Understanding Encephalitis Lethargica: A Historical Medical Mystery

In this article, you’ll discover some intriguing insights:

• Encephalitis lethargica (EL) is a puzzling neurological condition that affected millions between 1917 and 1930, resulting in various symptoms—like excessive sleeping, immobility, and in some cases, death.
• Surprisingly, even after extensive research over the past century, experts still can’t pinpoint the exact cause of the disease, nor can they predict whether it might resurface.
• Although the disease coexisted with the Spanish Influenza pandemic, which raises some questions about a possible connection, modern studies have explored other potential causes, including enteroviruses and autoimmune disorders.

Just over 100 years ago, the world was grappling with two pandemics, one well-known, the other, not so much. The Spanish Influenza (which actually began in the U.S.) swept the globe from 1918 to 1920, claiming over 50 million lives and affecting around one-fifth of the global population. Yet, during this chaotic period, a lesser-known but equally curious illness was on the rise: encephalitis lethargica, also termed “sleeping sickness.” While it didn’t have the same lethality as the flu, it still led to approximately 500,000 deaths and severely affected even more individuals.

After the EL pandemic faded from public memory, scientists classified it as one of the 20th century’s biggest medical enigmas. More than a century after Austrian physician Constantin von Economo first identified it, questions about its causes, modes of transmission, and whether a similar outbreak could occur again remain unanswered.

One of the more perplexing aspects of the EL outbreak was its inconsistent progression among various populations. In some cases, patients succumbed quickly; in others, they experienced prolonged illness. Hypersomnia was often a symptom, but some exhibited heightened activity. This variability is highlighted by von Economo’s observations early in the outbreak.

He noted, “We are dealing with a kind of sleeping sickness, having an unusually prolonged course,” describing an initial onset marked by headaches and malaise followed by a state of extreme sleepiness—or sometimes, an active delirium. This state could quickly lead to death or linger for an extended period.

Von Economo’s extensive work on EL laid the groundwork for a century of research into this complex disease. According to the American Society of Microbiology, EL can be divided into two main phases. The acute phase presents flu-like symptoms followed by profound sleepiness or, occasionally, mania. If a patient survived this, a chronic phase often featured symptoms reminiscent of Parkinson’s disease. About 50 percent of survivors experienced lasting effects, including changes in personality or psychosis.

A prominent theory linking EL to the Spanish Flu considers their simultaneous appearances and geographical overlap. Interestingly, reports of encephalitis also arose from an influenza epidemic in the late 19th century, exhibiting neurological symptoms akin to those of EL.

Yet, EL persisted for nearly a decade after the Spanish Flu vanished. Subsequent studies of historical brain samples from EL patients haven’t solidified a clearer connection between the two illnesses. A 2012 study proposed that the culprit might be an enterovirus, which is part of a family of RNA viruses including poliovirus, known to alter muscle function. Other hypotheses suggest an autoimmune disorder might be involved, but the widespread nature of the disease complicates that idea.

Despite the continued uncertainty around its origins, the repercussions of EL were severe. The final known survivor, Philip Leather, died in 2002 at 82, having spent most of his life in a mental hospital. Today, the disease is nearly forgotten, with just 80 reported cases in the last 85 years, according to the American Society for Microbiology.

This enigmatic disease is largely overlooked, but the question remains: will it continue to be a historical curiosity— or could it unexpectedly reemerge?