Limbic Encephalitis

A 30 yo male with history of OPP poisoning 5 years back since then mild cognitive decline. 

Findings:
Axial FLAIR images show faint hyperintensity involving  mesial temporal lobes.
Bilateral symmetrical involvement.
No restricted diffusion on Dw images.
Similar faint T2 hyperintensity involving bilateral para median thalami.

Imaging diagnosis : Limbic Encephalitis - Toxic. 
Closest DD:
Herpes; bilateral symmetrical involvement, without restricted diffusion, para median thalamic involvement with long term history goes less in favor of Herpes. 

Limbic encephalitis implies any inflammation in the limbic system.
The brain can be divided into three parts:
1. Brain stem, the most primitive part, plays a vital role in basic attention, arousal and consciousness. All information pass to and fro passes through the brain stem. The brain stem in houses vital centers for respiration, heart , sleep wake cycle, temperature control.
2. Limbic system, is wrapped around the brain stem. It includes the hippocampus, thalamus, hypothalamus and amygdala; involved in memory and much of the behaviour related to sex, hormones, food, fight or flight responses, the perception of pleasure and competition, higher emotions including the protection of the young and feelings such as love, sadness and jealousy.
3. Neocortex, the third and major part of the brain, provides logic and thought,  processes such as speaking, planning and writing.

It is therefore the Limbic Encephalitis is an anatomical diagnosis and not the etiological diagnosis. Not all cases share the same cause or very obvious cause every time.

As per the cause Limbic Encephalitis can divided into 3 main categories:
1. Infectious encephalitis :  usually a virus, number of viruses appear to target this area, mostly but not always include the herpes simplex.  Some people prefer the diagnosis of “limbic encephalitis” whilst others may be given the diagnosis “herpes simplex encephalitis ” for the same finding.
2. Autoimmune encephalitis :  Antibodies are made by all healthy individuals in response to infections or vaccination. The antibodies attack the infectious agent (such as a virus) and help the body’s immune system to get rid of the infection. Unfortunately, a few of these antibodies may “cross-react” with the patient’s healthy tissue proteins, attacking the tissue and causing an autoimmune disease. The antibody binds to the potassium channels, a protein, present in all brain tissue. This causes a reduction in the number of potassium channels, decreasing the control over electrical signals operating in the brain. Potassium channels particularly common in the hippocampus and other limbic areas of the brain explains affection of autoimmune diseases for limbic system.
There are two forms of autoimmune limbic encephalitis; A) paraneoplastic limbic encephalitis (PLE) and B) Non Paraneoplastic limbic encephalitis (NPLE).
A) Paraneoplastic limbic encephalitis (PLE) : occurs in patients with particular cancers like lung, thymus, the breast or testis.
B) Non-paraneoplastic limbic encephalitis (NPLE): recognised in the last five years only,  these patients who had the symptoms of paraneoplastic limbic encephalitis but who did not have any of the marker paraneoplastic antibodies or never developed a tumour. Moreover, some of these patients got better when treated immune suppressant.
3. Toxic and Metabolic.