The World Health Organization has declared the Zika virus and its potential link to birth defects a global health emergency. Scientists are studying if the spread of Zika in Latin America is linked to the increased rates of microcephaly, a condition in which babies are born with unusually small heads. Zika virus is transmitted mainly through the bite of an infected Aedes aegypti or Aedes albopictus mosquito.
Dr. William Dobyns and Dr. Ghayda Mirzaa are pediatric neurogeneticists and researchers at the Center for Integrative Brain Research at Seattle Children’s Research Institute who treat and study microcephaly. On the Pulse sat down with them to discuss microcephaly.
Q: What is microcephaly?
Mirzaa: Microcephaly is a condition in which a fetus or baby’s head size is abnormally small, defined as more than two standard deviations below average. The smaller head size reflects abnormal or decreased brain growth. Microcephaly affects about 2% of newborns, while severe microcephaly, defined as a head size more than three standard deviations below average, is seen in less than 0.1% of newborns.
Microcephaly occurs as the only birth defect in many children, but it can also occur with a wide range of additional abnormalities including other brain defects. When a baby has microcephaly, a neurologist or geneticist will order tests to determine the cause.
Q: What are the symptoms of microcephaly?
Dobyns: Some children with microcephaly, especially mild microcephaly, will have no developmental delay or other symptoms, and will appear unaffected. Others will have varying degrees of developmental problems depending on the severity and cause of the microcephaly. They may have intellectual disabilities, delayed speech and motor functions, vision and hearing problems, feeding problems, seizures, or other problems associated with brain abnormalities. A few may have short stature or birth defects of other parts of the body.
Q: What causes microcephaly?
Mirzaa: Microcephaly can be caused by numerous genetic disorders and metabolic disorders; certain viral infections during pregnancy; and brain injuries at or soon after birth such as lack of oxygen or bacterial meningitis. Researchers have identified defects in more than 50 genes associated with microcephaly.
Some of the non-genetic causes of microcephaly include:
- Traumatic injuries or major bleeding in a pregnant mother, especially during the second trimester.
- Fetal death of a twin. If a woman is pregnant with twins and one dies in the womb, the surviving twin has a higher risk of microcephaly.
- Certain infectious diseases. Microcephaly has been linked to cytomegalovirus, and less often to toxoplasmosis and rubella. Researchers are urgently studying the potential link to Zika virus.
- Toxins such as alcohol and radiation, which interfere with fetal development.
- Maternal metabolic diseases such as untreated phenylketonuria.
Q: How do doctors diagnose microcephaly?
Dobyns: About 25,000 children with microcephaly are diagnosed in the U.S. each year. Doctors may detect microcephaly in a fetus during ultrasounds in the second or third trimesters of pregnancy. In some cases, microcephaly is not evident until birth. If a baby is born with microcephaly, a doctor may order brain imaging and other tests to determine the cause and prognosis.
Q: What is the prognosis for a baby with microcephaly?
Mirzaa: There is a wide range of outcomes for microcephaly ranging from mild to severe. The outcome of the affected child depends on the overall clinical picture. There are a few key factors that will determine prognosis:
- Severity of the microcephaly.
- Association with other birth defects of the brain in imaging studies.
- Any additional abnormalities in the child.
Children with mild microcephaly may be normal or have only mild developmental problems. Children with severe microcephaly but no other abnormalities are said to have ‘primary microcephaly.’ Most have intellectual disability and require assistance. On the other end of the spectrum, especially when other birth defects occur, the prognosis can be more serious and can result in early developmental delay, feeding issues, severe intellectual disability and seizures.
While no treatments are available that can cure microcephaly, children can benefit from early intervention programs with physical, occupational and speech therapists who can help them reach their full potential and enhance their quality of life. Researchers have found that treating cytomegalovirus — which can cause microcephaly — in newborn babies for six months improved developmental and hearing outcomes. A child with suspected microcephaly should be assessed by a neurologist and geneticist as soon as possible.
Q: Is the Zika virus causing the increased rates of microcephaly in Latin America?
Dobyns: More research is urgently needed to answer this question. The information available to date suggests that exposure of women to the Zika virus during pregnancy can cause microcephaly, including severe microcephaly as well as other birth defects of the brain. We know that exposure to a few other viruses, such as cytomegalovirus, can cause microcephaly and other birth defects. Researchers are working to determine what the risk is when women are exposed to the virus at different stages of pregnancy.
There are many unanswered questions about the current increased incidence of microcephaly in Latin America. Women who are pregnant or concerned about risks should contact their doctor and monitor recommendations from the Centers for Disease Control, especially if they have traveled to Central or South America during the pregnancy.
Q: What research are you currently working on related to microcephaly?
Dobyns: At Seattle Children’s Research Institute, Dr. Mirzaa and I study the causes of microcephaly, especially severe microcephaly. Our current work is focused on many genetic causes and cytomegalovirus. We also assess babies and children with microcephaly in Seattle Children’s genetics clinic.