"We suspect it is a virus," but doctors have not yet confirmed its presence in all of the patients, he said.
Van Haren and Dr. Emmanuelle Waubant, professor of neurology at the University of California at San Francisco, are researching the cases and have found a virus, enterovirus-68, in the nasal swabs of two, but that sample is too small to draw conclusions, Waubant said. Enterovirus-68 is in the same family as the polio virus.
The polio vaccine does not protect against enterovirus-68, and there is no evidence that the vaccine causes the disease. Doctors Monday emphasized the disease is rare. It has been seen occasionally in the Bay Area, but the presence of five cases in the past 18 months is unusual, Van Haren said.
Other enteroviruses can cause similar symptoms. Another strain, enterovirus-71, has been implicated in similar cases of acute flaccid paralysis in southeast Asia and Australia, but that strain is not known here, Van Haren said.
The enterovirus-68 cases, which began in September 2012, have ranged from Monterey County to the North Bay, with others in southern California. There are no confirmed cases outside California.
Most of those stricken are ages 2 to 16 years old, but the disease has affected some adults, Van Haren said.
In some cases, the paralysis is preceded by a respiratory infection. In other cases, sudden muscle weakness rapidly progresses to flaccid paralysis, in which the limb hangs and cannot move.
"All of the children have permanent weakness," Van Haren said. "So far, we've seen modest to very little recovery."
But in most cases, the virus won't progress to paralysis, the doctors said.
"We think it's a very small number of patients infected with this virus who will have the neurological condition," Waubant said.
Any weakness that lasts for a few hours should be immediately looked at by a physician, the doctors said.
One case investigated at Lucile Packard involved a Berkeley girl, Sofia Jarvis, who is now 4 years old. In November 2012, Sofia, then 2, suffered from a respiratory infection and severe wheezing. Her parents took her to the doctor, and she was treated with Albuterol, an inhaler medicine that opens airways, her mother, Jessica Tomei, said. On the way home, Sofia started vomiting.
She was admitted to the hospital with breathing distress. Doctors thought she had asthma, her mother said. After four days, she returned home. The next day, she returned to her pediatrician for a follow-up appointment, as the doctor had not ruled out pneumonia.
After the visit, Sofia reached into the treasure box in the family's waiting room.
"I saw her left hand in mid-grasp stop working," Tomei said. "Over three days, she was not using her left arm."
Sofia's parents said they were speaking out publicly to alert other parents about what happened to their daughter.
"I know we are so lucky that she is here, and she is going to do amazing things," Tomei said.
Sofia, a lively girl with wavy red hair, calls her dangling arm "Lefty." It's a way to keep the arm integrated with the rest of her body as she tries to regain some movement, Tomei said.
The disease has not affected Sofia mentally, her mother said. She loves to dance, attends a Montessori preschool and is still academically advanced for her age. But she must learn alternate ways to dress herself and tie her shoes, Tomei said. She is currently in physical and occupational therapy.
Van Haren and Waubant will present their findings at the American Academy of Neurology's annual meeting in late April. The California Department of Public Health is also tracking the cases.
This story contains 652 words.
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