In recent years, incidence of autoimmune encephalopathies has increased. The diagnosis of the severe form of autoimmune encephalopathy is not difficult; however, milder forms can be misdiagnosed as general encephalopathies. We often treat Hashimoto's encephalopathy, which has diverse clinical symptoms and is often misdiagnosed as a psychosomatic disease. We have found that the neurological findings and symptoms of patients with Hashimoto's encephalopathy are similar to those of psychogenic diseases, such as giveway weakness and atypical sensory disorder. To understand the mechanism underlying these symptoms, we propose a new concept: neurological symptoms of diffuse brain damage. This theory is based on the premise that etiologically, symptoms observed were caused by diffuse, spotty, and shaded brain damage due to autoimmune encephalopathies. We also found similar neurological conditions in patients with anti-ganglionic acetylcholine receptor antibody-related encephalopathy, encephalopathies that developed after injection of the cervical cancer vaccine, and encephalopathies associated with Stiff person syndrome. In conclusion, the clinical features of autoimmune encephalopathy include the "neurological symptoms of diffuse brain damage" as well as the presence of antibodies. We could diagnose autoimmune encephalopathy more easily, using this new diagnostic concept.