The Centers for Disease Control and Prevention have confirmed 116 cases of a rare, polio-like condition so far this year, making it the highest amount the U.S. has seen since 2016. Officials have still not pinpointed a cause behind acute flaccid myelitis (AFM), but noted that the majority of patients are children.

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“It’s pretty concerning that it’s going up and we still haven’t figured out specifically how to prevent this or how to treat it,” Dr. Emmanuelle Tiongson, a pediatric neurologist at Children’s Hospital Los Angeles who has evaluated and treated patients with AFM, told Reuters.

The CDC has tracked an every-other-year spike in AFM cases, with 120 confirmed cases in 2014, but only 22 in 2015, and 149 confirmed cases in 2016, but only 33 total in 2017. The agency said most patients report mild respiratory illness or fever consistent with a viral infection before developing AFM, but they have not been able to determine why some patients develop the condition while others fully recover.

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The agency has been investigating 286 reports of AFM since August, with cases occurring across 31 states. The rare, serious condition affects the nervous system and may cause facial droop or weakness, difficult moving the eyes, drooping eyelids or difficulty swallowing and slurred speech. While there is no specific treatment, doctors may recommend intervention on a case-by-case basis. Many patients recover, but in rare cases the condition may be fatal or cause lifelong complications.