A New Way of Assessing Microcephaly in the Context of the Zika Virus Epidemic

 An international team led by Clara Menendez, director of the Maternal, Child and Reproductive Health Initiative at ISGlobal, describes a new way of defining microcephaly based on the ratio between the baby’s head circumference and body weight. The advantage is that it does not require accurate estimates of gestational age, something difficult to do in low and middle income settings.   

Although most infections by Zika virus are asymptomatic or provoke mild symptoms, infection during pregnancy can lead to severe fetal outcomes, including microcephaly. Microcephaly is defined as a smaller head circumference than expected in a baby of the same gestational age and sex, but the recent Zika epidemic has highlighted the challenge of identifying true microcephaly cases in a more accurate manner. WHO has published guidelines to define microcephaly that are useful for babies born at term but less so for premature or smaller than normal babies. In any case, the gestational age of the baby needs to be determined (preferentially by ultrasound), something difficult to achieve in low resource settings. Moreover, a small head is not always synonymous with microcephaly since the ratio with the body size is also important.   

In this study, the authors calculated the prevalence of microcephaly in babies born to mothers of two cohorts from Mozambique, and from studies carried out in Guatemala, Brazil and Colombia. They show how the prevalence estimate improves when taking into account the ratio of head circumference to body weight, even among small babies.   

“The advantage”, explains Clara Menéndez, “is that this new definition can also be applied to babies that are small for their age. Furthermore, it does not depend on methods used to determine gestational age, that have important limitations in low income countries”.   

The authors conclude that this strategy can be used for individual case management as well as for epidemiological surveillance of microcephaly in countries with active Zika virus transmission. 

Reference:

Quintó L, García-Basteiro AL, Bardají A, et al. The Challenge of Assessing Microcephaly in the Context of the Zika Virus Epidemic. J Trop Pediatr. 2017 Mar 10.