Question:

What is Limbic Encephalitis?

Answer:

The term limbic encephalitis should be restricted to cases in which the clinical features and ancillary studies indicate an isolated or predominant involvement of the limbic system.  ‘Limbic encephalitis’ (LE) describes the condition when limbic areas of the brain are inflamed (swollen) and consequently not functioning properly. The main regions of the limbic system include the hippocampus and amygdala, hypothalamus, cingulate gyrus, limbic cortex). The limbic areas of the brain control many functions including memory, learning, and emotions such as aggression. In addition, some of these limbic areas are susceptible to seizures, which are a common feature of limbic encephalitis.

THE CRITERIA FOR LIMBIC ENCEPHALITIS INCLUDE: 

(1) subacute onset of recent memory loss, seizures, or psychiatric symptoms.

(2) bilateral MRI fluid-attenuated inversion recovery (FLAIR)/T2 abnormalities highly restricted to the medial temporal lobes.

(3) CSF pleocytosis or EEG with epileptic or slow activity involving the temporal lobes.

(4) reasonable exclusion of alternative causes.

Demonstration of neuronal antibodies is not needed for the diagnosis if all the criteria are fulfilled. However, the determination of neuronal antibodies is still important because their presence supports the diagnosis, differentiates between immunological subtypes of limbic encephalitis, helps to predict response to treatment, indicates the likelihood of being paraneoplastic, and if so, guides the tumor search.

SYMPTOMS OF LIMBIC ENCEPHALITIS

The symptoms of LE include memory loss, seizures, confusion, disturbances of sleep, and psychological problems such as altered personality or behavior.

LIMBIC SYSTEM AFFECTED AREAS

Hippocampus:  Plays an important role in emotion, learning, and memory. It is critical for acquiring new memories and consolidating short-into long-term memory

Amygdala:  Plays a role in anger and aggression, emotional control, eating, drinking and sexual behaviors. It is also involved in reactions to stimuli such as fear (crucial for fear memories) and the fight or flight reactions. Influences our motivation and interpretation of non-verbal expressions.

Hypothalamus:  Monitors blood levels of glucose, salt, blood pressure, and hormones. Regulates temperature, sleeping and endocrine system.

Olfactory Cortex:  Receives sensory information from the olfactory bulb and is involved in the identification of odors.

Thalamus:  Relay sensory signals to and from the spinal cord and the cerebrum related to hearing, seeing, touching, and tasting.

CAUSES OF LIMBIC ENCEPHALITIS

Most forms of LE fall into two main categories:

1. Infectious encephalitis – caused by direct invasion of the limbic area of the brain by a bug, usually a virus.

2. Autoimmune encephalitis – caused by the person’s own immune system reacting against parts of the limbic system.

There are broadly two forms of autoimmune limbic encephalitis: paraneoplastic limbic encephalitis (PLE) and non-paraneoplastic limbic encephalitis (NPLE).

Paraneoplastic LIMBIC ENCEPHALITIS (PLE)

Sometimes when the immune system starts to react with the limbic areas, this happens because the person has a tumor in their body which activates the immune system. This activated immune system can, in turn, attack the brain. Doctors call this paraneoplastic limbic encephalitis as the tumor (neoplasm) that affects the brain from a distance, via the immune system.

Non-paraneoplastic limbic encephalitis (NPLE) NPLE has only been clearly recognized in the last few years when doctors began to identify patients with symptoms like those with PLE but who did not have any of the marker paraneoplastic antibodies in their blood and never developed a tumor. NPLE is far more common than PLE. It is becoming increasingly clear that NPLE is caused, at least in part, by specific antibodies in the patient’s blood that targets the patient’s brain, particularly the hippocampus and other limbic areas. Limbic encephalitis is the main manifestation of several of the autoimmune encephalitis syndromes that may occur without a cancer association. Many of these patients improve if they are treated with drugs that suppress the immune system and reduce the levels of the antibodies. These drugs include steroids, intravenous immunoglobulins, and plasma exchange.

Reference: IAES Limbic Encephalitis Fact Sheet