Investigating an increase in microcephaly diagnoses in British Columbia

To the Editor,

Microcephaly is a clinical condition, in which the brain fails to develop properly, resulting in small head size (1). Microcephaly is identified by using head circumference as a proxy measure of the volume of the brain (2). Newborns whose head circumference is below the third centile, compared with other newborns of the same sex and gestational age, are typically diagnosed with microcephaly (2,3). It is important to note that this criterion is necessary but not sufficient for the diagnosis of microcephaly as natural variation in head circumference can occur without pathology.

In 2016, perinatal health surveillance by the Public Health Agency of Canada detected an increase in microcephaly cases in BC. Further analysis by Perinatal Services BC with perinatal health surveillance data, from the British Columbia Newborn Record, identified an elevated rate of microcephaly at a single health care facility in British Columbia, Canada. This finding led to an investigation to verify that newborns with a clinical diagnosis of microcephaly had a head circumference below the third centile.

A retrospective cohort study of newborns with an International Classification of Diseases Tenth Revision, Canada code for microcephaly at the health care facility in fiscal years 2009/2010, 2010/2011, 2013/2014, or 2014/2015 was undertaken. Microcephaly diagnosis codes were generated when physicians noted microcephaly on the patient’s discharge summary, the newborn record, and/or the notice of birth. Birth head circumference, gestational age, and other newborn data were obtained from the British Columbia Perinatal Data Registry and medical record abstraction. We used the sex-specific Intergrowth-21st newborn standard to calculate newborn head circumference centiles. It is noteworthy that at the time of the study, newborn head circumference centiles were typically calculated based on a nonsex-specific chart provided on the provincially standardized newborn record (4).

We analyzed 151 newborns who met the study criteria of which 97 (65%) were female. Only 33 of the newborns (22%) had head circumferences below the 3rd centile, while 76 (50%) had a head circumference equal to or greater than the 10th centile. A single health care provider diagnosed 103 of the newborns with microcephaly (68%).

The main finding of our study was that many newborns whose medical chart included a diagnosis of microcephaly had a head circumference measurement greater than the third centile for their sex and gestational age. This coding was largely responsible for the increase in microcephaly cases noted in British Columbia (BC). Our findings highlight the importance of consistently applying a valid and reliable diagnostic standard for microcephaly to ensure comparability of microcephaly rates. This was especially timely given the evolution of the Zika epidemic, which resulted in heightened awareness of microcephaly and necessitated high quality surveillance of this congenital anomaly.

The high prevalence of female newborns diagnosed with microcephaly may be attributable to the newborn head circumference reference chart used by health care providers. The current reference chart used by BC health care providers is not sex-specific and based on a notably dated study (5). In contrast, the Intergrowth-21st chart was developed in 2014 and is sex-specific.

Continued education of health care providers involved in the first physical examination of the newborn infant is necessary to ensure the accuracy of these data. The development and implementation of a valid and reliable national diagnostic definition of microcephaly, relying upon the use of standardized sex- and gestational age-specific head circumference growth charts, will improve the accuracy of medical diagnoses and the quality of surveillance of microcephaly in BC and throughout Canada.

Conflict of Interest Declaration: The authors have no conflicts of interest to declare.

Disclaimers: None.

Source of Support: JAH was supported by New Investigator awards from the Canadian Institutes of Health Research and the Michael Smith Foundation for Health Research.

Funding: None.

Contributors: QC, CX, and LL developed the study methodology. QC and CX completed the data analysis and authored the initial manuscript. LL, KD, SM, and TJ acquired the data. JH, LL, KD, SM, and TJ assisted with interpreting the data. Each of the authors revised the manuscript critically for important intellectual content. QC and SC completed the manuscript revisions.