Summary

1.INTRODUCTION

Enormous efforts have been devoted to clarify determinants of neurodevelopment of humans. During the prenatal period, exposure of fetuses to heavy metals [lead (Pb), arsenic (As) and cadmium (Cd)] or deficiency of essential trace elements [zinc (Zn), selenium (Se) and copper (Cu)] were reported to be associated with prenatal and postnatal neurodevelopment. Maternal psychological conditions and in utero growth environment also affected the neurodevelopment during prenatal and postnatal period. During the postnatal period, growth environments (e.g., presence of appropriate toys, siblings, attitude of parents) are the prominent determinants of neurodevelopment.

The present study was conducted in Chitwan valley of lowland (Terai) Nepal. The author targeted this area because he suspected that the local ecosystem of the area was contaminated with Pb, Cd and As. Since Chitwan is located in the junction where a highway from Kathmandu connects to east-west highway, numerous vehicles are supposed to emit lead to the environment. In addition, for the Chitwan people, rice is the staple foods as well as the source of Cd and Pb, and the area is reported to be one of the "hot spots" of As contamination. The Zn deficiency reported to be the health problem in lowland Nepal.

Numbers of studies have evaluated the effect of cord blood lead on later neurodevelopment, but most of the studies evaluate such effect in developed countries. The dose-response relationship and time lag effect remains inconclusive for lead. Limited number of studies evaluated the effect of arsenic, cadmium, and copper on neurodevelopment in humans and the effect of deficiency of trace elements (Zn, Se and Cu) on later neurodevelopment (from birth to 6 months). The objectives of the present study were (1) to evaluate the in utero exposure to heavy metals (Pb, As and Cd) and trace elements (Zn, Se and Cu), pregnancy maternal conditions, and postnatal growth environments (analyzed as explanatory variables), (2) to evaluate the neurodevelopmental indicators of babies at birth [i.e., Brazelton neonatal behavioural assessment scale, third edition (NBAS III)] and at 6 months from birth (i.e., Bayley scale of infant development, second edition (BSID II) (analyzed as response variables), and (3) to investigate the association between in utero heavy metals and trace elements exposures, prenatal maternal conditions or postnatal growth environment, and neurodevelopmental indicators after birth.

2.METHODS

The data analyzed in the present study were collected in May 2008, September to October 2008, and March 2009. After the pilot study in May 2008, one hundred pregnant mothers were recruited in September to October 2008, and mother-newborn pairs were investigated just after the delivery. Mother-infant pair were again investigated after 6 months (n=98) in March 2009. Neurodevelopmental indicators were evaluated at birth (by NBAS III scores, and at six months from birth (by BSID II scale). Heavy metals, and trace elements in cord blood (n=94) were analyzed. The information for maternal psychological conditions and in utero growth environment, and postnatal growth environments were also evaluated using Rosenberg self esteem scale (RSES), birth weight and home observation of measurement of environment scale (HOME scale), respectively . Confounding variables were controlled. Cord blood samples, transported to the laboratory in Japan in October 2008, were analyzed by using ICP-MS in May 2009. Univariate analyses were conducted to examine the associations between explanatory variables (heavy metals and trace elements in cord blood, pregnancy maternal conditions, and postnatal growth environments) or confounding variables and response variables (NBAS III cluster scores at birth and BSID II cluster scores at 6 months after birth). Explanatory variables or confounding variables that were associated with response variables at p-value less than 0.1 were forcedly entered to multivariate model.

The study protocol was approved by the ethical committee of the Graduate School of Medicine, the University of Tokyo (No. 2244) and that of Bharatpur general hospital, Chitwan Nepal.

3.RESULTS

In multivariate regression model, neurodevelopment indicators at birth (those evaluated by seven cluster scores of NBAS) was significantly associated with the following explanatory variables: Rosenberg self esteem score (one of the indicators of prenatal maternal psychological conditions) was inversely associated (p=0.03) with habituation cluster score, while frequency of massage during pregnancy (one of the indicators of prenatal maternal psychological conditions) and incumbent length of babies (one of the indicators of nutritional status of babies at birth) was positively associated (p=0.04 and p=0.01, respectively) with the score. Cord blood concentrations of Se and As were inversely associated with state organization cluster score (p=0.03) and state regulation cluster score (p=0.01), respectively. Birth weight (one of the indicators of nutritional status of babies at birth) was positively associated (p<0.01) with state organization cluster (Table 1).

4. DISCUSSION

The neurodevelopment status (evaluated by NBAS III and BSID II) in the present study was similar to that reported in the previous studies, observed in "general" populations which do not have apparent risk factors (as defined by authors) for neurodevelopmental deficit.

Despite many papers reported the inverse associations between in utero Pb exposure and infants neurodevelopment indicators, the present study did not detect such association as reported in preceding studies. It is thought that the association between cord blood Pb concentration and neurodevelopmental indicators was not found due to low cord blood Pb.

Only in utero exposure of As grounds for the neurodevelopmental harm among the heavy metals examined in the present study population. The problem of As contamination in drinking water is worldwide (WHO, 2001). Recent studies suggested that As affects the central nervous system (Wright et al., 2006; von Ehrenstein et al., 2007; Calderon et al., 2001; Rocha-Amador et al., 2007; Wasserman et al., 2004). This study is the first to report the adverse effect of prenatal As exposure on neurodevelopmental indicator at birth in relatively low exposure level.

Regarding trace elements, Zn deficiency as reported by Andersen (2007) and Christian et al. (2006) from the region, was not confirmed in the present study population.

Cord blood concentration of Se was inversely associated with neurodevelopment indicators at birth but not at 6 months. Although endemic Se intoxication cases have been reported in China, the exposure among the participants in the present level was estimated to be much lower. The reason for the inverse association between Se and neurodevelopment stay unclear.

The present study confirmed the positive association between birth weight and neurodevelopment after 6 months as observed by Tofail et al. (2009), Torres-Sanchez et al. (2007) and Wu et al. (2008) among healthy, full term children.

Similarly, the present study demonstrated the positive association between neurodevelopmental indicator at 6 months and home environment as observed by Torres-Sanchez et al. (2007) in comparable home environment in Mexico cohort.

5. CONCLUSION

In a hospital-based cohort of infants and mothers in Terai region, lowland Nepal, relationship between the neurodevelopment of the infants at birth as well as at 6 months after birth and potential determinants of the development including metals exposure, essential trace elements, nutritional status at birth, and postnatal growth environment was examined. The cord blood As concentration was significantly associated with a part of scales of neurodevelopment of the newborns. It suggests that high As exposure during prenatal period in babies may induce retardation of in utero neurodevelopment. In utero growth environment was found to be associated with neurodevelopment at birth and after 6 months, while the postnatal growth environment (home environment) is a determinant of neurodevelopment at 6 months after birth.

Table 1. Multiple regression analysis of each cluster of NBAS III at birth 1 (n=94)

Table 2. Multiple regression analysis of each cluster of BSID II at 6 months after birth 1 (n=94)

本研究は、神経行動発達と臍帯血重金属・微量元素濃度との関連を明らかにすることを目的として、ネパール・チトワン郡のある病院で出産した100人の母親とその子供を対象におこなわれたものである。母親の臍帯血中の重金属(Pb、Cd、As)と微量元素(Zn、Se、Cu)濃度、妊娠中の母親の心理状態の指標、新生児の栄養状態と、新生児の神経行動発達指標(Neonatal Behavioral Assessment Scale IIIの各クラスター得点)との関連が検討された。さらに、6ヶ月後にフォローアップ調査をおこない、94組の母親とその乳児を対象に、乳児の神経行動発達指標(Bayley Scale of Infant Development IIの各スケール)、乳児の生育環境の指標(Home Observation of Measurement of Environment Scale)、母親の自己肯定感尺度 (Rosenberg Self Esteem Scale)についての評価をおこなった。交絡変数を調整した重回帰分析によって、以下の結果を得ている。

1.新生児のHabituationクラスターの得点(神経行動発達指標のひとつのクラスター)は母親の自己肯定感尺度と負の関連を示した(P=0.03)。一方、新生児の身長と妊娠中にマッサージを受けた頻度は、Habituationクラスターの得点と正の関連を示した(それぞれ、P=0.04、P=0.01)。

2.臍帯血中のSe濃度は、新生児のState Organization クラスター得点と負の関連を示し(P=0.03)、臍帯血中のAs濃度は、新生児のState Regulation クラスター得点と負の関連を示した(P=0.01)。

3.出生体重は、新生児のState Organization クラスター得点と正の相関を示した(P<0.01)。

4.出生から6ヶ月後の乳児の神経行動発達指標のなかで、Psychomotor Development Indexスコアは出生後の生育環境の指標と正の関連を示した(P=0.04)。

5.出生から6ヶ月後の乳児の神経行動発達指標のなかで、Behavioral Rating Scaleスコアは出生児体重と正の関連を示した。

本研究の結論として、胎児期における重金属への曝露あるいは微量元素の栄養状態は、出生時の神経行動発達指標と関連していること、胎児期における栄養状態は出生時だけでなく、出生から6ヶ月後の乳児の神経行動発達指標とも関連していることが示唆された。また、乳児の生育環境は、出生から6ヶ月後の乳児の神経行動発達指標と関連していることが示唆された。