Children who suffer a head injury are often routinely examined by a CT scan. Now, a Swedish team has found that radiation doses typically delivered by such a scan during infancy may harm intellectual capacity later in life.

While high doses of radiation to the developing human brain are known to cause mental retardation, it was not known if exposure to low doses "has more subtle effects on cognitive function," Dr. Per Hall from the Karolinska Institute in Stockholm and colleagues write in this week's British Medical Journal.

To investigate, they analyzed mental function and education in roughly 3000 18- to 19-year-old men who had been given relatively low doses of radiation before the age of 18 months to treat a type of birthmark called a cutaneous hemangioma.

Hall's group classified the subjects into four radiation dose categories, measured in milligrays (mGy): 1-20, 21-100, 101-250, and over 250 mGy.

They discovered that the percentage of boys who attended high school decreased with radiation doses greater than 100 mGy compared with the lowest dose of 1-20 mGy. The proportion of high school attendees decreased from roughly 32 percent among those with no exposure to radiation, to 17 percent among those who received > 250 mGy.

Radiation of the brain during infancy also had a negative effect on tests of learning ability and logical reasoning but not on tests of spatial recognition.

It is estimated that a head CT scan performed on an infant imparts a radiation dose of about 120 mGy.

"Irradiation of the brain with dose levels overlapping those imparted by CT can, in at least some instances, adversely affect intellectual development," Hall and colleagues write.

Based on their findings, they think that "the risks and benefits of CT scans in minor head trauma need re-evaluating."

SOURCE: British Medical Journal, January 3, 2004.

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