Lethargica

Lethargica, in medical usage often referred to as encephalitis lethargica, is a historical neurological syndrome characterized by profound sleep disturbance and a range of motor and neuropsychiatric symptoms. The term lethargica reflects the prominent lethargy or hypersomnolence seen during the acute phase of the illness.

The condition was first described in the 1910s and remains associated with an epidemic period from about 1916 to the early 1920s, with cases continuing sporadically afterward. Its exact cause is unknown; various hypotheses have implicated infectious agents or autoimmune mechanisms following a viral illness, particularly in the context of the 1918 influenza pandemic. No single pathogen has been definitively proven to cause encephalitis lethargica.

Clinical presentation varies, but the acute phase typically includes fever, malaise, and marked sleepiness with reduced responsiveness. Neurological features may include oculomotor abnormalities such as impaired vertical gaze, movement disorders (rigidity, tremor, chorea), and autonomic dysfunction. Psychiatric symptoms such as behavioral changes, apathy, or delirium can occur. Many survivors enter a chronic phase with long-term sequelae, most notably postencephalitic parkinsonism, as well as other movement disorders, cognitive impairment, and chronic fatigue.

Diagnosis relies on clinical history and exclusion of other causes, supported by neurological examination and imaging. There is no specific laboratory test. Treatment is supportive, focusing on management of symptoms. In later years, dopaminergic therapy has been used for postencephalitic parkinsonism with variable benefit. The overall prognosis is variable; many individuals recover partially or stabilize, but lasting disabilities are common among long-term survivors.