In 1917, people began falling into unusual states of extreme sleepiness. No warning. No cure. No clear explanation. Encephalitis Lethargica, often described as a form of “sleeping sickness,” swept parts of the world, affecting an estimated one million people and causing hundreds of thousands of deaths. Many survivors were left in a twilight state of neurological impairment. Then the epidemic gradually faded. Nobody knows exactly why.

What is Encephalitis lethargica?

Encephalitis lethargica (EL) is a rare inflammatory brain disease that emerged in the early 20th century. It affected an estimated one million people worldwide. The name Encephalitis lethargica means inflammation of the brain that makes one tired.

Encephalitis lethargica in children

Children who contracted EL were disproportionately affected by psychiatric and behavioural complications. Which leads to change in the personality often dramatic and distressing for families, Healthy children could also develop psychosis. Self-harming behaviour, including self-mutilation, was reported in severe paediatric cases. The severity of symptoms in children highlighted EL’s capacity to fundamentally alter brain function and behaviour.

Prodrome (early warning) symptoms:

•           Fever: often low-grade at onset, mimicking a common viral illness.

•           Sore throat: one of the earliest flu-like symptoms reported by patients.

•           Headache: persistent and often severe, accompanying general malaise.

•           Malaise and fatigue: generalised feeling of illness and exhaustion.

Neurological symptoms:

•           Double vision (diplopia): one of the earliest distinctive neurological signs.

•           Tremors: involuntary shaking that progressed with the illness.

•           Delayed response and slowed cognition: patients appeared mentally sluggish.

•           Drowsiness and lethargy: extreme sleepiness that could last for prolonged periods.

•           Coma: many patients fell into a complete, unresponsive state.

•           Extended unconsciousness: some patients remained comatose for months or even years without awakening.

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The Two Clinical Phases: Acute and Chronic

Phase 1: Acute Encephalitis Lethargica

The acute phase presented in three distinct clinical forms;

1. Somnolent-Ophthalmoplegic Form (Most Common)

•           Extreme sleepiness and prolonged somnolence.

•           Ophthalmoplegia: paralysis or weakness of the eye muscles.

•           Difficulty maintaining wakefulness: patients could fall asleep mid-conversation.

2. Hyperkinetic Form

•           Mania and agitation: hyperactive, restless behaviour.

•           Involuntary twitching and motor disturbances (chorea, myoclonus).

•           Severe insomnia: a complete reversal of the sleepiness seen in other presentations.

3. Akinetic Form

•           Severe muscular rigidity: the body becomes stiff and inflexible.

•           Akinesia: profound loss of voluntary movement.

•           Patients appeared “frozen”, unable to initiate movement on their own.

Phase 2: Chronic Encephalitis Lethargica and Post-Encephalitic Parkinsonism (PEP)

•           The chronic phase could develop years after the initial acute episode, even in patients who appeared to have fully recovered.

•           Post-Encephalitic Parkinsonism (PEP) was the most characteristic chronic manifestation of EL.

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Mysterious disappearance of the disease

The epidemic faded away, leaving more questions than answers. Encephalitis Lethargica had affected millions of lives worldwide and then gradually disappeared in its epidemic form by the late 1920s. The exact cause was never definitively identified, and no specific cure was found.

Scientists continue to study the disease even today, exploring possible infectious and autoimmune triggers and the link to post-encephalitic Parkinsonism. Until we fully understand what triggered it, the medical mystery remains unresolved.