Fallacies of international and national comparisons of disease occurrence in the epidemiology of neural tube defects

Describing the Prevalence of Neural Tube Defects Worldwide: A Systematic Literature Review

BACKGROUND

Folate-sensitive neural tube defects (NTDs) are an important, preventable cause of morbidity and mortality worldwide. There is a need to describe the current global burden of NTDs and identify gaps in available NTD data.

METHODS AND FINDINGS

We conducted a systematic review and searched multiple databases for NTD prevalence estimates and abstracted data from peer-reviewed literature, birth defects surveillance registries, and reports published between January 1990 and July 2014 that had greater than 5,000 births and were not solely based on mortality data. We classified countries according to World Health Organization (WHO) regions and World Bank income classifications. The initial search yielded 11,614 results; after systematic review we identified 160 full text manuscripts and reports that met the inclusion criteria. Data came from 75 countries. Coverage by WHO region varied in completeness (i.e., % of countries reporting) as follows: African (17%), Eastern Mediterranean (57%), European (49%), Americas (43%), South-East Asian (36%), and Western Pacific (33%). The reported NTD prevalence ranges and medians for each region were: African (5.2-75.4; 11.7 per 10,000 births), Eastern Mediterranean (2.1-124.1; 21.9 per 10,000 births), European (1.3-35.9; 9.0 per 10,000 births), Americas (3.3-27.9; 11.5 per 10,000 births), South-East Asian (1.9-66.2; 15.8 per 10,000 births), and Western Pacific (0.3-199.4; 6.9 per 10,000 births). The presence of a registry or surveillance system for NTDs increased with country income level: low income (0%), lower-middle income (25%), upper-middle income (70%), and high income (91%).

CONCLUSIONS

Many WHO member states (120/194) did not have any data on NTD prevalence. Where data are collected, prevalence estimates vary widely. These findings highlight the need for greater NTD surveillance efforts, especially in lower-income countries. NTDs are an important public health problem that can be prevented with folic acid supplementation and fortification of staple foods.

Global Birth Prevalence of Spina Bifida by Folic Acid Fortification Status: A Systematic Review and Meta-Analysis

BACKGROUND

Birth defects remain a significant source of worldwide morbidity and mortality. Strong scientific evidence shows that folic acid fortification of a region's food supply leads to a decrease in spina bifida (a birth defect of the spine). Still, many countries around the world have yet to approve mandatory fortification through government legislation.

OBJECTIVES

We sought to perform a systematic review and meta-analysis of period prevalence of spina bifida by folic acid fortification status, geographic region, and study population.

SEARCH METHODS

An expert research librarian used terms related to neural tube defects and epidemiology from primary research from 1985 to 2010 to search in EMBASE and MEDLINE. We searched the reference lists of included articles and key review articles identified by experts.

SELECTION CRITERIA

Inclusion criteria included studies in English or French reporting on prevalence published between January 1985 and December 2010 that (1) were primary research, (2) were population-based, and (3) reported a point or period prevalence estimate of spina bifida (i.e., prevalence estimate with confidence intervals or case numerator and population denominator). Two independent reviewers screened titles and abstracts for eligible articles, then 2 authors screened full texts in duplicate for final inclusion. Disagreements were resolved through consensus or a third party.

DATA COLLECTION AND ANALYSIS

We followed Preferred Reporting Items for Systematic Reviews and Meta-Analyses, or PRISMA, abstracting data related to case ascertainment, study population, folic acid fortification status, geographic region, and prevalence estimate independently and in duplicate. We extracted overall data and any subgroups reported by age, gender, time period, or type of spina bifida. We classified each period prevalence estimate as "mandatory" or "voluntary" folic acid fortification according to each country's folic acid fortification status at the time data were collected (as determined by a well-recognized fortification monitoring body, Food Fortification Initiative). We determined study quality on the basis of sample representativeness, standardization of data collection and birth defect assessment, and statistical analyses. We analyzed study-level period prevalence estimates by using a random effects model (α level of < 0.05) for all meta-analyses. We stratified pooled period prevalence estimates by birth population, fortification status, and continent.

RESULTS

Of 4078 studies identified, we included 179 studies in the systematic review and 123 in a meta-analysis. In studies of live births (LBs) alone, period prevalences of spina bifida were (1) lower in geographical regions with mandatory (33.86 per 100,000 LBs) versus voluntary (48.35 per 100,000 LBs) folic acid fortification, and (2) lower in studies of LBs, stillbirths, and terminations of pregnancy in regions with mandatory (35.22 per 100,000 LBs) versus voluntary (52.29 per 100,000 LBs) fortification. In LBs, stillbirths, and terminations of pregnancy studies, the lowest pooled prevalence estimate was in North America (38.70 per 100,000). Case ascertainment, surveillance methods, and reporting varied across these population-based studies.

CONCLUSIONS

Mandatory legislation enforcing folic acid fortification of the food supply lags behind the evidence, particularly in Asian and European countries. This extensive literature review shows that spina bifida is significantly more common in world regions without government legislation regulating full-coverage folic acid fortification of the food supply (i.e., Asia, Europe) and that mandatory folic acid fortification resulted in a lower prevalence of spina bifida regardless of the type of birth cohort. African data were scarce, but needed, as many African nations are beginning to adopt folic acid legislation.

Neural tube defect in a 4000-year-old Egyptian infant mummy: a case of meningocele from the museum of anthropology and ethnography of Turin (Italy)

This paper reports a paleopathological study of a severe neural tube defect in an ancient mummy, more specifically, a meningocele in an Egyptian infant from the XI dynasty (2100-1955B.C.). This is one of the most ancient cases of meningocele in mummified human remains described in paleopathological literature. Prehistoric and early historic examples of severe congenital defects of the vertebral column and neural tube are rare, because of the precarious preservation conditions of ancient human remains. Further, since the majority are only the skeletal remains, paleopathological and paleoepidemiological analysis based on the observation of bones is even more difficult. Hence, it is not easy to investigate this disease in the past in all its complexities and true diffusion. The case presented here is peculiar, since it concerns a mummy with almost all soft tissues preserved, thus allowing us to describe the defect in an infant. Only targeted, minimally invasive examinations were performed. An anthropological investigation with helical CT scan and histological analysis was used to diagnose the defect and identify post-mortem transformation processes. The analyses confirmed the diagnosis of meningocele in an approximately six-month-old infant.

Epidemiology of neural tube defects

The epidemiological investigation of the common open neural tube defects (NTDs), anencephaly, and spina bifida, has a long history. The most significant finding from these past studies of NTDs was the identification of the protective effect of maternal, periconceptional supplementation with folic acid. Fortuitously, the association between folic acid and NTDs became widely accepted in the early 1990s, at a time when genetic association studies of complex traits were becoming increasingly feasible. The confluence of these events has had a major impact on the direction of epidemiological, NTD research. Association studies to evaluate genes that may influence the risk of NTDs through their role in folate-related processes, or through other metabolic or developmental pathways are now commonplace. Moreover, the study of genetic as well as non-genetic, factors that may influence NTD risk through effects on the nutrient status of the mother or embryo has emerged as a major research focus. Research efforts over the past decade indicate that gene-gene, gene-environment, and higher-order interactions, as well as maternal genetic effects influence NTD risk, highlighting the complexity of the factors that underlie these conditions. The challenge for the future is to design studies that address these complexities, and are adequately powered to detect the factors or combination of factors that influence the development of NTDs.

Zinc-deficient sprouting blight potatoes and their possible relation with neural tube defects

Maternal nutritional zinc deficiency is blamed in the pathogenesis of neural tube defects. In animal and plant domains zinc is required for growth and development. The objective of the present study was to show that sprouting blighted potato tuber is zinc deficient. In five potato varieties, zinc was measured by atomic absorption spectrophotometry in wet-ashed paired slices of edible potato tuber and in its peel, in blighted potato tuber and in its sprout. Zinc contents were measured as the mean (+/- SEM) and the following values were found, 0.388 +/- 0.036, 0.623 +/- 0.059, 0.550 +/- 0.030 and 1.089 +/- 0.181 mg per 100 g wet weight, respectively. In conclusion, we believe that long-term consumption of zinc-depleted, blight potato tuber by pregnant woman could be potentially teratogenic with the consequent birth of a baby with neural tube defects.

Prevalence of anencephaly in the region of Rijeka, Croatia

This retrospective study determines the prevalence of anencephaly in the region of Rijeka, Croatia. Records of all spontaneous and therapeutic abortions terminated in medical institutions, all fetuses weighing more than 500 g or more than 22 weeks gestation (whether the product of abortion, therapeutic termination, stillborn or liveborn) and infants who died in the first year of life in the region of Rijeka, Croatia, during the 1963-2000 period were reviewed. There were 135,451 births; 22 of them were anencephalics (19 stillborn), which comprises 0.2% of all births and 2.1% of stillbirths. Annual prevalence of anencephaly varied in range from 0.00 to 7.42 per 10,000 births. In two cases pregnancy was electively terminated after ultrasonographic diagnosis of anencephaly. Fifteen anencephalics were female, six were male, and in one case sex was undetermined due to aplasia of genital organs. Associated congenital malformations were detected in 18 anencephalics. The importance of establishing national and international registers of congenital malformations in all countries is stressed. The authors suggested that the setting of obligatory reporting of all congenital malformations would be the first step toward this practice in Croatia, as well as in other developing countries.

Association between lower hair zinc levels and neural tube defects

Though folic acid supplementation has reduced the incidence of Neural Tube Defects (NTD), NTD still constitutes one of the important congenital malformations having wide medical, social and ethical implications. Zinc deficiency has been reported to produce NTD in animals. This study was designed to evaluate zinc status of the newborn babies with NTD and their mothers. Eighty newborn babies with NTD and their mothers served as cases. Eighty apparently normal newborn babies and their mothers served as controls. Serum and scalp hair zinc levels were analyzed by atomic absorption spectrophotometry. The mean (+/- SD) serum and hair levels in normal mothers were 74.1 +/- 4.1 micrograms/dl and 142.3 +/- 8.0 micrograms/g respectively. The mean (+/- SD) serum and hair levels of the mothers who delivered NTD babies were 75.7 +/- 5.6 micrograms/dl and 129.9 +/- 5.3 micrograms/g respectively. The mean (+/- SD) serum and hair levels in normal newborn babies were 77.8 +/- 5.3 micrograms/dl and 188.8 +/- 6.2 micrograms/g respectively. The mean (+/- SD) serum and hair levels in NTD babies were 80.1 +/- 12.9 micrograms/dl and 174.2 +/- 10.7 micrograms/g respectively. The hair zinc levels of the affected babies and their mothers were significantly lower (P < 0.001) than the controls. This study has found association between NTD and decreased hair zinc levels and large population based studies are recommended to confirm the association between zinc and NTD and to investigate whether zinc supplementation would reduce the overall incidence of NTD.

Prevalence of neural tube defects in Cape Town, South Africa

The aim of the study was to document the frequency of neural tube defects (NTD) over a 20-year period in Cape Town and to determine the effects of race, gender, maternal age, parity and season of conception on the prevalence. Multiple sources of ascertainment were used, including all maternity hospital records, neurosurgical and spinal defects clinic data, as well as those from the Human Genetics Department and Fetal Abnormality Group. The prevalence rates for NTD fluctuated between 1.74 and 0.63 per 1,000 births, but showed no significant trends over the 20-year period. Prevalence rates were highest for the white population group of 2.56 per 1,000 births compared to 0.95 per 1,000 for blacks and 1.05 per 1,000 for those of mixed ancestry. The higher rates in the whites, who are of British and European extraction and belong to the more affluent section of the community, would suggest that the possible effects of nutrition and infection are overshadowed by genetic factors. There was a female preponderance for both spinal bifida (M:F ratio 0.89) and anencephaly (M:F ratio 0.67). The highest NTD rates were found at both ends of the maternal age range (< 20 years and > 35 years of age). The prevalence was highest at the extremes of birth order (1.65 and 1.58 for birth order 1 and > 7, respectively, and 0.56 and 0.45 for birth order 5 and 6, respectively). A seasonal variation occurred which differed from that reported for the Northern Hemisphere and may reflect local climatic conditions.

The prevalence of anencephalus and spina bifida in New Zealand

Despite the plethora of epidemiological research conducted on anencephalus and spina bifida, few of the studies have used multiple source case ascertainment and controlled for the effect of possible confounding factors. This paper reports the results from a study of the relationships between various risk factors and the prevalence of anencephalus and spina bifida in New Zealand during 1978-82, using case data obtained from multiple sources and a national cohort of births as the denominator. The rates of anencephalus and spina bifida in New Zealand were 0.78/1000 and 0.94/1000 total births, respectively. The rate of a neural tube defect (NTD) birth for Maori parents was less than for their non-Maori counterparts. Paternal ethnic origin and maternal ethnic origin made similar contributions to the model of anencephalus rates, but the results suggest that paternal ethnic origin is a less important risk factor in the prevalence of spina bifida. The rate of both NTD was high among female infants and low among births to women born in countries other than the British Isles and New Zealand. The rate of anencephalus showed a distinct north-south gradient, but there was no evidence of effects for maternal or paternal age, parity, urban-rural place of residence, nuptiality, social class or season of birth in the prevalence of either NTD in New Zealand.

Neural tube defects after infertility treatment: a review

OBJECTIVE

To investigate whether an association exists between ovulation induction and neural tube defects (NTDs).

MATERIALS AND METHODS

Risk estimations in the medical literature were identified through Medline, and validity and power were assessed. Large in vitro fertilization-embryo transfer (IVF-ET) registries represent another source of information. The total number of NTDs and the total number of fetuses were computed from five registries. These data were expressed as proportions and compared with data from the general population.

RESULTS

Only one study could be identified as both valid and powerful, through literature review. This case-control study concluded there was no association between ovulation induction and NTDs. The pool of IVF-ET registry data represents another powerful epidemiologic tool. Analysis of the registry data confirms the findings of the case-control study.

CONCLUSIONS

Ovulation induction does not seem to represent a risk factor for NTDs in the offspring.

Teratological aspects of dental amalgam

The teratogenic effect is determined by four factors: (1) the agent, (2) the dose, (3) the stage of embryonic development, and (4) the genetic constitution of the embryo. The first two factors are of particular interest and warrant further comment. It should be emphasized that the mercury released from dental amalgam is mainly metallic mercury vapor. The dose of mercury vapor from dental amalgam fillings in the order of 5 micrograms/day is very low compared with the doses in a teratological study and is not likely to exceed the threshold necessary for a teratogenic effect to occur. The concentration of the teratogen at the target tissue is determined not only by the degree of placental transfer but also by other factors, such as the distribution within the maternal organism, the affinity to the fetal liver and blood, the hematocrit value, and the passage through the ductus venosus. These factors might help to explain toxicological mechanisms and species differences and have to be considered if the results of animal experiments are to be extrapolated to human conditions. Neither epidemiological data nor animal experimental data indicate that the release of metallic mercury vapor from dental amalgam therapy should cause teratogenic effects. A comparison with the incorporation of the fetotoxic methyl mercury might be justified.

Seasonal preovulatory overripeness ovopathy: a reappraisal of the concept

Seasonal Preovulatory Overripeness Ovopathy (SPOO) is one explanation for seasonal variations in the occurrence of congenital anomalies. This concept entails that during some seasons the preovulatory phase of the development of the human egg is lengthened, causing congenital anomalies. Although the concept of SPOO provides an innovative explanation for the etiology of congenital anomalies, its dissemination in the scientific world has been limited. The aim of this paper is to explain this limited dissemination through a critical examination of the concept itself. To do this we analyzed the degree in which this concept has a sufficient informative content and explanatory power, following criteria formulated by Popper. It is concluded that descriptions of the concept of SPOO are ambiguous. Thus the concept cannot be tested and has a limited informative content. The explanatory power of the SPOO concept remains unclear, in itself and relative to competing explanations. Further one of its basic assumptions, seasonal variation of the length of the preovulatory phase due to an endogenous mechanism, has been refuted. Future research should better be directed to causes of preovulatory overripeness ovapathy (OO) other than seasonality. This preovulatory OO offers an innovative view on the etiology of congenital anomalies.