Acute flaccid myelitis (AFM) is a syndrome characterized by rapid onset of flaccid weakness in one or more limbs and distinct abnormalities of the spinal cord gray matter on magnetic resonance imaging (MRI).
Beginning in the summer and fall of 2014, an apparent increase in reports of AFM occurred in the United States, and standardized surveillance was established in 2015 to monitor this illness and attempt to estimate the baseline incidence. Data collected since the establishment of standardized surveillance helped with the identification of another increase in reports nationally during 2016 and has provided additional valuable information on the clinical presentation to help better characterize the clinical features, epidemiology, and short-term outcomes of cases of AFM.