The condition is not new, but officials started to see a rise in cases in 2014; and there was another spike in 2016.
The CDC is not releasing a list of the 22 states with confirmed and suspected cases because of privacy issues.
Symptoms of AFM are similar to a severe respiratory illness along with a fever, but those progress into neurological symptoms.
There is no specific treatment for AFM, but neurologists who specialize in treating brain and spinal cord illnesses may recommend certain interventions, such as physical or occupational therapy, on a case-by-case basis. "Right now, we know that poliovirus is not the cause of these AFM cases". In extreme cases, paralysis and death can occur.
Among the concerns the CDC has are that the agency does not know the cause of AFM, why there has been an increase in cases since 2014, or the long-term effects of the illness. CDC has tested every stool specimen from every AFM patient.
"We understand that people, particularly parents, are concerned", said Dr. Nancy Messonnier, director for the CDC's National Center for Immunization and Respiratory Diseases, during a teleconference call with reporters.
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"As a parent myself I understand what it's like to be scared for your child", Messonnier said.
More broadly, she noted, "there is a lot we don't know about AFM". But some state health departments, such as Minnesota's, have reported the numbers.
She added that confirmation of each case requires a review of MRI images and symptoms, "so there is going to be a bit of a lag as we confirm those things".
About 90 percent of the cases are children who have suffered muscle weakness or paralysis.
States are reporting their cases to the CDC, Messonnier said.
"This is a pretty dramatic disease", Messonnier said.
In an email Tuesday, Maryland Department of Health spokeswoman Brittany Fowler said the CDC will "make a determination about the status of the cases" in Maryland "based on clinical and laboratory information".
The CDC says disease prevention steps should be followed, including staying up to date on vaccines, washing hands and using mosquito repellant.