Fludeoxyglucose positron emission tomography-computed tomography in limbic encephalitis

A 66-year-old male patient presented with low grade fever without chills associated with body ache and weakness, irrelevant talk, episodes of anxiety and depression, gradual worsening of memory, and simple partial seizures. The patient's symptoms did not correlate with each other, and his condition seemed to worsen. He was advised whole body positron emission tomography-computed tomography scan. Figure 1 demonstrates increased metabolism in bilateral caudate nuclei and putamen and in anterior and medial aspect of both temporal lobes. Increase in metabolic activity in the limbic system. Figure 2 shows that there was no evidence of active disease anywhere else in the body. This picture along with his symptoms helped us conclude that the findings represent ongoing inflammatory pathology most likely limbic encephalitis. The patient's cerebrospinal fluid analysis was performed, which showed an increase in white blood cell and proteins, which confirmed our suspicion.

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Figure 1 Increased metabolism in bilateral caudate nuclei and putamen with increased uptake in anterior and medial aspect of both temporal lobes. Increase in metabolic activity in the limbic system

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Figure 2 There is no evidence of active disease anywhere else in the body

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The name limbic encephalitis was coined by Corsellis et al.[1] Limbic encephalitis could be caused either by direct invasion of the brain by an infectious agent, usually a virus (infectious encephalitis) or by the persons own immune system reacting against itself (auto-immune encephalitis). Limbic encephalitis is a rare disorder characterized by personality changes, irritability, depression, seizures, memory loss, and sometimes dementia. The diagnosis is difficult because clinical markers are often lacking. There are two forms of autoimmune limbic encephalitis: Paraneoplastic limbic encephalitis (PLE) and non PLE.

In PLE, most individuals will turn out to have cancer of the lung, thymus gland, lymphatics, the breast, or the testis. The condition may improve or at least stabilize if the cancer is detected and treated effectively; but, unfortunately in many cases, the tumor proves difficult to identify or the treatment does not cure the patient's neurological symptoms. Sixty percent of the time, limbic encephalitis is paraneoplastic in origin.[2] In fact, it is thought that up to 1 in 100 people with cancer have PLE which is commonly misdiagnosed as neurological diseases such as Alzheimer's.[3] The anti-Hu antibody is directed against RNA-associated neuronal proteins and is known to cause paraneoplastic encephalomyelitis/sensory neuronopathy syndrome mostly when associated with small cell lung cancer.[4]

Nonparaneoplastic limbic encephalitis has only been clearly recognized in the recent years. It includes patients who had the symptoms of PLE, but who did not have any of the marker paraneoplastic antibodies in their blood and never developed a tumor. Moreover, some of these patients got better if they were treated with drugs that suppress the immune system. The cause of this is believed to be an antibody that binds to a protein, present in all brain tissue: The potassium channel. This causes a reduction in the number of potassium channels, decreasing the control over electrical signals operating in the brain. Potassium channels are proteins that lie in the surrounding membrane of nerve cells in the brain and in the nerves that lead to the muscles of the skeleton, the gut, and the heart.[5] They are particularly common in the hippocampus and other limbic areas of the brain.

In the setting of limbic encephalitis, fluorodeoxyglucose-PET imaging can be of value from the standpoints: (1) Detection of the occult malignant focus (paraneoplastic neurological syndrome) and (2) objective assessment of the presence and extent of the functional abnormality in the brain and correlation of the imaging findings with the clinical features and disease activity.[6]