CDC Confirmation of Cases of Acute Flaccid Myelitis: Gray Matter vs White Matter Damage

Cases of Acute Flaccid Myelitis continue to peak in summers of even years—and it’s critical that all cases are reported by local health departments to CDC so the full scope of this disease can be realized. Not every case sent to CDC is confirmed, for a variety of reasons. A longstanding concern held by the AFM community is how CDC confirms cases where both there is damage to both the gray and white matter. The team of neurologists at CDC has put together an explanation walking through their process of MRI review.

For case classification of AFM, we rely less on quantifying how much gray vs white matter involvement there is, and give greater importance to the MRI lesion pattern. We evaluate each patient for both imaging and documentation of flaccid weakness, to see if they fit the surveillance definition of AFM. There isn’t necessarily a certain amount of white matter that would make a case “not a case”. While we consider AFM a disease of the gray matter, there may be reasons that we see white matter involvement. There could be “collateral damage” which can occur when the swelling of the gray matter extends out to the white matter. When we review images what we are really looking for are specific features and patterns that we see commonly in AFM patients.


The timing of the MRI is also critical to our classification process. We know that the spinal cord can look different in the initial phase of the illness compared to later in the course of disease, but often only one MRI is done per patient. There may be other issues that could impact our ability to classify a case, including the quality of the image that we receive. We try to take these factors into account, but this can make it challenging to be certain if a patient meets our case definition, which is why we have multiple neurologists review each case, especially the more challenging ones.


It important to note that our “confirmation” is not a clinical diagnosis. As clinicians, we understand that it can be confusing for parents to separate the clinical diagnosis of AFM from CDC’s classification process. But we do want to stress that those are two separate processes.


If your child’s physician has diagnosed your child with having AFM, we encourage you to stick with their doctor’s diagnosis. It may be that our team at CDC did not have enough required features or information necessary to confirm it based on our more stringent criteria. We are learning more and more about AFM as time goes on, but it may take more time to understand the full spectrum of the disease.

If you have not heard about the status of your case, we urge you to reach out to your local health department for clarification. More information about CDC Case Classification can be found here.