Encephalitis lethargica (EL) is a form of encephalitis which is largely characterized by sleep and movement disorders, and postencephalitic parkinsonism in survivors. It is a rare disease. Notably, between 1917 and 1926, there was an outbreak of EL.
Encephalitis lethargica (EL) is a rare form of encephalitis that garnered attention during an epidemic of the disease between 1917 and 1926. The cause of EL is still not known . It is sometimes referred to as the sleepy sickness or sleeping sickness.
The illness was mainly characterized by lethargy and somnolence, although there were significant variations in presentation. Some patients presented with insomnia, mania, psychosis, or even coma. Patients with EL suffered for years. It was estimated that the disease affected over a million people  .
Common clinical features included movement disorders which in the acute phase of the illness included muscular rigidity, tremor, and abnormal eye movements. Other associated manifestations were fever, visual disturbances, myalgias, weakness, and slow physical and cognitive functioning. In patients that survived the disease, one of the most prominent sequelae was postencephalitic parkinsonism (PEP). PEP could be distinguished from Parkinson's disease (PD) based on an early age of onset and because the associated tremor was not the typical pill-rolling tremor seen in PD. PEP also caused pyramidal signs and respiratory distress.
Since the aforementioned epidemic, there have been a few sporadic cases recorded . Speculations about the etiology of EL have mostly revolved around a possible link with the Spanish influenza virus that caused a pandemic around the same time as EL. Some atypical enteroviruses have also been suspected. EL is diagnosed clinically, although there are brain changes that can be visualized by magnetic resonance imaging (MRI). There were as many as 28 proposed types of the disease, based on clinical presentation and symptomatology .
Although the diagnosis is clinical, there are measurable and observable changes in the nervous system. About half of patients with encephalitis lethargica have lymphocytes in their cerebrospinal fluid (CSF), as well as elevated proteins . Autoantibodies against the basal ganglia cells are found in EL. Often there is evident grey matter inflammation on magnetic resonance imaging (MRI). Nuclear medicine has also been utilized for diagnostic purposes.
EL can be initially diagnosed as a psychiatric or neurological condition such as substance intoxication or epilepsy. Consequently, many patients with EL were regarded as having a psychiatric disorder before the diagnosis of EL became clear.
Literature has proposed certain criteria for the diagnosis of EL and these include acute or subacute encephalitis, flu-like symptoms, hypersomnia, ophthalmoplegia, and psychiatric symptoms. These features should be present with no other illness that could explain their existence.