Viral encephalitis overview

# Overview
Encephalitis is an acute inflammation of the brain, commonly caused by a viral infection. Sometimes, encephalitis can result from a bacterial infection, such as bacterial meningitis, or it may be a complication of other infectious diseases like rabies (viral) or syphilis (bacterial). Certain parasitic or protozoal infestations, such as toxoplasmosis, malaria, or primary amoebic meningoencephalitis, can also cause encephalitis in people with compromised immune systems. Brain damage occurs as the inflamed brain pushes against the skull, and can lead to death.

# Classification
It can be classified into primary and secondary post-infectious encephalitis (immune reaction to infection).

# Causes
It is an acute inflammation of the brain, commonly caused by a viral infection. Sometimes, encephalitis can result from a bacterial infection, such as bacterial meningitis, or it may be a complication of other infectious diseases like rabies (viral) or syphilis(bacterial). Certain parasitic or protozoal infestations, such as toxoplasmosis, malaria, or primary amoebic meningoencephalitis, can also cause encephalitis in people with compromised immune systems.

# Risk Factors
Increased risk factors include increased age, weakened immune system, certain geographical locations, outdoor activities and some season of the year (summer and fall).

# Natural History, Complications, and Prognosis
Its natural history, complications and prognosis depends on age of patient, immune status, type of organism and time to initiate medical therapy. Thus, depending on these factors it may present with complications like seizures, shock, cranial nerve palsy, and coma.

# Diagnosis

## History and Symptoms
Adult patients with encephalitis may present with acute onset of fever, headache, confusion, weakness and sometimes seizures. Younger children or infants may present with irritability, anorexia and fever. Less commonly, photophobia, stiffness of the neck can occur if meningeal involvement occurs simultaneously with brain involvement. Rarely, patients may present with stiffness of the limbs, slowness in movement and clumsiness, and hallucination depending on which specific part of the brain is involved. The symptoms of encephalitis are caused by the brain's defense mechanisms activating to get rid of the infection.

## Physical Examination
It may present as focal or diffuse neurological manifestations. The signs may include altered mental status, change in personality, meningismus, ataxia, seizures, and cranial nerve involvement. Stiff neck, due to the irritation of the meninges covering the brain, indicates that the patient has either meningitis or meningeoncephalitis.

## Laboratory Findings
Diagnosis is often made with detection of antibodies against specific viral agent (such as herpes simplex virus) or by polymerase chain reaction that amplifies the RNA or DNA of the virus responsible. Other lab tests that might be useful are  complete blood count with differential, coagulation profile, serum electrolyte, urine electrolyte, serum glucose, blood urea nitrogen and serum creatinine levels (to rule out confusion due to dehydration), liver function test (to rule out hepatic involvement), lumbar puncture and CSF examination.

## CT
CT scan should be performed in all patient with suspected encephalitis. It helps in excluding ass lesion, bleeding and infarction that could be the possible cause for the encephalopathic state. It should always be performed before lumber puncture in case manifestations suggestive of increased intra-cranial pressure is there.

## Other Diagnostic Studies
EEG and brain biopsy are other tests that can be done in patients with encephalitis. In patients with herpes simplex encephalitis, electroencephalograph may show sharp waves in one or both of the temporal lobes.

# Treatment

## Medical Therapy
Treatment with acyclovir with or without steroids and antibiotics should be initiated as soon as possible. Antiviral agent like acyclovir has been useful in treatment of encephalitis due to herpes simplex virus and varicella zoster. Treatment for other causative agents of encephalitis is mostly supportive.