Journal Club: Dental radiation and birth weight

An article published in the April 28 issue of JAMA tries to make a link between dental radiation and infant low birth weight.

It's been making the news lately and stirring up a lot of fuss.

At first I didn't think much of it and wasn't going to pay much attention to it. Just another study trying to link tiny insignificant exposures of radiation to a much larger problem (correlated effect vs causal relationship). But after seeing it make the news I thought I'd take a closer look and see what all the commotion was all about.

The main hypothesis behind the paper is that dental x-rays result in non-trivial radiation dose the the thyroid, which affects thyroid function, which in turn has an effect on infant birth weight. There is no radiation exposure to the fetus from dental radiation.

One thing I have to take issue with is their radiation dosimetry. There doesn't appear to have been any effort to do any dose reconstruction for the study. Rather, the authors looked at what dental radiation procedures people in the study got, looked up radiation doses for those procedures from a 1993 NEXT survey of dental exposures and said "This is the radiation exposure these patients got". And if a patient had multiple x-ray procedures, they just got added up.

Now, NEXT is a fine tool to see how x-ray exposures compare for different types of procedures or equipment. The FDA (or state delegates) sends out their inspectors to randomly selected hospitals and clinics every now and then, they make some radiation measurements with various phantoms, and all of it gets compiled into the survey report.

Have a look at the 1999 NEXT Dental survey. Intraoral skin entrance exposures using D speed film (page 9) averages 194.6 mR, but with a seriously wide distribution (standard deviation 103.5). If dental units in Washington State (where the study population comes from) actually produce lower exposures than what's published in the NEXT survey, then radiation exposures are seriously overestimated in the study.

Would more accurate dosimetry affect the results of the study? Not sure. I'm also not too clear on how the authors accounted for other risk factors associated with low birth weight (smoking, alcohol consumption, hypertension, etc).

I think the most interesting table of the paper is Table 2 which lists the dental procedures, and the numbers of low birth weight (LBW), term low birth weight (TLBW) and normal birth weight (NBW) infants. The last two columns give P values for comparisons between LBW/NBW and TLBW/NBW, although I'm not sure if this is a Student's T-test correlation or some other type of test. It's not stated in the paper as far as I can see. A strong correlation between LBW/TLBW vs NBW is shown for thyroid exopsures > 0.4 mGy, although n is very low for this group. But there is also a strong correlation for TLBW vs NBW for no radiation exposure (P = 0.02) where n is much larger for both groups.

As for the main hypothesis, the study does give some support to the idea that dental radiation might have an effect on thyroid function, affecting birth weight. I don't think it's anywhere near definite though. I think to really see something, you'd have to do thyroid function tests and check hormone levels on a whole lot of people receiving dental x-rays and be able to accurately measure the radiation dose to the thyroid in order to get any sort of valid correlation. I think it's more likely that dental radiation to the thyroid is a contributing factor rather than a causal factor in low birth weight, acting in conjunction with other low birth weight risk factors.

Antepartum Dental Radiography and Infant Low Birth Weight JAMA. 2004;291:1987-1993

Abstract:

Context Both high- and low-dose radiation exposures in women have been associated with low-birth-weight offspring. It is unclear if radiation affects the hypothalamus-pituitary-thyroid axis and thereby indirectly birth weight, or if the radiation directly affects the reproductive organs.

Objective To investigate whether antepartum dental radiography is associated with low-birth-weight offspring.

Design A population-based case-control study.

Participants and Setting Enrollees of a dental insurance plan with live singleton births in Washington State between January 1993 and December 2000. Cases were 1117 women with low-birth-weight infants (<2500 g), of whom 336 were term low-birth-weight infants (1501-2499 g and gestation 37 weeks). Four control pregnancies resulting in normal-birth-weight infants (2500 g) were randomly selected for each case (n = 4468).

Main Outcome Measures Odds of low birth weight and term low birth weight by dental radiographic dose during gestation.

Results An exposure higher than 0.4 milligray (mGy) during gestation occurred in 21 (1.9%) mothers of low-birth-weight infants and, when compared with women who had no known dental radiography, was associated with an adjusted odds ratio (OR) for a low-birth-weight infant of 2.27 (95% confidence interval [CI], 1.11-4.66, P = .03). Exposure higher than 0.4 mGy occurred in 10 (3%) term low-birth-weight pregnancies and was associated with an adjusted OR for a term low-birth-weight infant of 3.61 (95% CI, 1.46-8.92, P = .005).

Conclusion Dental radiography during pregnancy is associated with low birth weight, specifically with term low birth weight.