The Centers for Disease Control and Prevention said Tuesday it has received reports of 127 suspected cases of a polio-like condition called acute flaccid myelitis or AFM.
So far, 62 cases have been confirmed in 22 states, the CDC said. That makes 2018 look like it might end up being a year with more cases than usual, the CDC's Dr. Nancy Messonnier told reporters.
It's still not clear what is causing the condition, which can develop after a viral infection.
"We have not been able to find the cause of the majority of AFM cases," Messonnier said. "We have detected enterovirus in several of these individual cases."
None of the specimens has tested positive for polio virus, however. Polio was eradicated in the U.S. by vaccination. "AFM can be caused by other viruses such as enterovirus and West Nile virus as well as environmental toxins and autoimmune disease," Messonnier said.
"We are actually looking at everything. We are looking beyond the normal infectious diseases that can cause this."
AFM grabbed attention in 2014, when 120 cases were reported nationally. The CDC says it counted 22 cases in 2015, 149 cases in 2016 and 33 cases in 2017. That appears to indicate an every-other-year pattern.
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The CDC does not name states with confirmed or suspected cases. NBC News had done its own unofficial survey of state health officials and found 87 confirmed or suspected cases in 26 states.
It takes some time to investigate each case, Messonnier said. To confirm a case requires not only that a child have symptoms of limb weakness or paralysis, but a magnetic resonance imaging (MRI) scan that shows specific damage in the spinal cord.
Depending on which part of the spine is damaged, different muscles can become weak or paralyzed.
This can cause a range of symptoms, from difficulty lifting an arm to severe weakening of the muscles used in breathing. That can require use of a ventilator to help the patient breathe.
Infection with many different viruses can cause neurological complications. AFM may be one of them.
Most cases are in children. "Ninety percent of the cases are in those less of 18 years in age," Messionnier told NBC News.
The good news is that the complication is "incredibly rare", Messonnier said. "Overall, the rate of AFM since 2014 is less than one in a million," she said.
Nonetheless, it's important to get patients to a doctor right away if there are any symptoms of limb weakness.
"AFM is a rare condition. It's also a serious condition. So we want to encourage parents to seek medical care right away if you or you child develop symptoms of AFM such as sudden weakness or paralysis of the arms and legs," she said.
Some children may have mild symptoms, but AFM doesn't just sneak up on its victims.
"This is actually a pretty dramatic disease. These kids generally have a sudden onset of weakness," Messonnier said.
Some children with AFM recover completely, but others are disabled for years.
There is no specific treatment for AFM, the CDC says. Once a virus has attacked the nervous system, there’s no known medical intervention that can reverse the effects. Rehabilitation therapy can help patients regain function.
Since 2014, doctors have been trying to find if there is a way to prevent or treat the damage that leads to AFM. They found no hard evidence that corticosteroids, use of intravenous immune globulin or IVIG, a type of blood filtering called plasmapheresis, use of an immune booster called interferon, antiviral drugs or other medical treatments helped.
"I want parents to know how hard we're working to try to identify the cause of AFM and when we get information that will help them understand it, we're going to make that information available," Messonnier said.