Weygandt's On the Mixed States of Manic-Depressive Insanity: a translation and commentary on its significance in the evolution of the concept of bipolar disorder.

Wilhelm Weygandt's Uber die Mischzustände des manisch-depressiven Irreseins (On the Mixed States of Manic-Depressive Insanity) describes and conceptualizes mixed states of mood, behavior, and thinking commonly found in manic-depressive disorders. These ideas emerged from Weygandt's service in the 1890s at the Psychiatric Clinic of the University of Heidelberg, directed by Emil Kraepelin. In the sixth (1899) edition of Kraepelin's influential textbook, the concept of manic-depressive illnesses underwent a fundamental shift from a complex group of syndromal subtypes to a single integrated disorder, widely known from the 1921 English translation of the eighth (1920) edition. In the 1899 edition, Kraepelin acknowledged Weygandt for a new section on mixed manic-depressive states within the new integrated view of manic-depressive disorder. We provide biographical notes on Weygandt, a little-known but historically important figure, as well as the first English translation of his monograph and interpretive summaries of his findings. We also consider whether Weygandt's important insight that the same person could be both manic and depressed not only at different times but even at the same time served as an important stimulus to Kraepelin's unified manic-depressive disorder concept, which survives as bipolar disorder a century later.