What is Acute Flaccid Myelitis?

Acute flaccid myelitis (AFM) is a rare and serious neurological condition that causes certain muscles and reflexes to become weak (flaccid). The symptoms come on suddenly and can also affect your ability to breathe.

AFM causes inflammation in the gray matter of your spinal cord. You have grey matter in your brain and spinal cord (central nervous system). Of all the types of central nervous system tissue, it plays the most significant part in allowing you to function normally. Gray matter in your spinal cord specifically plays a role in controlling movement.

AFM is a relatively new diagnosis — researchers first described it in 2014. Previously, healthcare providers and researchers thought it was a type of transverse myelitis.

What is the difference between acute flaccid myelitis and Guillain-Barré syndrome?

Acute flaccid myelitis and Guillain-Barré syndrome (GBS) are both rare neurological conditions that cause muscle weakness, but they’re different.

In GBS, your body has an autoimmune response that damages peripheral nerves — the nerves that connect your central nervous system (brain and spinal cord) to your limbs and organs. AFM affects the gray matter of your spinal cord.

The muscle weakness in GBS usually begins in your feet and legs and travels up your body. In AFM, muscle weakness can begin in your legs or arms.

Another difference is that AFM typically affects children, whereas GBS most commonly affects adults aged 40 or older.

Who does AFM affect?

About 90% of acute flaccid myelitis cases affect young children between the ages of 1 and 7, but adults can get AFM, too.

How common is acute flaccid myelitis?

Acute flaccid myelitis is rare. Researchers estimate that fewer than 1 person per 1 million people per year in the U.S. develop AFM. However, the number of cases has been rising.

Cases have usually occurred in clusters in certain geographical areas with a distinct seasonal biennial pattern (every other year). For example, there was a cluster of cases in California in 2012 and a cluster in Colorado in 2014.