[Neurological syndromes, encephalitis]

Abstract

The remote effects of malignant tumors in most cases of paraneoplastic neurological syndromes(PNS)are mediated by autoimmune processes against antigens shared by the tumor cells and the nervous tissue(onconeural antigens). Onconeural (or paraneoplastic)antibodies are broadly categorized into two groups according to the location of the corresponding onconeural antigens, inside or on the surface of neurons. Antibodies established as clinically relevant diagnostic markers for PNS are designated as well-characterized onconeural antibodies (or classical antibodies)that target intracellular antigens(Hu, Yo, Ri, CV2/CRMP5,Ma2, and amphiphysin). They also serve as useful markers in detecting primary tumors. Recent identification of new antibodies as markers of subtypes of limbic encephalitis has also expanded the concept of autoimmune limbic encephalitis. These autoantibodies are directed to neuronal cell-surface antigens including neurotransmitter receptors(NMDA, AMPA, and GABAB receptors)and ion channels(VGKC). They are less frequently associated with cancer, so that they cannot be used as specific markers for PNS. Autoimmune limbic encephalitis with anti-neuronal cell surface antobodies and paraneoplastic limbic encephalitis with classical antibodies overlap in some clinical features but are pathophysiologically distinct. Classical antibodies are not simple tumor markers. They seem to be closely related to the disease mechanisms because specific intrathecal synthesis has been shown in PNS patients. However, attempts to produce an animal model of PNS by passive transfer of these antibodies have been unsuccessful, and there is no direct evidence demonstrating the pathogenic role of classical antibodies. Instead, some circumstantial evidence, including pathological studies showing extensive infiltrates of T cells in the CNS of the patients, supports the hypothesis that cytotoxic-T cell mechanisms cause irreversible neuronal damage. On the other hand, humoral immune response is probably the principal mechanism in autoimmune encephalitis associated with antibodies against neuronal cell-surface antigens. Those antibodies are supposed to mediate neural dysfunction which may be reversed by immunosuppression therapy, while the exact mechanism remains to be elucidated. Further accumulation of the cases and longer observation would be necessary to delineate the clinical spectrum of each type of newly-identified autoimmune limbic encephalitis. Early diagnosis and optimal oncological treatment is a prerequisite for better prognosis of PNS patients. Detection of the primary tumor at very early stages including carcinoma in situ is a challenging issue. Optimization of immunosuppression/ immunomodulation therapy for each patient according to the underlying pathophysiological processes is another important clinical issue.