A historical analysis of the relationship between encephalitis lethargica and postencephalitic parkinsonism: a complex rather than a direct relationship.

Postencephalitic parkinsonism has been considered unique among disorders with parkinsonian features because it is believed to have a unitary etiology associated with the virus that presumably caused encephalitis lethargica. Careful analysis of the historical record, however, suggests that this relationship is more complex than commonly perceived. In most cases, the diagnosis of acute encephalitis lethargica was made post hoc, and virtually any catarrh-like illness was considered to have represented encephalitis lethargica, often after an oral history-taking that was undoubtedly subject to patient recall and physician bias. Also, postencephalitic parkinsonism and oculogyric crises were not recognized as sequelae to encephalitis lethargica until well after other sequelae such as movement disorders and mental disturbances had been identified (see previous paper). We suggest here that the relationship between encephalitis lethargica and postencephalitic parkinsonism is not simplistic, i.e., encephalitis lethargica was not solely responsible for the etiology of postencephalitic parkinsonism, thus aligning the latter with most other parkinsonian disorders that are now believed to have multiple causes.