Letter to the Editor

Epidemiologic Methods in the Search for Causesof Human Birth Defects

To the Editor:We are responding to the Editor’s invitation to com-

ment on the basis for identifying a human teratogen(Holmes ’99a), with a special focus on epidemiologicstudies.

Epidemiology is the basic science of public health. Assuch, the classical description of a disease or congenitalanomaly in ‘‘time, place, and person’’ might lead topractical means of prevention (e.g., the withdrawal ofthalidomide). Detailed research might also identifydisease association (i.e., ‘‘risk factors’’) as clues to theetiology of the disease and possibilities of diseasereduction by a variety of interventions. Such epidemio-logic studies often provide useful information to scien-tists in basic biologic disciplines, who in turn possessnew biologic information on which further epidemio-logic studies could be designed.

The work of Andaloro et al. (’98) might have providedan opportunity for such interaction between basic andepidemiologic sciences. We have become aware frommodern developmental studies—many in chick em-bryos—that abnormal cardiogenesis may originate inthe pre-organogenetic phase (Manner et al., ’96, ’99;Pexieder, ’95a), indicating that new etiologic categoriza-tions of cardiovascular malformations might have to beconsidered. Such an orderly development of progressiveresearch would follow Pexieder’s model of ‘‘the integra-tion of cellular and molecular biology with cardiacembryology and teratology, each partner taking advan-tage of the other partner’s expertise so that one canconsider with sufficient confidence the development offirst experimental and later clinical primary preven-tion’’ (Pexieder, ’95b).

Because recent controversial letters and commentson ‘‘Fetal Effects of Dextromethorphan in Ovo’’ inTeratology (Brent et al., ’99; Holmes, ’99a; Rosenquist,’99) quoted our epidemiologic study, the Baltimore–Washington Infant Study (BWIS), it is relevant tocomment on the background and methods of this re-search project and on our findings on the drug inquestion.

The need for this large population-based study be-came evident 25 years ago in a controversy similar to

the one we are now witnessing, on a possible terato-genic role of maternal hormone therapies. Severalstudies failed to reach any conclusions because ofmethodologic problems and lack of consideration of themultifactorial etiology of cardiac malformations (Chez,’78; Ferencz et al., ’80). We also experienced intrusionby the media and lawyers, attacks based on improperuse of statistics (Wilson, ’84), and regrettable attemptsto ‘‘burn each other’s books’’ (Ferencz, ’84). No scientificbenefit resulted. However, it became obvious that therewas no systematic knowledge on the distribution ofcardiovascular malformations by familial and environ-mental factors. Thus, a descriptive case-control studywas necessary to describe the occurrence of congenitalheart defects in a population and to generate etiologichypotheses.

For this new study, rigorous epidemiologic methodswere used. The design and progress of the BWIS wasreviewed in six sessions of the NIH Epidemiology andDisease Control Study Section and in one multidisci-plinary site visit, which was followed by a MeritoriousExtension in Time (MERIT) award for the concludinganalyses and reporting of the results (Ferencz et al., ’93,’97). This study was conducted over a 10-year period(1981–1989) with the collaboration of all six pediatriccardiology centers in the state of Maryland, the Districtof Columbia, and northern Virginia, comprising 53hospitals and nearly 1,000 physicians. The study in-cluded all liveborn cases of congenital heart disease inthe region confirmed by cardiologic studies (n 5 4,390)compared with a representative sample of the regionallivebirth cohort (n 5 3,572) from a birth population of906,626 infants. Home interviews of 3,377 case and3,572 control families used a structured questionnairethat provided information on the family history ofcardiac and noncardiac malformations, maternal medi-

*Correspondence to: Dr. Christopher Loffredo, Baltimore-WashingtonInfant Study, Department of Epidemiology and Preventative Medi-cine, University of Maryland, 660 West Redwood St., Baltimore, MD21201-1596.

Received 5 November 1999; Accepted 11 November 1999.

TERATOLOGY 61:243–245 (2000)

r 2000 WILEY-LISS, INC.

cal history, use of therapeutic drugs, and parentalexposures to environmental substances in lifestyle,home, and occupations during the periconceptionalperiod.

Multivariate logistic regression analysis of the ge-netic and environmental risk factors for the entire casegroup compared with controls did not reveal an associa-tion with dextromethorphan. However, the drug ap-peared as a ‘‘candidate’’ risk factor in some diagnosis-specific subgroups. We found three associations withantitussives, that is, a category of over-the-counter,multi-ingredient medications containing dextrometho-rphan as the common ingredient. Among infants withlaterality and looping defects, there were 3 of 104exposed case mothers, and among infants with non-chromosomal atrioventricular septal defect (AVSD)there were 3 of 76 exposed case mothers. Relative to 23control mother exposures (0.6%) the adjusted odds ratio(OR) for these diagnoses was 6.31 (99% confidenceinterval [CI] 5 1.3–31.5) for laterality and looping, and8.9 (99% CI 5 1.7–45.2) for AVSD. A third associationwith muscular ventricular septal defect (3 of 87 moder-ate size defects, OR 5 4.7) was significant only at the95% CI level. The rarity of the exposure and of themalformations with which they were associated sug-gests caution: the possible association needs furtherstudy in other populations and in biologic systems.

The multidisciplinary teratology community is ide-ally suited to initiate collaborative studies using high-quality methods in biology and in population studies.However, to date many population studies of teratologycontinue to have serious conceptual and methodologicflaws. Some of these were described in a recent Edito-rial and in Letters to the Editor in Teratology. Holmes(’99b) called attention to the importance of specific andcomparable diagnostic criteria of congenital anomalies.Martinez-Frias and coworkers highlighted the inappro-priate use of malformed infants as ‘‘controls’’ (Prietoand Martinez-Frias, ’99), the problems of bias in datafrom the Teratogen Information Services (Martinez-Frias and Rodriguez-Pinilla, ’99a), and the seriousresponsibility of risk assessment, in which inappropri-ate methodologies might negate the judgment of safetyof a potential teratogen (Martinez-Frias and Rodriguez-Pinilla, ’99b). Our Spanish colleagues should be stronglycommended for their principled stand for excellence inpopulation studies. Our past publications also pointedout some of these limitations (Ferencz and Correa, ’91,’95; Ferencz et al., ’93).

Sample size and design issues are critical in planningstudies and in interpreting results. It is scientificallyinappropriate to report as ‘‘negative’’ the results ofcohort comparisons without analyzing and reportingthe statistical power of the study to detect importantdifferences in outcomes. This limitation clearly appliesto the pharmaceutical registries quoted by Holmes(’99c). The National Birth Defects Prevention Study,

mentioned in that same commentary, is a case-controlstudy; i.e., identification of the exposures will be ‘‘retro-spective’’ in mothers of malformed and nonmalformedinfants, as in the BWIS. In such studies, a search for apotential teratogen must have a specific focus on ahomogeneous malformation subgroup and a sufficientbreadth of information on maternal characteristics,which might influence exposures. Our evaluation ofdrug use in pregnancy revealed a number of predispos-ing characteristics: women who reported nonprescrip-tion drug use were more likely to be white, older, andmarried; to have a higher income and private medicalcare; and to have more chronic and acute illnesses(Rubin et al., ’93). This example illustrates that there isno ‘‘quick and easy’’ approach to the complex question ofthe origins of human birth defects.

It is regrettable that the recent controversy over apossible association between dextromethorphan andspecific cardiovascular malformations has hamperedthe fundamental need for collaboration among basicand applied teratological sciences. An opportunity toexchange ideas and move the field forward, in the bestsense of Pexieder’s model (quoted above), may yetemerge through a constructive dialogue.

CHARLOTTE FERENCZCHRISTOPHER LOFFREDO*P. DAVID WILSONDepartment of Epidemiology and

Preventative MedicineUniversity of MarylandBaltimore, Maryland

LITERATURE CITED

Andaloro VJ, Monaghan DT, Rosenquist TH. 1998. Dextromethorphanand other N-methyl-D-aspartate receptor antagonists are terato-genic in the avian embryo model [comments]. Pediatr Res 43:1–7.

Brent RL, Shepard TH, Polifka JE. 1999. Response to Dr. Rosenquist’scomments pertaining to the paper by Andaloro et al. (’98) ‘‘Dextro-methorphan and other n-methyl-d-aspartate receptor antagonistsare teratogenic in the avian embryo model’’ and letters to the editorby Polifka JE and Shepard TH (’98) and Brent RL (’98) [letter].Teratology 60:61–62.

Chez RA. 1978. Progesterone, progestins, and fetal development.Fertil Steril 30:16–26.

Ferencz C. 1984. The teratogenicity of progestational agents [letter].Teratology 29:135–136.

Ferencz C. 1993. Congenital heart disease: an epidemiologic andteratologic challenge. In: Ferencz C, Rubin JD, Loffredo CA, MageeCA, editors. Epidemiology of congenital heart disease: the Baltimore–Washington infant study, 1981–1989. Mt. Kisco, NY: Futura. p 1–15.

Ferencz C, Matanoski GM, Wilson PD, Rubin JD, Neill CA, GutberletR. 1980. Maternal hormone therapy and congenital heart disease.Teratology 21:225–239.

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Ferencz C, Correa-Villasenor A. 1995. Epidemiology overview: theepidemiologic approach to the study of congenital cardiovascularmalformations. In: Clark EB, Markwald RH, Takao A, editors.Developmental mechanisms of heart disease. Armonk, NY: Futura.p 629–638.

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Ferencz C, Loffredo CA, Correa-Villasenor A, Wilson PD. 1997.Genetic and environmental risk factors of major cardiovascularmalformations: the Baltimore–Washington infant study, 1981–1989. Armonk, NY: Futura.

Holmes LB. 1999a. Studies of the fetal effects of dextromethorphan inovo [editorial]. Teratology 60:56–60.

Holmes LB. 1999b. Need for inclusion and exclusion criteria for thestructural abnormalities recorded in children born from exposedpregnancies [editorial]. Teratology 59:1–2.

Holmes LB. 1999c. Commentary. Pediatr Res 45:286–287.Manner J, Seidl W, Steding G. 1996. Experimental study on the

significance of abnormal cardiac looping for the development ofcardiovascular anomalies in neural crest-ablated chick embryos.Anat Embryol 194:289–300.

Manner J. 1999. Does the subepicardial mesenchyme contributemyocardioblasts to the myocardium of the chick embryo heart? Aquail-chick chimera study tracing the fate of the epicardial primor-dium. Anat Rec 255:212–26.

Martinez-Frias ML, Rodriguez-Pinilla E. 1999a. Problems of usingdata from teratology information services (TIS) to identify putativeteratogens. Teratology 60:54–55.

Martinez-Frias ML, Rodriguez-Pinilla E. 1999b. First-trimester expo-sure to topical tretinoin: its safety is not warranted [letter]. Teratol-ogy 60:5.

Pexieder T, 1995a. Conotruncus and its septation at the advent of themolecular era. In: Clark EB, Markwald RR, Takao A, editors.Developmental mechanisms of heart disease. Armonk, NY: Futura.

Pexieder T. 1995b. Proper laboratory practice in experimental studiesof abnormal cardiovascular development. In: Clark EB, MarkwaldRR, Takao A, editors. Developmental mechanisms of heart disease.Armonk, NY: Futura.

Prieto L, Martinez-Frias ML. 1999. Case-control studies using onlymalformed infants: are we interpreting the results correctly? [let-ter]. Teratology 60:1–2.

Rosenquist TH. 1999. Author’s reply: studies of the fetal effects ofdextromethorphan [letter]. Teratology 60:56–60.

Rubin JD, Ferencz C, Loffredo C. 1993. Baltimore–Washington infantstudy group. 1993. Use of prescription and non-prescription drugs inpregnancy. J Clin Epidemiol 46:581–589.

Wilson PD. 1984. Sample size and power in case-control studies[letter]. Teratology 29:137–138.

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