Higher summer heat may cause congenital cataracts, a birth defect.

Dec 18, 2012

Van Zutphen, AR, S Lin, BA Fletcher and S-A Hwang. 2012. Population-based case-control study of extreme summer temperature and birth defects. Environmental Health Perspectives http://dx.doi.org/10.1289/ehp.1104671.

Synopsis by Cheryl Stein

Higher summer heat is associated with increased risk of a rare birth defect that can lead to blindness. A 5-degree increase in temperature was associated with a 51 percent increase in congenital cataracts. The strongest link in this new study was found during a specific time of pregnancy when the eye is developing. The finding could be worrisome given climate change and increases in extreme hot-weather events.

A 5-degree increase in apparent temperature during a specific time of pregnancy was associated with an increased risk of congenital cataracts, a rare birth defect, according to a study from New York State.

Apparent temperature is a universal measure that incorporates temperature, humidity and other weather such as vapor pressure and wind speed to account for how hot it actually feels. 

The strongest association between higher summer temperature and congenital cataracts occurred during gestational weeks four to seven, which is when the fetal eye is developing. This compatibility with biology adds credence to the study's findings.

Similar associations between the hotter weather and the cataracts were also found with other measures of heat, including heat waves and number of heat waves. The increased risk of congenital cataracts at higher temperatures is worrisome in the context of extreme weather events and climate change.

Congenital cataracts are rare, occurring in about three out of every 10,000 live births. With cataracts the lens of the eye is cloudy and vision is blurry. In babies this blurriness can lead to permanent blindness, because the brain doesn’t receive the visual stimuli it needs to develop vision pathways. Surgically removing the cataracts can protect the infant's sight.

Increased body heat during pregnancy – such as occurs with a fever – can cause birth defects including neural tube defects, eye defects and abdominal wall defects. Having a fever during pregnancy does not mean that the baby will have a birth defect, but sometimes the elevated body temperature happens during a critical period and fetal development is affected.

The researchers from the New York State Department of Health wanted to explore whether other factors that increase body temperature – like high summer heat – could also cause birth defects.

This study, published in the journal Environmental Health Perspectives, used New York State birth certificate data, birth defects registry data and routinely collected weather data to examine the association between summer temperature and birth defects during June, July and August from 1992 - 2006. Researchers analyzed data on 6,422 babies with birth defects and 59,328 babies without birth defects who were at weeks 5 to 20 of gestation during the summer months. Average daily weather data were assigned to the 15-week pregnancy periods based on where the mother lived at the time of delivery.

A 5-degree increase in the apparent temperature was associated with a 51 percent increase in risk of congenital cataracts. In fact, risk of congenital cataracts was associated with all measures of temperature studied: heat wave, the number of heat waves and the number of days when temperatures were at their highest – in the top 10 percent of all temperatures measured. Conversely, the same 5-degree increase in apparent temperature was associated with a reduced risk of a different type of eye birth defect. 

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