It starts with just a droopy eye and a slightly slumped face. Within just a few days, reflexes begin to lose their sharpness, while the arms and legs become weaker. If left without therapy, the nervous system continues to become damaged, the ability to breath becomes strained, and paralysis can take hold.

More and more children are falling sick with this polio-like illness. However, there are currently few solid ideas about how it develops, how it can be treated, or even what it is exactly.

Reporting in the journal mBio, a recent study by the US National Institutes of Health has reviewed the current evidence about the condition and its unusual reemergence, concluding that more research into the condition needs to be carried out urgently.

Known as acute flaccid myelitis (AFM), it's characterized by sudden muscle paralysis of healthy children, although there are a handful of cases that have occurred in adults. The condition appears to damage the nervous system, specifically the area of the spinal cord called gray matter.

Due to the similarity of the symptoms, the condition was long believed to be caused by the poliovirus. However, the world is fast approaching an eradication of polio, yet the rates of AFM are continuing to rise.

A boom of AFM cases was documented in 2014 across the US, the UK, and continental Europe. From August 2014 to January 2019, there were 551 confirmed cases of AFM, with 2018 seeing one of the biggest seasonal waves of the recent epidemic. The CDC notes it's still extremely rare, affecting less than one to two in a million children in the US in a given year.

This rarity makes the disease hard to study and understand. Environmental toxins and genetics were once suspected to play a key role in the disease, but an important clue emerged when researchers noted AFM outbreaks tended to coincide with outbreaks of two enteroviruses, EV-A71 (the cause of hand-foot-and-mouth disease) and EV-D68. The new study claims this evidential link between AFM and the enteroviruses is “circumstantial” but “strong”.

Nevertheless, the study is still dotted with unanswered questions.

“As it unfolds, the AFM story seems to be getting more complicated,” the study authors write. “Could we be entering some kind of new epidemic era, in which fundamental but unappreciated determinants of enterovirus evolution and spread are changing?“

“The trajectory of AFM over the past 5 years suggests that the problem is getting worse, and so it is critical that we galvanize our efforts to learn more about, and respond adequately to, this ubiquitous, often crippling, continually reemerging group of viruses.”