Liz Szabo

USA TODAY

A new study out Tuesday links the Zika virus to a rare joint condition that can make it difficult or impossible for children to straighten their arms and legs.

Children with the condition, called arthrogryposis, have limited movement in joints such as the elbows or knees. In some cases, their joints are stuck in a one position and can't be unbent.

Arthrogryposis is not usually painful in babies, said pediatric neurologist Vanessa van der Linden, an author of the study and a researcher at the Association for Assistance of Disabled Children in Recife, Brazil, a city that has been the epicenter of the Zika epidemic in that country. As children grow, certain conditions, such as dislocated hips, can cause pain, she said.

Zika is already known to cause a variety of birth defects, including blindness and microcephaly, in which babies are born with abnormally small heads and, in most cases, incomplete brain development.

Van der Linden and her colleagues studied detailed brain and joint images of seven babies with the condition who are believed to have been infected with Zika in the womb. All had brain calcifications, or lesions similar to scars that indicate past damage by a virus or other cause.

Four of the babies' mothers had a rash — a common Zika symptom — while pregnant, according to the study, published in The BMJ, formerly The British Medical Journal.

Two babies tested positive for a past Zika infection. Doctors didn't test the others, because the only test that can detect past infections isn't routinely available in Recife, van der Linden said.

All of the babies were born with dislocated hips, in which the femur has come out of the hip socket. Six of the children have microcephaly. Six had trouble swallowing; two needed feeding tubes, as well as tubes in their windpipes to help them breathe. All five boys in the study had testicles that failed to descend from the abdomen to the scrotum.

Most of the children had joint problems in both their arms and legs. Six of the babies had clubfoot, in which the foot is twisted so that the sole can't be placed on the ground. Six had fingers that were permanently bent. Five also had abnormalities in their eyes.

Doctors tested six of the children's hearing. One had hearing problems in one ear, while a second had hearing problems in both ears.

Doctors used special tests to study electrical activity in the children's muscles. They weren't able to fully study every nerve and muscle in all children because some of the babies were irritable, a problem that van der Linden blamed on problems such as inflammation of the esophagus, epilepsy or "hyperexcitability." Reports from Brazil have noted that many babies with Zika-related microcephaly are irritable and cry incessantly. Van der Linden said the babies' irritability improved after when they were four or five months old, after the inflammation or epilepsy were treated.

Doctors found no abnormalities in the children's joints themselves, suggesting their conditions could have been caused by problems in nerve cells that send signals to the infants' muscles, van der Linden said.

Doctors already know the Zika virus can damage nerve cells in the brain. Now, doctors will take a closer look to see if Zika also can damage nerves in the spinal cord, said Ganeshwaran Mochida,a pediatric neurologist and researcher at Boston Children’s Hospital. Although Mochida was not involved in the new study, he examined some of the babies involved in it while visiting Recife.

Prenatal tests can detect about half of arthrogryposis cases, said Irina Burd, founder of the Johns Hopkins Integrated Research Center for Fetal Medicine. Other cases become apparent only after delivery.

Arthrogryposis can result when a fetus doesn't move while in the womb, Burd said. Because of the lack of movement, the fetus' limbs become stuck in one position. Many things can prevent a fetus from moving in the womb, she said. Damage to nerve cells could prevent muscles from being stimulated to move. Problems with the placenta could lead to a lack of fluid inside the uterus, which prevents fetuses from moving freely.

Treatment for arthrogryposis can include physical and occupational therapy; splinting or casting to restore more normal joint position; or surgery to release a muscle or tendon to improve the range of motion, according to Seattle Children's Hospital.

Some children with arthrogryposis do better than others. Those who have a problem in only one joint can often be treated successfully, said James Bale, Jr., professor of pediatric neurology at the University of Utah School of Medicine, who was not involved in the study.

Children with more severe arthrogryposis can also improve with therapy, but are often left with "lifelong disability," Bale said.

All babies with Zika infections should be examined for possible joint problems, the study's authors said.

One of the biggest barriers to treating children with arthrogryposis in Brazil is access to care, Mochida said. He met families who traveled eight hours to get to the rehabilitation center in Recife, often several times a week.