Narcolepsy doesn't strike at random. After studying the medical records of a large group of Chinese narcoleptics, researchers concluded that their symptoms--sudden naps, constant sleepiness, hallucinations--were most likely to have started in the month of April. In fact, each year's new cases of narcolepsy appeared in a cyclical pattern, following the seasons. Could narcolepsy be a delayed reaction to the flu?
The brains of narcoleptics fail to produce enough hypocretin (also called orexin), a hormone that regulates wakefulness. It's a disorder that most often begins in childhood, and sometimes runs in families. In popular culture, narcolepsy sufferers have a tendency to fall asleep at comically inconvenient moments. But this view ignores narcolepsy's most ridiculous symptom, called cataplexy. Not all narcoleptics experience cataplexy, but those who do may lose all strength in their muscles during moments of strong emotion. A fit of anger or laughter can leave a person paralyzed on the floor for several minutes.
Though it's not known what causes the onset of narcolepsy, one theory is that it's an autoimmune disorder: The immune system, overreacting to some trigger, attacks hypocretin-producing neurons in the brain. (Similarly, multiple sclerosis is an autoimmune disease that damages neurons throughout the body.) In the event of an immune system uprising, certain people's genetics might guide the attack to the brain's hypocretin neurons. Eventually, enough neurons are damaged or lost that narcoleptic symptoms set in.
After the H1N1 pandemic, there were suggestions that flu vaccinations might have triggered new cases of narcolepsy. To examine this claim, researchers from Beijing University People's Hospital and Stanford University examined the medical records of 906 narcoleptic patients who had been diagnosed in Beijing between 1998 and 2010. For 629 of the patients, researchers knew the month and year in which symptoms began. (These dates were reported after the fact, which could introduce error. But since most of the patients were young children when their symptoms began, the months and years in their records would have come from their parents, whom you'd expect to have a pretty accurate idea of when their child began falling asleep spontaneously in the middle of the day.)
The researchers found that new narcolepsy cases followed a yearly cycle that peaked in April. The fewest new cases occurred in November. And it wasn't a small difference between the top and bottom of the cycle--nearly seven times as many new cases occurred in April as in November. They also saw a huge leap in narcolepsy cases in 2010, with three to four times as many new patients as normal.
Comparing these trends to government data, the authors found that the narcolepsy cycle lagged about six months behind the seasonal cycle of influenza and colds. The spike in 2010 had come six months after the height of the H1N1 pandemic in China.
There was no connection between new narcolepsy cases and flu vaccines, though. Researchers surveyed patients who had developed narcolepsy after the H1N1 pandemic, and found that only 5% had received a vaccine.
Although the researchers couldn't test it directly, the results fit with the theory that infection by influenza can trigger narcolepsy in vulnerable individuals. The infection may set off an autoimmune reaction that, around six months later, has destroyed enough hypocretin neurons to bring on serious symptoms. Though the correlation between the flu cycle and narcolepsy cases could be circumstantial, the spike after the H1N1 pandemic adds some weight to the idea. Previous studies found a possible connection between narcolepsy and strep throat infections. The authors mention, too, that after the 1918 flu pandemic, there was an increase in cases of a brain-lesion disorder.
Even if the infection theory is true, some questions remain. Is narcolepsy inevitable in individuals who develop it? When the disorder sets in during childhood, does it mean the brain was doomed from the start, and the child was one serious infection (or other trigger) away from becoming sick? Or can some narcolepsy cases be avoided? The Chinese data appear to show a slow but steady rise in new cases between 1998 and 2010, which might suggest that environmental factors are outweighing genetic ones. If the disorder is avoidable, further research may show us how to prevent narcolepsy, so that during the flu epidemics of the future we can protect our brains as well as our bodies.
Han, F., Lin, L., Warby, S., Faraco, J., Li, J., Dong, S., An, P., Zhao, L., Wang, L., Li, Q., Yan, H., Gao, Z., Yuan, Y., Strohl, K., & Mignot, E. (2011). Narcolepsy onset is seasonal and increased following the 2009 H1N1 pandemic in china Annals of Neurology DOI: 10.1002/ana.22587