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An alarming rise in Brazil in the number of cases of babies born with smaller-than-normal heads — a condition known as microcephaly — has public health officials racing to determine if the cause is a mosquito-borne virus known as Zika.

Officials are still trying to determine if Zika definitively causes microcephaly in babies born to women infected with the virus during their pregnancies. But as Zika cases pop up in more places around the world, and global health agencies issue travel warnings for pregnant women, concern about microcephaly is on the rise.

So what actually is microcephaly? And if children are born with it, what does that mean for them? Here, STAT answers those questions and more.


What is microcephaly?

Microcephaly is not a specific disease. Instead, it’s a generic term used to describe any case in which someone’s head is a certain size smaller than the average. In babies born with microcephaly — which is known as congenital microcephaly — physicians identify it simply by measuring the circumference of the head and comparing that to growth charts. In some instances, it can be diagnosed during pregnancy with an ultrasound. (Babies born with normal-sized heads can also show signs of the condition later in infancy.)

Microcephaly is derived from the Greek words meaning “small” and “head,” but what it really means is that the brain has not formed properly or has stopped developing.


Typically, as a fetus matures, its head gets larger to accommodate the budding brain, so “if the brain doesn’t grow, the skull doesn’t grow,” said Dr. Sumit Parikh, a pediatric neurologist at Cleveland Clinic Children’s.

Zika virus, which can be transmitted by mosquitos, has exploded onto the world stage in recent weeks. Alex Hogan/STAT

What causes microcephaly?

A lot of things. Microcephaly is a physical manifestation of a variety of underlying problems. “It’s like saying fever … there’s a billion causes,” Parikh said. Causes range from genetic mutations to injuries to infection. “Anything that arrests the cells from dividing or differentiating can result in a smaller brain,” said Dr. Avindra Nath, the clinical director of the National Institute of Neurological Disorders and Stroke.

Fetuses can have strokes during a pregnancy, cutting the blood supply from reaching the brain. Genetic disorders such as Down syndrome can throw off how the brain forms. Drug and alcohol use by pregnant women can cause microcephaly, as can malnourishment, chemical exposure, and certain medications.

What is the outlook for children born with microcephaly?

It depends on the severity and how the brain is affected. Some newborns have heads small enough to be labeled as cases of microcephaly, but their neurological function isn’t impaired. More severe cases result in stillbirths or infants who die soon after birth.

Many of the problems associated with microcephaly — including developmental delays; problems with vision, hearing, or speech; and epilepsy — are actually the result of the underlying cause.

Babies born with microcephaly can have trouble moving their arms and legs and problems with feeding because of difficulties sucking and swallowing. A smaller head at birth is also associated with mental retardation.

But people with neurological problems “can live close to a normal life span,” Nath said, particularly if they have a strong support network as they grow up. One problem is that even in milder cases, microcephaly can be linked to seizures, which can reduce life expectancy.

Does it affect boys and girls at the same rates?

Because there are so many causes, it’s hard to say for sure, researchers said. But if there is a difference in rates between boys and girls, it’s a small one.

What kind of infections cause or are thought to cause microcephaly?

The infections associated with congenital microcephaly are those that can cross the placental barrier, which is meant to protect the fetus and blocks certain infections. When viruses break through the placenta, they can enter the fetus’s nervous system and halt the brain’s development.

Why do researchers think the Zika virus could be causing the surge in microcephaly cases in Brazil? What do the cases in Brazil look like?

Researchers have found Zika virus RNA in the brains of infants born with microcephaly in the past few months in Brazil, according to the US Centers for Disease Control and Prevention, an indication that Zika can bust through the placental barrier and enter the fetus’s nervous system.

Before the Zika outbreak, microcephaly in Brazil appeared at rates of about 0.5 cases for every 10,000 births, although some public health experts have said underreporting contributed to an artificially low rate. Still, even adjusting for possible underreporting, Brazil likely saw no more than one to two cases per 10,000 births. In the second half of 2015, Brazil was seeing rates of 20 cases per 10,000 live births, according to a CDC report released last week. The country has reported almost 3,900 cases of microcephaly since October.

Brazil is also seeing severe cases of microcephaly, which are more likely to be paired with greater developmental delays. “Most of what is being reported out of Brazil is microcephaly with other associated abnormalities,” Dr. Denise Jamieson, medical officer in CDC’s division of reproductive health, told STAT this month.

An important note: If a pregnant woman contracts the Zika virus, her child will not necessarily have microcephaly. Public health officials are still trying to determine what percentage of pregnant women infected with Zika will have a baby with microcephaly, if the virus does in fact cause the birth defect.

When does microcephaly present itself during fetal development? Does the timing affect the severity of the condition?

Initial evidence indicates that mothers in Brazil who gave birth to infants with microcephaly tended to be infected with the Zika virus in their first trimesters, the CDC said. But “we do have some evidence that the risk can continue into the second trimester,” Dr. Cynthia Moore, the director of the CDC’s division of birth defects and developmental disabilities, told reporters this month.

Dr. Ganeshwaran Mochida, a pediatric neurologist at Boston Children’s Hospital, said that congenital infections often strike the fetus between the second and fourth month of a pregnancy, when most neurons are formed. “This is the critical period for neurogenesis,” Mochida wrote in an email. “You cannot ‘catch up’ if not enough neurons are made during this period.”

If an infection occurs later in the pregnancy, it may be less likely to trigger microcephaly, researchers said. And when cases of microcephaly occur later in pregnancy, infants may be better off in the long run because their brains will have had more time to develop properly.

Still, CDC officials have said that pregnant women in any trimester should think twice before traveling to an area with Zika transmission. In the case of Brazil, they have also said there is evidence of brain tissue that was forming in fetuses being destroyed.

What are researchers looking at?

Most research connected to microcephaly is focused not on the physical condition, but on the problems that cause it. Scientists are also trying to stem the conditions tied to microcephaly. A group of Dutch researchers, for example, is exploring the use of amino acid replacement therapy to halt seizures associated with a rare neurometabolic disease that also causes microcephaly.

What can be done for babies born with microcephaly?

Unfortunately, not much in the large majority of cases. With congenital microcephaly, “the injury has already occurred, and we’re just dealing with the consequences of the injury,” Parikh said.

Parents who have a child with microcephaly caused by a genetic mutation are encouraged to get genetic counseling so they can have a sense of the microcephaly risk for future children. As for the children, speech, physical, and, eventually, occupational therapy can improve their ability to function day to day. Associated complications like seizures can be addressed, as well.

But assisting people with microcephaly requires teams of highly trained caregivers. So if the Zika outbreak in Brazil is indeed causing the surge in microcephaly cases, researchers said, the country is going to need to grow its workforce of people who can support the children who survive beyond their first few days and, for some of them, eventually enter adulthood.

Helen Branswell contributed reporting.

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