Clinical History – 68 years old female presented with acute onset of altered sensorium. No history of fever or seizures.

Imaging findings – Cortical thickening with T2/FLAIR hyperintensity in the right medial temporal lobe which shows increased signal on DWI and no signal loss in ADC–Limbic Encephalitis.

CSF analysis was aseptic. Patient is a known case of CA Breast, post MRM status, hence diagnosis of paraneoplastic limbic encephalitis was made.

Discussion:

The term “autoimmune encephalitis” generally refers to a family of closely related disease processes that share overlapping clinical features and neuroimaging findings but are ultimately differentiated by the specific antibody subtypes driving the underlying immune-mediated attack on different CNS structures.

It is divided into two categories namely paraneoplastic or non-paraneoplastic as per the availability of a fundamental malignancy.

• paraneoplastic limbic encephalitis: Generally antibodies are not favourable to intracellular antigens and there is poor response to immunotherapy.
• non-neoplastic autoimmune limbic encephalitis: antibodies are not in favour of extracellular antigen and are generally present with a reversible neuronal dysfunction and better results.

Paraneoplastic syndromes having a negative impact on the CNS are usually considered to develop in cancer wherein antigens shared by tumor cells and native non-neoplatic neuronal cells end up in an antibody-mediated attack on earlier immune-privileged neuronal structures. The presence of paraneoplastic syndromes is generally observed in small-cell lung cancer. However, it can be noticed of different types of cancers including neuroblastoma, breast cancer, immature ovarian teratomas etc.

There is an obvious preference for antigens in autoimmune encephalitis within the limbic system.

Imaging Findings – MRI

During the initial period of the disease, there may not be any particular findings.The most prevalent location of engagement is the mesial temporal lobes and limbic systems, generally manifested by cortical thickening and raised T2/FLAIR signal intensity of these regions. Bilateral presence is generally seen, although it is mostly asymmetric. The lateral temporal lobe and insula are not mostly engaged, whereas the basal ganglia, in contrast, are commonly engaged. Though it is not much common, fundamentally any part of the central nervous system can be associated. Patchy areas of improvement may be seen after contrast. True diffusion restriction (i.e. low ADC values) and hemorrhage are not common.

T2

FLAIR

DWI                                                                             ADC

Differential Diagnosis

• Herpes simplex encephalitis
o acute, generally dramatic time period
o fever
o psychiatric symptoms are not much present
o bilateral asymmetrical association of the limbic system, medial temporal lobes, insular cortices and inferolateral frontal lobes.
o basal ganglia not considered
o Restricted diffusion is generally found

• Status epilepticus
o acute, generally dramatic time course
o Common areas involved are cerebral cortex and subcortical white matter, hippocampi and mesial temporal lobes, thalamus.

• Tumor
o Low-grade astrocytoma
• Appearances can be pretty same when localized with regard to the temporal lobe
o Gliomatosis cerebri
• diffuse T2 hyperintensity consisting of multiple contiguous lobes
• no preference for the limbic system

Conclusions

Autoimmune encephalitis is an important diagnostic consideration in patients presenting with new onset of altered mental status of unclear etiology. It includes a myriad of clinical conditions that have a common pathophysiology (ie, antibodies directed against CNS structures). The 2 distinct groups (group I, intracellular directed antibodies, and group II, cell-surface directed antibodies) have overlapping clinical and imaging features. Neuroimaging findings will most often involve the limbic structures, but involvement of the striatum, diencephalon, or rhombencephalon can be seen. A subset of patients with autoimmune encephalitis will have no neuroimaging findings despite profound neuropsychiatric dysfunction, but serum antibody testing can still ultimately lead to the diagnosis of autoimmune encephalitis.

Dr. Shabna Jasmin K
Radiologist
Kauvery Hospital