Limbic encephalitis with acute onset and Hu antibodies treated with rituximab: Paraneoplastic or non-paraneoplastic disorder?

Paraneoplastic limbic encephalitis (PLE) associated with Hu antibodies is a rare autoimmune disorder usually characterized by subacute onset of slowly progressive neurocognitive symptoms. Small cell lung carcinoma is the most frequent PLE-associated cancer, which negatively affects the prognosis of the disease. We report on a patient with acute onset of confusional state and disorganized speech. Cerebrospinal fluid analysis and brain MRI temporal lesions corroborated the diagnostic suspects toward infectious or autoimmune encephalitis but testing for onconeural antibodies suggested the alternative diagnosis of PLE, in the absence of cancer (total-body CT and PET were negative). The patient's serum was positive for Hu antibodies, thus leading to a diagnosis of PLE. First line immunotherapies were ineffective on the neurocognitive symptoms, which improved after rituximab. Six months later, a retropharyngeal peri-jugular mass was histopathologically diagnosed as a metastasis of lung neuroendocrine tumor. Still clinically improved, the patient died from the oncological disease-related complications. Testing for onconeural antibodies should be considered in patients with clinico-radiological features of acute infectious or autoimmune encephalitis.