The Department of Public Health has confirmed the second Massachusetts case this year of a rare but potentially devastating illness that strikes mostly children and causes muscle weakness or paralysis.
The other case of acute flaccid myelitis, or AFM, occurred in August, and four others are under investigation in Massachusetts.
First recognized in 2014, AFM remains mysterious. It follows a peculiar seasonal pattern, with the highest incidence occurring every other year in late summer or fall. No one knows what causes AFM, although a virus or viruses are suspected.
When AFM strikes, patients experience sudden weakness, as well as loss of muscle tone and reflexes in the arms and legs. Some also experience facial droop, difficulty moving the eyes, drooping eyelids, difficulty swallowing, or slurred speech.
But this illness is exceedingly rare, affecting fewer than one in a million people.
Massachusetts has seen 16 confirmed cases in children since 2014, plus one probable case in an adult. The Department of Public Health declined to provide the children’s ages or town of residence, citing the need to protect privacy.
The US Centers for Disease Control and Prevention reported Tuesday that 62 cases had been confirmed in 22 states this year, and that an additional 65 are under investigation nationwide. All told, 386 cases of AFM have been confirmed since 2014, more than 90 percent in children younger than 18. The average age of patients was 4.
“We have not been able to identify a cause for the majority of these cases,” said Dr. Nancy Messonnier, director of the CDC’s National Center for Immunization and Respiratory Diseases.
While she said she couldn’t predict how 2018 will turn out, Messonnier said the number of cases this year seems to be in line with the previous two waves in 2014 and 2016.
The CDC says the illness could be caused by viruses, environmental toxins, genetic disorders, or a condition in which the immune system attacks body tissues.
Some victims have been infected with viruses, but researchers have been unable to identify a single virus responsible for all cases. Nor can they explain why only a handful of infected children developed AFM.
The first cases of AFM coincided with an unusually severe outbreak of a common virus that affects the lungs or gut. But that bug, enterovirus D68, could not be definitively linked to the illnesses.
Although symptoms resemble polio, no evidence of poliovirus has been found in specimens from any case diagnosed so far, Messonnier said.
“Right now we know that poliovirus is not the cause of these AFM cases,” she said. West Nile virus is also not a culprit, she said.
Many other aspects of the illness remain unknown, including what factors put a person at risk of becoming ill and the long-term consequences.
“We know that some patients diagnosed with AFM have recovered quickly, and some continue to have paralysis and require ongoing care,” Messonnier said. In 2017, one person died of AFM.
There is no treatment specifically for AFM, but affected children can undergo physical and occupational therapy to maximize their strength and adapt to their limitations.
“As a parent myself, I understand what it’s like to be scared for your child,” Messonnier said. “Parents need to know that AFM is very rare even with the increase in cases we’re seeing right now.”
She recommended that parents take routine protections against viral infections, such as keeping up to date on immunizations, washing hands, and avoiding mosquito bites.
“We recommend seeking medical care right away if you or your child develops sudden weakness of the arms and legs,” she said.
Boston Children’s Hospital has evaluated all of the confirmed and suspected AFM cases that occurred in Massachusetts this year.
Dr. Leslie Benson, assistant director of Children’s pediatric neuroimmunology program, said AFM patients show a wide spectrum of symptoms. Some merely have difficulty moving the eye or the face; some have a single weakening limb. In the most severe cases, the muscles that control breathing are weakened to the point the patient requires a ventilator. Doctors at Children’s have not seen an AFM patient needing a ventilator this year, Benson said.
Most children with AFM see subtle improvements over time, but “most have long-term weakness,” she said.
“It seems most likely that the virus infects the motor nerves in the spinal column,” Benson said. Once damaged, such nerves don’t regenerate.