Health authorities in the United States said this week that they were investigating an unusual spike in cases of a rare condition that causes limb paralysis and severe muscle weakness in children.

At least five states — Illinois, Washington, Colorado, Minnesota and Pennsylvania — reported that they were investigating the cause or trying to confirm new cases of the illness, known as acute flaccid myelitis. Some health authorities in those states have said the condition coincides with seasonal respiratory illnesses.

A.F.M. affects the nervous system and causes, mostly in children, paralysis similar to polio. The signs include sudden weakness in the arms or legs; neck weakness or stiffness; a drooping face or eyelids; difficulty swallowing; and slurred speech, health officials say.

Recent Cases

Since mid-September, six cases of A.F.M. in children under 10 years old have been reported to the Minnesota Department of Health, the agency said. Another two possible cases are pending confirmation, officials said.