Etiology of Severe Microcephaly in Infants: A Multinational Surveillance Study

BACKGROUND AND OBJECTIVES: Severe microcephaly, or head circumference at least 3 standard deviations below the mean for age and sex, is a rare condition with diverse etiology, making diagnosis challenging. Following the 2015 to 2016 Zika virus outbreak, surveillance studies in Australia, Canada, New Zealand, and the United Kingdom and Ireland were conducted to monitor severe microcephaly. We describe the etiology, clinical features, and diagnostic investigations of severe microcephaly among children aged younger than 1 year. METHODS: We pooled reports of patients with severe microcephaly detected through 4 national active surveillance studies, through the International Network of Paediatric Surveillance Units. Incident cases were reported voluntarily between June 2016 and October 2018 by networks of pediatricians totaling more than 8000 members. Etiology was categorized as genetic (confirmed/suspected), acquired (infection, ischemia/hypoxia, prenatal alcohol exposure, placental insufficiency), or unknown. Anonymized data were pooled and analyzed using descriptive statistics. RESULTS: Overall, the cases of 118 patients with severe microcephaly were analyzed, including 59 from the United Kingdom and Ireland, 34 from Canada, and 25 from Australia (n < 5 cases from New Zealand were not analyzed). Median age at diagnosis was 17 days (IQR 1-119), and mean head circumference-for-age Z-score was -4.0 (SD 1.1). Genetic causes were determined for 50% (n = 59) vs 18% acquired (n = 21) and 32% unknown (n = 38). Common investigations included brain magnetic resonance imaging (70%), DNA microarray (69%), brain ultrasonography (53%), and cytomegalovirus screening (48%). CONCLUSIONS: At least one-half of severe microcephaly cases are attributable to genetic causes. One-third had unknown etiology, highlighting a need for a systematic approach to diagnostic investigation, including genomic sequencing and brain imaging for all children with severe microcephaly.