Encephalitis 

What is encephalitis?

Encephalitis is inflammation of the active tissues of the brain caused by an infection or an autoimmune response. The inflammation causes the brain to swell, which can lead to headache, stiff neck, sensitivity to light, mental confusion and seizures.

Encephalitis strikes 10–15 people per 100,000 each year, with more than 250,000 patients diagnosed in the last decade alone in the U.S. The condition can affect anyone, but more often occurs in younger people.

Encephalitis Causes and Types

Encephalitis can be caused by infections or autoimmune conditions where the body’s own immune responses attack the brain. Even with extensive testing, the specific cause of encephalitis remains unknown in about 30%–40% of cases.

Cases of encephalitis are likely to continue to rise as more and more autoimmune causes are recognized. Emerging infections such as Zika, chikungunya and Powassan viruses can also contribute to this trend.

Infectious Encephalitis

Infectious encephalitis is typically caused by a viral infection. Vaccines for measles, mumps, rubella and chickenpox have reduced the rate of encephalitis from these diseases, but other viruses can cause encephalitis. The most common causes of viral encephalitis are herpes simplex virus types 1 and 2, varicella zoster virus and enteroviruses, which cause gastrointestinal illness.

Encephalitis can also result from certain viruses carried by mosquitoes, ticks and other insects or animals such as:

• West Nile virus
• Japanese encephalitis virus
• La Crosse virus
• St. Louis virus
• Equine viruses
• Powassan virus
• Zika
• Chikungunya

Other infectious microorganisms such as bacteria, fungi and parasites can also cause encephalitis, though rarely.

Autoimmune Encephalitis

Autoimmune encephalitis occurs when a person’s own antibodies or immune cells attack the brain. Antibodies may target specific proteins or receptors in the brain, which determine the type of autoimmune encephalitis:

• In anti-NMDA receptor encephalitis, the immune system targets the NMDA receptors in the brain.
• In VGKC-complex antibody encephalitis, the immune system targets the VGKC brain protein complex that includes subtypes LGI-1 and CASPR2.
• GABA-A and GABA-B receptors may also be targeted.

While the causes of autoimmune encephalitis are not well understood, it can sometimes result from a tumor (benign or cancerous). Some types of autoimmune encephalitis such as acute disseminated encephalomyelitis (ADEM) are typically triggered by an infection (post-infectious encephalitis).

Encephalitis Symptoms

Acute encephalitis shows up with an onset of symptoms that get worse over the course of days to weeks. Infectious encephalitis often starts with flu-like symptoms or headache and evolves to altered mental status and problems with thinking, remembering and reasoning. Autoimmune encephalitis typically progresses over the course of weeks.

Symptoms of encephalitis vary according to the area of the brain affected and, in autoimmune encephalitis, depending on the associated antibody.

Physical Symptoms

• Fever
• Seizures
• Headache
• Movement disorders
• Sensitivity to light
• Sensitivity to sound
• Neck stiffness
• Loss of consciousness

In severe cases, encephalitis symptoms may include:

• Weakness or partial paralysis in the arms and legs
• Double vision
• Impairment of speech or hearing
• Coma

Cognitive Symptoms

• Excessive sleepiness
• Confusion and disorientation
• Irritability
• Anxiety
• Psychosis
• Hallucinations
• Memory loss
• Other behavioral changes
• Cognitive impairment