Heat waves and other extreme heat exposures during an embryo's critical period were positively associated with congenital cataracts, results of a population-based case control study showed.
Development of congenital cataracts had a significant association with the occurrence of heat waves (adjusted odds ratio [AOR] 1.97, 95% CI 1.17 to 3.32), number of heat waves (AOR 1.45, 95% CI 1.04 to 2.02), number of days with temperature above the 90th percentile (AOR 1.09, 95% CI 1.02 to 1.17), and 5-degree increases from the mean daily minimum temperature (AOR 1.51, 95% CI 1.14 to 1.99), according to Alissa Van Zutphen, PhD, of the New York State Department of Health, and colleagues.
Inconsistent, but positive, associations also were seen between several heat indicators and renal agenesis/hypoplasia, Zutphen and co-authors wrote online in Environmental Health Perspectives.
Although hyperthermia is known to be a teratogen in animals, and maternal fever in humans has been associated with birth defects in their offspring, little data are available on the relationship between high environmental temperatures and birth defects, the authors noted in their introduction. To explore the issue further, the researchers examined the association between high ambient temperatures during the critical period of embryogenesis and the rate of birth defects.
The study gathered birth certificate data on 6,422 patients in the New York State Congenital Malformations Registry from 1992 to 2006, as well as 59,328 control patients born in New York state during the same time period, amounting to a 10% sample of the New York state population, excluding New York City.
Meteorological data was collected through the National Climate Data Center and overlaid onto 14 weather regions that New York state was divided into based on homogeneous weather and ozone exposures. The regions were then assigned to mothers based on residence at the time of the child's birth.
Birth certificates contained maternal age, race/ethnicity, education, date of last menses, prenatal care, and smoking and similar characteristics, as well as infant date of birth, sex, birth weight, and gestational age.
Weather data included daily average temperature values in degrees Fahrenheit. Weather analyses were restricted to summer months. Heat waves were identified as three or more consecutive days of mean temperature above the 90th percentile.
Congenital cataracts remained positively and significantly associated -- after adjustment for detection bias -- with a number of warm weather extremes, including:
- Five-degree increase in mean daily minimum temperature (AOR 1.79, 95% CI 1.28 to 2.51)
- Five-degree increase in mean average daily temperature (AOR 1.76, 95% CI 1.25 to 2.46)
- Five-degree increase in mean daily maximum temperature (AOR 1.76, 95% CI 1.25 to 2.46)
- Heat wave (AOR 1.96, 95% CI 1.09 to 3.51)
- Number of heat waves (AOR 1.52, 95% CI 1.05 to 2.19)
- Number of days with temperature above 90th percentile (AOR 1.11, 95% CI 1.03 to 1.2)
Additionally, the adjusted odds ratio for congenital cataracts and mean daily minimum temperature showed significant increased "susceptibility from week 1 to weeks 6 and 7, and no exposure by week 10," the researchers wrote.
Renal agenesis/hypoplasia was significantly and positively associated with a 5-degree increase in mean daily minimum temperature (AOR 1.17, 95% CI 1.00 to 1.37), but did not remain significant when associated with mean or maximum 5-degree mean increase in temperature.
The authors also noted a negative significant association between 5-degree increase in mean daily minimum temperature and anophthalmia/microphthalmia (AOR 0.71, 95% CI 0.54 to 0.94), although there were no other significant negative associations.
The authors noted that their study was limited by lack of heterogeneity of indoor and outdoor temperatures, missing air conditioner use data, and potential chance effect on statistically significant findings.
They added that future studies on cataract development during the critical gestational period should involve different study populations.
The study was funded by the Centers for Disease Control and Prevention.
The authors declared no conflicts of interest.
- Reviewed by Robert Jasmer, MD Associate Clinical Professor of Medicine, University of California, San Francisco and Dorothy Caputo, MA, BSN, RN, Nurse Planner
Environmental Health PerspectivesSource Reference: Van Zutphen AR, et al "A population-based case-control study of extreme summer temperature and birth defects" Environ Health Perspect 2012; DOI: 10.1289/ehp.1104671.