Encephalitis

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 protozoal infestations, like by toxoplasma, 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.

Features
Patients with encephalitis suffer from fever, headache, photophobia. The patients could also suffer from weakness, seizure, and less commonly, stiffness of the neck. Rarely, the patients may have stiffness of the limbs, slowness in movement and clumsiness, depending on the specific part of the brain involved. The symptoms of encephalitis are caused by the brain's defense mechanisms activating to get rid of the infection.

Etiology
Victims are usually exposed to viruses resulting in encephalitis by insect bites or food and drink. The most frequently encountered agents are herpes simplex viruses, arboviruses (carried by arthropods such as mosquitoes or ticks, see also tick-borne meningoencephalitis) and enteroviruses (coxsackievirus, poliovirus and echovirus). Some of the less frequent agents are measles, rabies, mumps, and varicella. Incidentally type 3 Lyssavirus (Mokola virus), found in Australia, causes a lethal encephalitis which hardly resembles rabies. Numerically, the most important cause of encephalitis worldwide is probably Japanese encephalitis, as it produces 50,000 cases a year, with about 15,000 deaths. Japanese encephalitis affects East and Southeast China, Korea, Japan, Taiwan, Southeast Asia, Papua New Guinea, South Asia and even Northern Australia. The most widespread cause of encephalitis worldwide, however, is Herpes Simplex encephalitis. The herpes simplex virus causes inflammation on the temporal lobe of the brain, and if not treated, half to three quarters of the patients succumb to the disease. In very young children, however, the virus could affect any part of the brain, even sparing the temporal lobe.

An interesting cause of viral encephalitis is the Nipah virus. It was first discovered in Malaysia in 1998. A virus from the same genus, the Hendra virus, was discovered in September 1994 in northern Australia when 13 horses and their owner died.

Diagnosis
Adult patients with encephalitis present with acute onset of fever, headache, confusion, and sometimes seizures. Younger children or infants may present with irritability, anorexia and fever.

Neurological examinations usually reveal a drowsy or confused patient. Stiff neck, due to the irritation of the meninges covering the brain, indicates that the patient has either meningitis or meningoencephalitis. Examination of the cerebrospinal fluid obtained by a lumbar puncture procedure usually reveals increased amounts of protein and white blood cells with normal glucose, though in a significant percentage of patients, the cerebrospinal fluid may be normal. CT scan often is not helpful, as cerebral abscess is uncommon. Cerebral abscess is more common in patients with meningitis than encephalitis. Bleeding is also uncommon except in patients with herpes simplex type 1 encephalitis. Magnetic resonance imaging offers better resolution. In patients with herpes simplex encephalitis, electroencephalograph may showed sharp waves in one or both of the temporal lobes. Lumbar puncture procedure is performed only after the possibility of prominent brain swelling is excluded by a CT scan examination. 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.

Treatment
Treatment is usually symptomatic. Reliably tested specific antiviral agents are available only for a few viral agents (e.g. acyclovir for herpes encephalitis) and are used with limited success for most infection except herpes simplex encephalitis. In patients who are very sick, supportive treatment, such as mechanical ventilation, is equally important.

Encephalitis lethargica
Encephalitis lethargica is an atypical form of encephalitis which caused an epidemic from 1917 to 1928. There have only been a small number of isolated cases since, though in recent years a few patients have shown very similar symptoms. The cause is now thought to be either a bacterial agent or an autoimmune response following infection.

Limbic System Encephalitis
In a small number of cases, the pathogens responsible for encephalitis attack primarily the limbic system (a collection of structures at the base of the brain responsible for basic autonomic functions), often causing memory deficits similar to those observed in Alzheimer's disease or Creutzfeldt-Jakob disease (although limbic encephalitis does not cause the same cognitive deficits as these diseases.

Paraneoplastic limbic encephalitis (PLE) is a particularly severe form of limbic encephalitis caused by neoplasms most commonly associated with small cell lung carcinoma. Whereas the majority of encephalitides are viral in nature, PLE is associated with cancer.