Microcephaly is a condition in which the brain does not develop properly resulting in a physical deformity, specifically, a smaller than normal head.
It is one of the most common outcomes of Zika virus infection in newborns, but the infection cannot be accurately diagnosed in babies solely based on screening for the brain defect. According to researchers, in order to detect Zika virus infection in newborns, signs and symptoms of brain abnormalities, not just head circumference, should be included in the screening criteria.
After studying all suspected cases of microcephaly in newborns in Brazil, the researchers discarded 3 out of 5 cases because they ended up being normal newborn babies with small heads.
“However, one in five definite or probable Zika cases had head circumference values in the normal range, said lead author Professor Cesar G Victora from the Universidade Federal de Pelotas in Brazil. “Therefore, the current focus on microcephaly screening alone is too narrow.”
Of the 1501 live born cases investigated, 899 were discarded. However, Zika infection among 602 cases were categorized as probable or definite. Among these cases, the babies had small head circumference at birth and their mothers were more likely to have experienced a rash during pregnancy. The definite or probable cases were also 4 times more likely to die in the first weeks of life.
More than 100 of these cases had head circumferences with normal range, however. As such, they would not have been included in the analysis for Zika if the researchers had used smaller cutoffs for head circumference.
“Although we believe that the underreporting of microcephaly cases is rare during the epidemic, newborns infected with the virus late in pregnancy may go unreported due to their head size being within normal range,” Victora said.
According to the researchers, their findings suggest that some, but not all fetuses in pregnancies affected by Zika will have brain abnormalities and microcephaly. However, other fetuses will have abnormalities with normal head sizes, and others still will not be affected. A surveillance system should not just focus on microcephaly or even rash during pregnancy, as for one-third of definite or probable cases there was no history of rash.
Unfortunately, the researchers have not yet determined the ideal cut-off point for head circumference.