More children are becoming suddenly paralyzed — and health officials still don’t know why

Ninety people have been diagnosed in the U.S. this year with the sudden paralysis known as acute flaccid myelitis, and 162 additional cases are being investigated, federal health officials said Tuesday.

This year’s paralysis outbreak could be the biggest the country has ever seen if the cases under investigation are confirmed, experts say.

The condition, known as AFM, closely resembles polio and is most likely to afflict children. The average patient’s age is 4.

In 2014, more than 100 children in the U.S. were diagnosed with AFM after suddenly becoming paralyzed. In 2016, another outbreak paralyzed even more children. Two years later, the devastating illness is back — and worse.

“I can certainly understand why parents are worried. I’m concerned about this increase in AFM,” Nancy Messonnier, director of the National Center for Immunization and Respiratory Diseases at the federal Centers for Disease Control and Prevention, told reporters Tuesday. “It’s important for parents to realize that this is a relatively rare condition.”

The chance of a child getting AFM is less than 1 in 1 million, according to federal health officials.

CDC officials say they don’t know what is causing the paralysis and that scientists and doctors are continuing to investigate.

“As a mom, I know what it’s like to be scared for your child,” Messonnier told reporters Tuesday. “Right now, the science doesn’t give us an answer.”

Some doctors have criticized the CDC’s response to the AFM outbreaks, saying the paralysis is probably caused by a summer virus and the CDC’s decision to call it a “mystery” illness is wrongly stoking fears among parents. The AFM outbreak in 2014 coincided with an outbreak of enterovirus D-68, a cousin of poliovirus.

Messonnier said Tuesday that doctors have found the viruses EV A-71 and EV D-68 in the spinal fluid of two patients. Officials had said in the past that finding the viruses in the spinal fluid was key — proof that they had traveled to the spinal cord and caused the motor damage characteristic of AFM.

Yet Messonnier said Tuesday that finding the viruses in two patients’ spinal fluid did not explain all of the cases. She said experts are looking into whether the condition is an autoimmune response to a virus.

“We’ve learned a lot about AFM since 2014, but there are things that we still don’t understand,” she said.

soumya.karlamangla@latimes.com

Twitter: @skarlamangla