The borderland with neurasthenia ('functional syndromes').

At the end of the 19th century, neurasthenia and hysteria were considered distinct diseases. Specifically, neurasthenia was regarded as a disease of the body, whereas hysteria was regarded as a disease of the psyche. However, immediately before World War I, due to their common characteristics, both hysteria and neurasthenia were thought to be 'functional diseases'. Moreover, it was suggested that heredity and the presence in both of the predisposing condition called 'nervous weakness' were other shared factors. Nervous weakness was considered essential for the definition of neurasthenia, but it was also considered a precondition for the development of hysteria. Because of this, it is still difficult to demarcate a line between neurasthenia and hysteria; therefore, the two diseases should be considered as sharing a common borderland with each other.