Abstract: The present invention relates to a method for diagnosing a disease, comprising the step detecting in a sample an autoantibody binding to the alpha 3 subunit of the human neuronal Na(+)/K(+) ATPase, an isolated and/or recombinant polypeptide comprising the alpha 3 subunit of the human neuronal Na(+)/K(+) ATPase, a use of a membrane-associated human Na(+)/K(+) ATPase or a variant thereof for the diagnosis of a disease, an isolated and/or recombinant polypeptide comprising the alpha 3 subunit of the human neuronal Na(+)/K(+) ATPase or a variant thereof, for use in the treatment of a disease, an autoantibody binding to said polypeptide and a method for isolating such autoantibody, a pharmaceutical composition, medical or diagnostic device or test kit comprising the polypeptide.