usually begins with the acute onset of fever, headache, vomiting and stiff neck. Encephalitis is suggested by drowsiness, paralysis, coma, convulsions, ataxia, psychosis, and papilledema.
• The electroencephalogram is usually diffusely abnormal with arboviral encephalitis. The CSF may vary between several hundred and several thousand white blood cells. Generally there is a lymphocyte predominance but Eastern Equine Encephalitis may have a polymorphonuclear predominance, especially in the early stages. The protein is usually elevated to 300-400 mg per dl. The glucose is usually normal (but may be depressed with H. simplex).
• Herpes simplex encephalitis frequently has a temporal lobe focus on EEG, brain scan, CAT scan, MRI or arteriogram. The CSF frequently shows several thousand red blood cells in addition to a mildly elevated WBC. The glucose may be slightly depressed.
• Because viral encephalitis involves the brain diffusely, such processes like secretion of inappropriate ADH may occur with resultant hyponatremia.

1.4. Diagnosis

• Presence of HSV DNA in CSF detected by PCR is best means to make a diagnosis of HSV encephalitis. Arboviral infection detected by serum antibody of CSF, culture of CSF or serum antibody. Enteroviruses can be cultured from CSF, stool, and throat wash.

1.5. Differential Diagnosis

• The differential diagnosis requires the exclusion of tumors, vascular malformations, a fungal or tuberculous process, toxoplasmosis, brain abscess, bacterial meningitis with an encephalitic component, Lyme disease, syphilis, or lead intoxication.

1.6. Prognosis

• In contrast to aseptic meningitis the morbidity and mortality for encephalitis is considerable. The prognosis depends somewhat on the specific causative agent. For example, the overall mortality for arboviral encephalitis in the United States between 1955 and 1971 was 50% for Eastern Equine Encephalitis (EEE), 8% for St. Louis Encephalitis (SLE), 2.9% for Western Equine (WE) and 0.4% for California (CE). include source?
• The mortality for untreated Herpes simplex encephalitis is 70%.With current therapy, acyclovir, the mortality can be reduced to about 15-20% but there may be considerable morbidity with long term neurologic sequelae, especially if the onset of treatment is delayed.
• In children with WEE who were < 1 year of age at the onset of infection, 57% developed either mental retardation, paralysis, seizures, spasticity, speech difficulties, hearing defects.
• The most treatable form of viral encephalitis is Herpes simplex.
• There has been the introduction of a new course of viral encephalitis into US from Middle East-West Nile Encephalitis. It is a flavivious like St. Louis Encephalitis, Japanese Encephalitis and Dengue. Clinical manifestations similar to other viral encephalitides except profound weakness, lower motor neuron symptoms common. Most severe illness in elderly. Expanding pool of virus activity throughout US over 90 deaths last year.

6.1. Readings