Need for developing case definitions and guidelines for data collection, analysis, and presentation for congenital microcephaly as an adverse event following maternal immunisation:
Congenital microcephaly, also referred to as primary microcephaly due to its presence in utero or at birth, is a descriptive term for a structural defect in which a fetus or infant’s head (cranium) circumference is smaller than expected when compared to other fetuses or infants of the same gestational age, sex and ethnic background.
Congenital microcephaly can be diagnosed either postnatally or prenatally and is usually defined by the measurement of occipital-frontal circumference (head circumference) that is more than 2 standard deviations (SDs) below the mean for age and sex or less than the 3rd percentile for age and sex , , . Severe microcephaly is defined as head circumference more than 3 SDs below the mean for age and sex , , , .
Congenital microcephaly may occur as an isolated structural birth defect or in combination with other birth defects. Physiologically, congenital microcephaly is a disorder of reduced brain size and volume resulting from abnormal fetal development. Microcephaly has been associated with intellectual disability .
In addition to congenital microcephaly, there is also an acquired form of microcephaly in which an infant’s head circumference falls within the normal range at birth with subsequent development of microcephaly over time due to deceleration of brain growth. Classifying microcephaly as either congenital or acquired is the currently favored nomenclature rather than the past designations of “primary,” “pure,” or “true” for congenital microcephaly versus “secondary” or “syndromic” for acquired microcephaly. We have focused on congenital microcephaly for this case definition and will not address acquired microcephaly.