Background
In 2016, the World Health Organization (WHO), for the fifth time in its history, declared a Public Health Emergency of International Concern due to the recognition of the Zika Virus Congenital Infection [
1]. After a “pandemic that surprised the world” [
2], several studies reported an association between Zika virus (ZIKV) infection during pregnancy and congenital abnormalities [
3‐
6]. Microcephaly is considered the “tip of the iceberg” in Congenital Zika syndrome (CZS), which defines a more complex spectrum of anomalies related to ZIKV congenital infection [
7,
8]. When present, microcephaly indicates a neurogenesis failure that varies in severity [
9,
10].
Brazil saw the largest outbreak of ZIKV infection and was the first country to investigate the relationship between ZIKV congenital infection and microcephaly. Between November 2015 and November 2018, almost 17,000 suspected cases of CZS were reported to the Brazilian Ministry of Health. Of those, 2819 were confirmed cases – either tested by laboratory methods or based on clinical-epidemiology evidence [
11]. Brazilian data revealed a frequency of microcephaly up to 24 times higher following Zika virus infection during pregnancy (PZIK) [
12]. In 2016, a study that reviewed data from the 2013–2015 outbreak of ZIKV in French Polynesia estimated a microcephaly risk ratio of 53.4, caused by ZIKV infection in the first trimester of pregnancy [
13]. In the Hawaiian Islands, one of the territories closest to French Polynesia, there was a threefold increase in the microcephaly rate between 2005 (4.8 cases per 10,000 total births in 2005) [
14] and the 2007–2013 period (14.7 per 10,000 total births) which coincided with the known outbreaks of ZIKV in the Pacific that started in 2007 [
13,
15,
16]. Worldwide, a systematic review estimated a prevalence of microcephaly of 2.3% among all ZIKV infections during pregnancy [
17].
Although the association between PZIK and microcephaly is considered a “scientific consensus” [
6,
18], variations on the risks within geographical areas and population groups have been observed [
17,
19]. It has been discussed that some factors may act as effect modifiers, increasing the risk of neurological damage [
20]. However, there is still a lack of evidence on cofactors or component causes that act as associated risks or preventive factors in the development of birth defects [
21‐
23]. To address this gap in knowledge, this systematic review and meta-analysis aim to identify maternal and fetal prognostic factors associated with microcephaly in fetuses and newborns from mothers infected with ZIKV during gestation.
Methods
Protocol and registration
The systematic review protocol was registered on February 21, 2018, in the PROSPERO (International Prospective Register of Systematic Reviews) database under the number CRD42018088075 [
24]. The Preferred Reporting Items for the Systematic Reviews and Meta-Analysis (PRISMA) [
25] checklist was filled out and can be found in the Additional Table
1.
Table 1Characterization of the selected studies
Aragão, MFVV et al. (2017) [ 26] | Case-control | Brazil | Dec 2015 – Nov 2016 | “to review neuroimaging of infants to detect cases without microcephaly and compare them with those with microcephaly” | Laboratory evidence: ZIKV IgM in cerebral spinal fluid and/or serum samples | Satisfactory |
Schaub, B et al. (2017) [ 27] | Case-control | Martinique | Jan 2016 – Nov 2016 | “to describe the early ultrasound markers and progression of the fetal cerebral insults during the pregnancy” | Laboratory evidence: ZIKV RNA (RT-PCR) or ZIKV IgM or IgG in serum, amniotic fluid, placenta, amnion, cerebrospinal fluid, or brain samples | Satisfactory |
Krow-Lucal, ER et al. (2018) [ 28] | Case-control | Brazil | Aug 2015 – Feb 2016 | “to assess the association of microcephaly and Zika vírus” | Laboratory evidence: ZIKV IgM in blood samples. Presumed infection also acceptable. | Satisfactory |
Honein, M A et al. (2017) [ 29] | Cohort | USA | Dec 2015 – Sep 2016 | “to estimate the preliminary proportion of fetuses or infants with birth defects after maternal Zika virus infection by trimester of infection and maternal symptoms” | Laboratory evidence: ZIKV RNA (rRT-PCR), ZIKV IgM (PNRT ≥10) and either a DenV- IgM or a DenV PRNT< 10 (or both) in serum, placenta or other tissue samples | Good |
Kumar, M et al. (2016) [ 30] | Case-control | USA | 2009–2012 | “to find a link between ZIKV infection and babies born with microcephaly” in Hawaii | Laboratory evidence: ZIKV IgM and IgG in serum samples | Good |
Brasil, P et al. (2016) [ 31] | Cohort | Brazil | Sep 2015 – May 2016 | “to describe clinical manifestations in mothers and repercussions of acute ZIKV infection in infants” | Laboratory evidence: ZIKV RNA (RT-PCR) in serum and/or urine samples | Good |
Pomar, L et al. (2017) [ 32] | Cohort | French Guiana | Jan 2015 – Jul 2016 | “to establish the incidence of fetal central nervous system (CNS) anomalies (including microcephaly), signs of congenital infection and fetal loss in pregnant women infected with Zika virus (ZIKV) and noninfected pregnant women in western French Guiana” | Laboratory evidence: ZIKV RNA (RT-PCR) or ZIKV IgM or PRNT in serum, placenta, urine, amniotic fluid and fetal samples | Satisfactory |
Sanz Cortes, M et al. (2018) [ 33] | Cohort | Colombia | Dec 2015 – Jul 2016 | “(1) to assess the prevalence of microcephaly and the frequency of the anomalies that include a detailed description based on ultrasound and magnetic resonance imaging in fetuses and ultrasound, magnetic resonance imaging, and computed tomography imaging postnatally, (2) to provide quantitative measures of fetal and infant brain findings by magnetic resonance imaging with the use of volumetric analyses and diffusion-weighted imaging, and (3) to obtain additional information from placental and fetal histopathologic assessments and postnatal clinical evaluations” | Laboratory evidence: ZIKV IgM or IgG in serum samples, if positive ZIKV RNA (RT-PCR) in serum and amniotic fluid offered | Satisfactory |
Shiu, C et al. (2018) [ 34] | Cohort | USA | Jan 2016 – Dec 2016 | “to assess clinical outcomes and challenges associated with Zika virus screening and testing” | Laboratory evidence: ZIKV RNA (rRT-PCR), ZIKV IgM in serum, placenta or other tissue samples | Satisfactory |
Vargas, A et al. (2016) [ 35] | Case series | Brazil | Aug 2015 – Oct 2015. | “to describe the first cases of microcephaly possibly related to Zika virus in live born babies reported in the Metropolitan Region of Recife, Pernambuco State, Brazil” | Presumed infection | Satisfactory |
França, G V A et al. (2016) [ 36] | Case series | Brazil | Nov 2015 – Feb 2016** | “to describe these newborn babies in terms of clinical findings, anthropometry, and survival” | Laboratory evidence: ZIKV RNA (RT-PCR) or ZIKV IgM or IgG in serum samples.Presumed infection also acceptable * | Low |
Ventura, L O et al. (2017) [ 37] | Cross-sectional | Brazil | May 2015 – Dec 2015 | “to describe the visual impairment associated with ocular and neurological abnormalities in a cohort of children with congenital Zika syndrome (CZS)” | Laboratory evidence: ZIKV IgM in cerebral spinal fluid samples | Good |
The search strategy aimed to find pertinent data in theses and dissertations in addition to published studies. The following databases were searched by a university librarian on January 8, 2019: MEDLINE via OvidSP (1946 onward), Embase via OvidSP (1947 onward), Cochrane Central Register of Controlled Trials or “Cochrane CENTRAL” via OvidSP (1991 onward), the Cumulative Index of Nursing and Allied Health Literature or “CINAHL” via EBSCOhost (1981 onward), Web of Science Core Collection (1900 onward), ProQuest Dissertations and Theses Global (1861 onward), and Latin American & Caribbean Health Sciences Literature or “LILACS” (1982 onward). The full electronic search strategies for all databases can be accessed via QSpace, Queen’s University’s research repository service [
http://hdl.handle.net/1974/24246]. No language or date restrictions were applied. The reference list of systematic reviews and reports were searched for additional studies.
We searched ProQuest Dissertations and Theses and thesis databases from Brazil, Colombia, Canada, USA, and Europe on January 8, 2019, using the terms “zika” or “zikv” or “zyca” or “zyka”. The authors of editorials, correspondence, and conference abstracts that met the inclusion criteria were searched online for original papers.
Additionally, we identified studies that we believed concern PZIK, the prognostic factors of interest, and microcephaly data, but had not published results (i.e. conference abstracts). The first and last authors’ curricula were screened online, and the first authors of each of these studies were contacted. In total, we contacted 17 study investigators at least twice. Responses to seven of the requests were received, all of which informed us of an inability to provide full results.
Eligibility
We included published data from original research that studied microcephaly as a congenital effect of ZIKV in humans. We included randomized controlled trials, prospective, retrospective or descriptive cohorts, case series (only the case series whose primary data allowed the formation of groups that were prospectively compared regarding outcome), and cross-sectional and case-control studies that could answer the review question.
Studies were excluded if they: (1) were not in humans; (2) did not report our primary objectives; (3) were in vitro/cell studies; (4) were not original research, such as literature reviews, guidelines and manuals, protocol summaries, editorials, opinion pieces, or book chapters; (5) duplicated publications from the same sources (data from the same sources, but published in different papers); or (6) were case reports, epidemiological analyses/bulletins, or case series that included outcomes for only one group.
To avoid double-counting in cases where the same individuals or data were reported in more than one publication, we evaluated publications from the same author or study place (hospital-based, city-based, or state-based population). If there was possible duplication, we used the publication with the most complete information available to extract the data.
Initial triage of articles was based on the screening of titles and abstracts by two independent evaluators to assess the relevance to the objective of the review. Then, a full-text read of the selected studies was conducted to determine inclusion according to eligibility criteria and the study question. Data extraction and quality assessment were conducted by two independent authors using a standardized instrument. Disagreements at any stage were resolved by consensus or discussion with a third author.
Data extracted included the name of first author, year of publication, period of study, country, language, study design, size, population, and any potential patient-related prognostic factors. These included demography (maternal age, ethnicity, deprivation, education level, marital status, social support); lifestyle factors (smoking habits, drug use); patient history (including comorbidity, family history); symptom type; health care usage (including screening); presenting behaviour; symptom knowledge; and characterization of outcomes.
The methodological quality and risk of biases in the studies were assessed by two independent reviewers using the Newcastle-Ottawa Quality Assessment Scale (NOS) [
38]. The studies that received 7 stars or more were considered to be of high quality, those that received 6–5 stars were considered to be of satisfactory quality, and those with 4 or less stars were considered to be of low quality [
39].
Synthesis of results and summary measures
To ensure comparable and accurate results to perform the meta-analysis, we contacted the corresponding authors of all the included studies in the systematic review up to three times to ask whether they would be willing to provide results. A standardized table (Additional Table
2) was sent to all the authors to complete using their primary data, considering as a population of interest only cases of PZIK, and the outcome of interest being the presence or absence of microcephaly.
Table 2Population characteristics of the studies included in the meta-analysis
Aragão, MFVV et al. (2017) [ 26] | U* | 19 | 16 | – | – | – | – | |
Schaub, B et al. (2017) [ 27] | 14 | 14 | 9 | 5/9 | 4/3 | 9/9 | – | 26.78 (6.33) |
Krow-Lucal, ER et al. (2018) [ 28] | U* | 115 | 43 | – | – | – | – | |
Honein, M A et al. (2017) [ 29] | 442 | 55 | 18 | – | – | – | – | |
Kumar, M et al. (2016) [ 30] | 4 | 4 | 3 | 1/3 | 0/1 | 3/3 | 1/1 | 27 (5.57) |
Brasil, P et al. (2016) [ 31] | 134 | 134 | 4 | – | – | – | – | |
Pomar, L et al. (2017) [ 32] | 301 | 278 | 28 | 15/28 | 126/250 | 27/28 | 244/250 | 28.08 (7.75) |
Sanz Cortes, M et al. (2018) [ 33] | 12 | 9 | 7 | – | – | – | – | |
Shiu, C et al. (2018) [ 34] | 8 | 87 | 5 | – | – | – | – | |
Vargas, A et al. (2016) [ 35] | U* | 40 | 40 | 20/43 | 5/14 | 12/43 | 2/14 | 23.5 (8) |
França, G V A et al. **(2016) [ 36] | 1501 | 602 | 330 | 244/567! | 221/691! | 495/567! | 591/691! | 24.79 (6.668) |
Ventura, L O et al. (2017) [ 37] | U* | 32 | 29 | 54/148 | 9/148 | – | – | 27.36 (7.28) |
We asked about maternal and infant data, considering the following variables: population size (number of newborns/fetuses); maternal age; maternal ethnicity; maternal schooling; maternal symptoms compatible with ZIKV infection during gestation; maternal smoking habits and/or use of alcohol and/or other drugs; presence of comorbidities in the mother; trimester of pregnancy when infection occurred; prior yellow fever vaccine exposure; prior exposure to other vaccines; fetus sex; and newborn gestational age at birth. The results provided by the authors were also used to complement the description of the studies in the qualitative synthesis.
Since the characterization of microcephaly can vary among countries, for the meta-analysis we used the definition recommended by WHO: the measure of the head circumference (HC) of less than two Standard Deviations (SD) below the average for sex and gestational age [
40]. Also, as a reliable laboratory diagnostic test of ZIKV was not available during the first part of the outbreak, we used the ZIKV case definition as reported by the study. All the authors were advised to follow these criteria when providing the data.
Meta-analyses were performed if sufficient results were available. Summary measures [Relative Risk (RR) and Odds Ratio (OR)] were calculated for each different variable included. The random effects model was used when the I2 test demonstrated a large degree of heterogeneity between studies (I2 higher than 0.5). When the heterogeneity between studies was equal or lower than 0.5, fixed-effect model was used for the analysis. We used the RevMan software to execute the calculation of summary measures and heterogeneity of the studies, and to generate the figures of the meta-analysis (Forest plots).
Discussion
To our knowledge, this is the first systematic review to evaluate maternal and fetal prognostic factors that may contribute to the presence of microcephaly in newborns and fetuses when the mother was infected with ZIKV during pregnancy. Our meta-analysis showed that infection in the first trimester of pregnancy may increase the risk of microcephaly by 41% when compared to other trimesters, and female fetuses have a lower risk of developing microcephaly. Our study did not show differences between groups regarding maternal age, ethnicity, presence of comorbidities, and consumption of alcohol or other substances during gestation.
In relation to trimester of infection, other STORCH infections also confer differential risk of congenital defects according to the stage of pregnancy in which infection occurs [
41‐
44]. These events are related to both the development of the central nervous system (CNS) and the fetus immune response [
45]. Even though ZIKV infection in the second and third trimester of pregnancy seemed to be a lower risk compared to ZIKV infection in the first trimester, it is important to highlight that this infection carries a risk for the development of microcephaly and other adverse pregnancy outcomes throughout the full duration of pregnancy [
4,
46,
47]. There is still a lack of knowledge on the magnitude of the risk of newborns infected by ZIKV developing microcephaly later in childhood [
48,
49].
The relationship between fetal gender and adverse pregnancy outcome is controversial [
50,
51]. The male sex, especially in low-risk pregnancies, seems to have an effect on adverse pregnancy outcomes [
52] such as preterm births [
53,
54] and stillbirths [
55]. During the ZIKV outbreak on Yap Island, a higher prevalence of IgM antibody against ZIKV was found in men as compared to women [
15]. It should be pointed out that our meta-analysis (point estimates) of both prospective and retrospective studies showed that females had a lower risk of developing microcephaly than males. However, the OR was not significant (0.54, IC95% 0.08, 3.66), probably due to the small number of individuals included in the retrospective studies in the meta-analysis. Additionally, the studies of Pomar et al. [
32] and Vargas et al. [
35], both of satisfactory quality, did not show a statistically significant relation. Nonetheless, our findings reinforce previous studies that support a male-biased incidence in infectious diseases, [
51,
56] pointing out a probable relationship between microcephaly and fetal sex, with males being at a higher risk than females.
It is important to stress that the symptoms of ZIKV infection, both in men and women, are often mild and infrequent [
4,
15,
57,
58]. There is still uncertainty about whether symptoms can be addressed as reliable indicators of vertical transmission or disease severity [
59‐
61]. Other infections that lead to congenital malformation, such as cytomegalovirus, also have a high number of asymptomatic cases, but, when present, symptoms might indicate an adverse outcome [
62].
Our meta-analysis results suggest an association of microcephaly with symptoms, probably restricted by the heterogeneity of the studies. Even so, this association may be influenced by recruitment and selection, given that tree studies [
27,
36,
37] performed the recruitment based only on the infants, thus increasing the microcephaly rate compared to the microcephaly rate observed in studies that included all pregnant women infected with ZIKV. Also, asymptomatic ZIKV infection in pregnant women could decrease the sensibility of the microcephaly detection, specifically in areas where ZIKV surveillance was inadequate. Furthermore, two studies [
35,
36] used cases of presumed ZIKV infection that were diagnosed based on clinical-epidemiological evidence and not laboratory tests. In this sense, although the low viremia induced by ZIKV infection increases false negative results [
63], the most reliable diagnostic test is RT-PCR in the newborn sample.
Regarding socioeconomic, demographic, and environmental factors linked to adverse pregnancy outcomes such as maternal ethnicity, low family income, maternal schooling, and maternal age, this review was unable to determine if these can act as additional prognostic factors in microcephaly development, as only a few of the included studies assessed them. The risk ratio – point estimates of maternal ethnicity (non-white, RR 0.91, CI 95% 0.77, 1.08) and the absence of alcohol, tobacco, and/or other substance consumption during pregnancy (RR 0.84, CI 95% 0.55, 1.29) suggest that these are protective factors for microcephaly, yet differences were not statistically significant.
Our results suggest that maternal age and ethnicity are not prognostic factors for microcephaly. On the other hand, the presence of comorbidities and substance consumption during pregnancy may have been influenced by the small sample size of the studies included in the meta-analysis, restricting our results, since those factors have been reported in the literature as being capable of interaction with other prognostic factors increasing the risk of adverse outcomes [
64‐
67].
The inconsistency between the studies also influenced the analysis concerning health factors such as microcephaly history in the family, maternal comorbidities, and nutritional status. Nevertheless, our review was able to indicate that the presence of comorbidities might raise the risk of microcephaly, although we were not able to find statistically significant differences. The presence of antibodies for other flaviviruses such as dengue and yellow fever, which may act as modulators of adverse outcomes, as well as prior exposure to the yellow fever vaccine, were not explored in most of the studies, thus hampering the analysis of possible effect modifiers. This is likely related to the urgency of assessing the relationship between PZIK and intergenerational effects [
68‐
70] at the time when most of the studies were conducted.
Regarding the methodological quality of the studies included in the meta-analysis, França et al. [
36] was the only low-quality study, but because of the size of its population, this study received the highest weight among the prospective studies included. This study used secondary data and was able to collect information on 1258 ZIKV-infected pregnant women – the highest number of individuals in the included studies. In the same perspective, Pomar et al. [
32] was deemed to be as of satisfactory quality and included 278 ZIKV-infected participants. Their confidence interval was often large, as it included the estimates found in the other studies, reducing the I
2 of the meta-analysis of prospective studies. On the other hand, Kumar et al. [
30], a retrospective study, included four cases and was assessed as good quality. This study also had a large confidence interval, incorporating the estimates of Schaub et al. [
27], therefore reducing the I
2.
The complexity of the prognostic factors associated with microcephaly due to ZIKV infection during pregnancy and the broader socioeconomic context in which it occurs, including an increased social and economic impact caused by the Congenital Zika Syndrome, must be considered when designing preventive programs or providing health care. Although this review and meta-analysis only approached individual-level factors, the most appropriate interventions might be on the ecological level, especially in low-income countries addressing pathways of infection by mosquito control and protective measures against sexual transmission.
Limitations of this systematic review
Common sources of bias in any meta-analysis are publication bias and heterogeneity between studies. We assessed the publication bias by reviewing the grey literature, looking for recent published manuscripts by authors who published theses and presented conference abstracts, and contacting them. Despite this effort, no additional data were found. Thus, the meta-analysis only included published studies. Because of the small number of included studies, we were not able to perform tests to detect publication bias. Since nonsignificant results have a decreased likelihood of publication, we believe that the included studies might be reporting a higher association between PZIK and birth defects than may generally be the case. However, in terms of associated prognostic factors for the development of microcephaly – our exposures of interest –, it is unlikely that publication bias would affect our results. Regarding summary measures, we understand that our data also reflect the number of participants in each study (and not the methodological quality of them), as is observed in all unweighted summary measures.
Concerning the design of the studies, the case-series studies did actually include the entire available populations, using surveillance strategies, tending to have the characteristics of a descriptive cohort study. However, due to the small number of participants, they were designed and assessed as case-series. Moreover, the small number of individuals included in the retrospective studies restricted the power of the meta-analysis.
Specific limitations of our review were the non-inclusion of in vitro studies in the eligibility criteria – some authors have reported that the ZIKV strain can be related to varied outcomes [
71,
72] –, and the inherent differences between studies, especially in regard to the ZIKV infection definition and data collection. There is still a lack of consensus on diagnostic strategies for ZIKV [
63], and the studies identified in this review used different tools for this purpose. Also, as cited, most of the studies did not assess the possible effect modifiers that this review was seeking to analyse.
It is necessary to point out that the systematic review might have some duplication of cases, as the study of França et al. [
36] included all the notified cases in Brazil from October 2015 to February 27th, 2016. Other studies using this time frame may have used individuals as they were notified in the national system. However, the data used in the meta-analysis does not overlap in time or place, so this was not considered a hurdle for our results.
Our study design was formulated in such a way that we would not exclude any study after its quality assessment. The methodological quality of included studies was considered predominantly satisfactory and four studies were assessed as good quality [
28‐
30,
37]. The only study [
36] that scored as low quality had the largest population and, therefore, the smallest confidence intervals. Although the weight of the study may have influenced our summary measures, the estimate of the other prospective studies [
32,
35] went in the same direction, reinforcing our conclusions.
Finally, our findings are supported by observational studies only. The small number of included studies reflected a lack of adequate studies in the literature for the investigation and understanding of the prognostic factors related to the association of microcephaly and congenital zika infection. Additionally, since there were different case definitions for ZIKV infection across the studies and laboratory tests are still not fully reliable, the studies may have included non-ZIKV infections in the analysed groups, therefore introducing a possible measurement bias which could sway our results to either side, but most probably towards a null hypothesis. For these reasons, our results should be interpreted cautiously so as not to influence prenatal care or health surveillance strategies used to detect and prevent new cases.
Conclusions
This systematic review and meta-analysis found that the male sex, the occurrence of ZIKV infection in first trimester, and symptomatic infection increase the risk of microcephaly. The available evidence does not establish maternal age and ethnicity as prognostic factors. The prognostic effect of previous antibodies for other flaviviruses, family history of microcephaly or other congenital abnormality, family income, schooling level, and civil status remain unclear. These findings should be interpreted cautiously because ZIKV is an emergent disease and its effects are still under study. Still, they can be used to reduce false alarms regarding maternal age and ethnicity as prognostic factors; to increase preventive strategies to ZIKV infection, especially in the first trimester; and to understand that, due to the lack of reliable diagnostic tests, specifically after the viremic period, the presence of symptoms may be a good indicator of ZIKV infection, and that pregnant women reporting them should receive more attentive antenatal care.
This study only reviewed prognostic factors for microcephaly related to ZIKV, but the effects of Congenital Zika Syndrome is inconstant and other factors may be associated with the different outcomes of ZIKV infection during pregnancy. Although there has been a high demand for and a high production of studies to understand the pathogenicity of ZIKV in the last three years, the studies conducted show a high heterogeneity in both methods and data collection. This highlights the need for dialogue between researchers seeking to investigate an emergent problem in public health. Future research needs to homogenize definitions of relevant outcomes, test hypotheses of potential disease modulators, include other aspects of the Congenital Zika Syndrome other than microcephaly, and include other variables related to birth defects.
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