Olfactory vector hypothesis for encephalitis lethargica.

Viruses have long been implicated in the pathogenesis of classical encephalitis lethargica, which was first described by Constantin von Economo in 1917. In this article, I propose the hypothesis that an airborne virus travels along the olfactory conduit to infect the olfactory bulb; this local infection or induced neuroinflammation, in turn, retrogradely targets certain neuronal populations with sleep-wake regulatory functions in the hypothalamus and midbrain, leading to the development of wakeful inactivity, a hallmark clinical feature of the disease. Furthermore, the olfactory vector hypothesis may also explain the pathomechanism of the debilitating complication of the disease, i.e., postencephalitic parkinsonism, in terms of a recently discovered nigro-olfactory projection.