Longer-term follow up
Nielsen at UCLA and M. Elisabeth Lopes Moreira, MD, of the Oswaldo Cruz Foundation in Rio de Janeiro, are following a cohort of more than 100 children born in Rio de Janeiro during Brazil’s 2015-2016 epidemic to mothers with symptomatic, PCR-confirmed Zika infections during pregnancy. In December 2018, their team reported that rates of severe neurodevelopmental delay and sensory dysfunction – 14% of 131 children aged 12-18 months – were higher than those found in earlier studies.
In August 2019, the team described neurodevelopmental, vision, and hearing outcomes in 216 Zika-exposed children 2 years after birth. They used the Bayley-III Scales of Infant and Toddler Development to assess cognitive, language and motor skills in 146 of the children. Forty percent of them were below or very below average in development, more than one third (35%) had language delays, 12% percent had hearing loss, and 7% had abnormal eye anatomy, such as underdeveloped retinas.
In two of the eight children in the cohort with microcephaly, the abnormality unexpectedly resolved. Although that finding received a lot of press, Dr. Nielsen pointed out that “not all microcephalies are created equal.”
In one case, a child born small for gestational age had proportional microcephaly: the baby›s head circumference met the criteria for microcephaly, but the infant›s head was proportional to the body so, as the child grew, the apparent microcephaly disappeared.
In the other case, the child was born with craniosynostosis, in which the skull sutures fuse too early – another effect seen with prenatal Zika exposure, Dr. Nielsen said. After corrective surgery, the child’s head circumference no longer met the definition of microcephaly, but the child still had symptoms related to congenital Zika: a developmental delay and calcifications in the brain. Meanwhile, two other children in the Rio cohort developed secondary microcephaly.
In another follow-up study of children up to age 4, Dr. Nielsen and colleagues found that both clinicians and family may think that Zika-exposed infants without microcephaly are developing normally, but that may not be true. Nearly 70% of children without microcephaly had neurologic abnormalities on physical examination, and more than half had failure to thrive because of poor feeding related to neurologic abnormalities.
Initially, some children may be able to mask subtle problems. A study published in January from Sarah B. Mulkey, MD, PhD, of Children’s National Hospital in Washington, DC, and colleagues described neurodevelopmental outcomes in 70 Colombian children up to 18 months old who had been exposed to Zika in utero. The children had a normal head circumference at birth and a normal fetal MRI, but – compared with typically developing peers – their communication, social cognition, and mobility scores on standardized assessments tended to decline as they got older.
“Especially in a very young child, there’s always going to be a possibility that you can compensate for a deficit, and it appears that at least some of these children are doing so,” said William J. Muller, MD, PhD, associate professor of pediatrics at Northwestern University, Chicago. When the children are older, certain behavioral effects will become easier to assess.
“With these children now approaching school age, understanding the full spectrum of neurodevelopmental abnormalities has important public health and educational system implications,” Dr. Muller and Dr. Mulkey wrote in a commentary about one of Dr. Nielsen’s studies.
Researchers face multiple barriers to understanding the long-term effects of fetal Zika infection. Many infants known to have been exposed in utero never received the recommended early assessments and haven’t been followed long-term. Particularly in Brazil, poverty, poor access to healthcare, and overcrowding all complicate surveillance efforts, Dr. Muller said. Stigma related to children’s neurodevelopmental problems also can potentially reduce a mother’s willingness to attend all follow-ups and assessments.
Some children may have been exposed but were never recognized as such, making it difficult for researchers to track their development and assemble a complete picture of prenatal Zika infection outcomes. Asymptomatic infection occurs in about 80% of Zika infections, though it’s not clear if that number holds for infections during pregnancy as well, according to Dr. Muller and Dr. Mulkey. Because nearly all the current research involves children whose mothers had symptomatic infections, the studies’ generalizability may be limited.
Those likely asymptomatic infections are also a major reason none of the cohorts have comparison groups.
“There are literally hundreds of things that can contribute to or cause developmental problems,” said Dr. Moore of the CDC, who noted that it would be nice to have a comparison group so as to know what Zika may not be responsible for. That said, it would be difficult-to-impossible to create a control group with similar geographic and demographic characteristics as the exposed children, a group who researchers can be certain weren’t exposed.
Neurodevelopmental disabilities occur in about 15% of the general population, making it difficult to determine whether Zika causes any or all long-term, less severe developmental findings in exposed children. The difficulty only compounds with time: the older a child is when a developmental problem is recognized, the harder it is to go back and say the problem is a result of something that occurred before birth, Dr. Moore said. “It’s a challenging field to say, this is what caused that outcome.”