An unknown illness has reportedly caused various degrees of paralysis among 51 children in 23 states, as the Centers for Disease Control and Prevention (CDC) continues efforts to identify the cause of the illness.

The agency began asking states to track the condition after a cluster of cases were reported in Colorado in August and September, according to a media release from the American Physical Therapy Association (APTA).

The New York Times reports that CDC officials continue to describe the cases as “extremely rare,” and they are not supporting a connection between the polio-like condition and respiratory virus enterovirus 68, although some doctors “suspect a link.” The NYT article quotes Mark Pallansch, director of the division of viral diseases at CDC, as saying, “We don’t have a single clear hypothesis that’s the leading one at this point.”

Early reports cited the condition as appearing in 40 states. Since those initial reports, CDC ruled out several cases that do not meet its definitions for inclusion, which require the presence of spinal lesions largely in the gray matter, among other factors.

The condition seems to strike younger children and advances rapidly, becoming most acute within 1-3 days of initial symptoms of weakness. The paralysis varies in severity, and most recovery is made within the first 2 months, with a slowdown in gains as time progresses. A pediatric neurologist quoted in the NYT story describes recovery as “highly variable,” saying that “some patients recover very well, others not.”

[Source: American Physical Therapy Association]